ANATOMY

BONE ANATOMY

Bone is an osseous tissue that makes up the part of the skeleton in most humans. It is well versed in resisting tensile and compact forces. By virtue of their name, the skeletal muscles attach to the skeleton and operate on the bone to cause movement. The markings on the bones (the bony markings) are the specific locations where muscles attach.

When a fetus is 8 weeks old, its skeleton is nothing but rubbery tissues, called cartilage. When the fetus grows and the trimester advances, these cartilages start getting converted to solid bones, this process is called ossification. At birth, there are approximately 300 bones present; many of these fuse together during development, leaving a total of 206 separate bones in the adults. Bone is actively constructed and remodeled throughout life by special bone cells i.e osteoblasts, osteoclasts, etc.

(*Osseous tissues are a major structural and supportive connective tissue consisting mainly of a honeycomb-like collagen matrix which is internally mineralized with bound minerals crystals like calcium and phosphorus. The combination of flexible collagen and hard mineral crystals makes this tissue hard without making it brittle, thus this way the bones are made super strong. Osseous tissues are of two major types: the compact bone and the spongy bone tissues.)

BONE TISSUES

In simplest words, bone tissue is an aggregation of cells with similar functions meant to perform a specific function. Even our bone is itself a tissue but if we go with a broad picture and watch out for the number of jobs bone does, we can actually say that bone is an organ, simply because there is lots of stuff inside the bone, it is made up of nervous tissues, connective tissues, cartilage, blood vessels, etc. Inside the bone, the bone tissues are finely tuned to enhance their mechanical strength.

Bone also contains mineralized tissues, which are biological tissue that incorporates minerals into soft matrices, and these tissues form a protective shield that also imparts structural support. These mineralized tissues, combine stiffness, low weight, and high strength due to the presence of certain minerals i.e calcium, phosphate, etc. Due to structural layering formed inside the tissues, they are able to transfer the loads and stresses throughout the bone length which results in the indulgence of energy within the arrangement. Our bone structure has five major types of tissues i.e woven tissues, lamellar tissues, marrow, periosteum tissues, and endosteum tissues, let’s discuss them one by one.

WOVEN TISSUES

Woven tissues or simply woven bones are composed of loosely and randomly arranged collagen bundles containing numerous cells that vary in shape and size.

Woven tissue is a weak type of bone tissue that is still developing and its matrix is still unorganized and contains a matrix of orderless crisscross texture.

Woven bone tissues are found mostly in the embryonic skeleton (an early stage of development, it is the part of a life cycle that begins just after fertilization and continues through the formation of body structures such as tissues and organs) and also in cortical and cancellous bones at the state of rapid bone growth.

As the bone grows the woven bone tissues are replaced in the normal skeleton by lamellar bone tissues.

LAMELLAR TISSUE

Lamellar bone tissues are the main and mature type of bone tissues present in a
well-developed skeleton.

Lamellar bone tissue is a hard type of bone tissue that is characterized by an orderly arrangement of collagen bundles and their cells. Its matrix is organized, uniform, and regularly distributed.

Lamellar bone tissues are formed from the woven bone tissues to remodel their mechanical strength and toughness.

Lamellar bone tissues are of two types:-
1. Compact bone (or cortical bone)
It serves a mechanical function i.e support, protection, etc. This is the area of bone to which ligaments and tendons attach. It is a thick, dense, and ultra-hard outside layer of bones. It contains 80% of the total bone mass of the adult skeleton.
2. Spongy bone (or trabecular bone or cancellous bone)
It serves a metabolic function such as the exchange of calcium ions. This type of bone is located between layers of compact bone and fills the interior of many bones. It is thin and porous. It contains only 20% of the total bone mass of the adult skeleton.


BONE MARROW

Bone marrow is a semi-solid, nutrient-dense, soft connective tissue that is found inside a cavity (within the spongy bone), called the marrow cavity. Bone marrow contains nearly 5% of total body mass in healthy adults which produces roughly 500 billion blood cells per day, which join the blood circulation. People need healthy bone marrow and blood cells to live.

There are two types of marrow in adults which are:-

1. Red bone marrow (myeloid tissue)
It consists of a fine, highly vascular fibrous tissue containing hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.
Red blood cells (erythrocytes): These transport oxygen around the body.
White blood cells (leukocytes): These help fight infection and disease.
Platelets (thrombocytes): These help with blood clotting after injury.

2. Yellow Bone Marrow (fatty tissue)
Yellow bone marrow contains mesenchymal stem cells or marrow stromal cells. It also aids with the production of fat, bone, and cartilage. Importantly, it helps with the storage of fats in cells, known as adipocytes, which helps provide sustenance and maintain the correct environment for bones to function.

(Stem cells are immature cells that can turn into a number of different types of cells)

Why do we need healthy bone marrow?

As white blood cells survive from a few hours to a few days, platelets for about 10 days, and red blood cells for about 120 days. Bone marrow needs to replace these cells constantly, as each blood cell has a set life expectancy. Bone marrow also produces and releases more white blood cells in response to infections and more platelets in response to bleeding. If a person experiences serious blood loss, yellow bone marrow can activate and transform into red bone marrow in order to produce red blood cells.

Red Bone Marrow OR White Bone Marrow, which one is predominant?

The composition of marrow is dynamic, as the mixture of cellular and non-cellular components shifts with age and in response to systemic factors. All marrow is red in newborns until about the age of seven, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones. While, yellow bone marrow tends to be located in the central cavities of long bones and is generally surrounded by a layer of red bone marrow within a sponge-like reticular framework, and during life-threatening events, serious emergencies, where our body has experienced rapid blood loss then here, the yellow bone marrow essentially transforms into the red bone marrow to produce blood cells and keep us alive. The red bone marrow produces all red blood cells and platelets and also around 60-70% of lymphocytes in human adults. Other lymphocytes begin life in the red bone marrow and become fully formed in the lymphatic tissues, including the thymus, spleen, and lymph nodes.

PERIOSTEUM

The periosteum is a membranous tissue that covers the surfaces of your bones except those surrounded by cartilage and where tendons and ligaments attach to the bone.

These periosteum tissues consist of an outer fibrous layer composed mostly of an elastic fibrous material i.e collagen and an inner cellular layer (also called as cambium layer).

The outer layer contains nerve fibers that cause pain when the tissue is damaged. It also contains many blood vessels to contribute to the blood supply within bones, which can pass into the dense and compact layer of bone tissue below, to supply the osteocytes (a type of bone cells).

The inner layer of the periosteum contains osteoblasts (bone-producing cells) and is most prominent in fetal life and early childhood when bone tissues are still developing. These osteoblasts are responsible for increasing the width of a long bone. and the overall size of the other bone types.

When a bone gets fractured, the progenitor cells develop into osteoblasts and chondroblasts, which are essential to make bones healthy again.

The inner layer of the periosteum becomes thinner with age. This thinning begins in childhood and continues through adulthood.

ENDOSTEUM

Endosteum tissues are the type of connective tissue mainly composed of reticular connective tissue which lines the inner surface of the bone, forming the medullary cavity of long bones to enclose the bone marrow.

Endosteum tissues form a highly vascular membrane lining that has a network of reticular fibers, made of type III collagen. (Type III collagen is fibrillar forming collagen and is expressed in early embryos and throughout embryogenesis. In the adult, type III collagen is a major component of the extracellular matrix in a variety of internal organs and skin)

Endosteum tissues are classified into three types based on their location:-
(i) Cortical endosteum: It lines the bone marrow cavity
(ii) Osteon endosteum: Endosteum lining the osteons mainly contains nerves and blood vessels.
(iii) Trabecular endosteum: Lines the trabecula near the developing part of the bone. It plays a role in the growth and development of the bone.

Osteoclasts cells are present in the endosteum tissues in the regions of active bone resorption. During bone growth, the width of the bone increases as osteoblast cells lay new bone tissue at the periosteum tissue region. To prevent the bone from becoming unnecessarily thick, osteoclasts cells resorb the bone from the endosteal side.


BONE CELLS

The bone is a metabolically very active tissue, when it comes to growing, shaping, and maintenance of bones, there are four basic types of bone cells are present in our bone tissues osteoblasts, osteocytes, osteogenic cells, and osteoclasts which help to maintain optimal bone functioning.

Osteoblasts
- Osteoblasts cells are bone-forming cells with a single nucleus.

- It makes a mineralized bone matrix.

(They make a protein mixture that is composed primarily of collagen and creates the organic part of the matrix.)

- Osteoblasts cells also release calcium and phosphate ions that form mineral crystals within the matrix.

- Osteoblasts cells produce hormones like prostaglandins that also play a role in the mineralization of the matrix.

Osteocyte
Osteocytes cells are the most commonly found bone cell in mature bone tissue and can live as long as the organism itself.
Osteocytes cells are formed from osteoblasts that have become entrapped within their own bone matrix.
Osteocytes cells help regulate the formation and breakdown of bone tissue.

Osteocytes cells have multiple cell projections that are thought to be involved in communication with other bone cells.
Osteocyte cells are relatively inert cells, they are capable of molecular synthesis and modification.
The osteocyte cells are an important regulator of bone mass and a key endocrine regulator of phosphate metabolism.


Osteogenic
Osteogenic cells are the only bone cells that can divide.
Osteogenic cells can get altered and develop into osteoblast cells which, in turn, are responsible for forming new bones.
Osteogenic cells are mitotically active stem cells and are found in the deep layers of the periosteum and the marrow.

Osteoclast
Osteoclasts cells are bone cells with multiple nuclei and are found in pits of the bone surface.
Osteoclasts cells are responsible for the breakdown of bones by the process of bone resorption. (This process also helps regulate the level of blood calcium. New bone is then formed by the osteoblasts cells. Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts)
Osteoclasts cells can dissolve the minerals in bone and release them into the blood whenever needed, or when the bone becomes extra thick. This function is critical in the maintenance, repair, and remodeling of bones.

BONE COMPOSITION

Our bones are made up of living cells that are enclosed in a mineralized organic matrix. Basically our bones are composed of both organic and inorganic compounds.

ORGANIC COMPOUNDS:- The main organic component of human bone is type I collagen. Approximately 30% of the component of bone consists of organic matter. The organic component of human bone gives bone its tensile strength.

INORGANIC COMPOUNDS:- The main inorganic component of human bone is hydroxyapatite having a composition of Ca10(PO4)6(OH)2 Approximately 70% of the component of bone consists of inorganic matter. The inorganic component of human bone gives bone its compressive strength.

BONE DEVELOPMENT

Bone development begins with the replacement of collagenous mesenchymal tissue (refers to cells that develop into connective tissue, blood vessels, and lymphatic tissues) by bone. This results in the formation of woven bone, a primitive form of bone with randomly organized collagen fibers that are further remodeled into mature lamellar bone, which possesses regular parallel rings of collagen. Lamellar bone is then constantly remodeled by osteoclasts and osteoblasts.
There are two different methods by which bone is produced from mesenchymal tissue:-

1. Endochondral Ossification
It is the process by which cartilage is progressively replaced by bone at the epiphyseal growth plates. This occurs in long bones, the vertebrae, and the pelvis.

2. Intramembranous Ossification
It is the process by which mesenchymal tissue is directly replaced by bone without an intermediate cartilage step. It occurs most notably in the bones of the skull.

*Ossification is simply the process of bone formation.

BONE REMODELLING

In adults, after growth has come to an end, bone is formed by the osteoblasts cells only where it was previously resorbed by the osteoclast cells. This follows a specific sequence of events, and takes about three months in total to complete:-

1. Activation: In the process of activation, osteoblast cells induce osteoclast cells to break down the bone matrix. This process lasts for approximately 3 days.

2. Resorption: In resorption, the ruffled border of the osteoclast forms a sealing zone that isolates the area of bone erosion. Organic acids and lysosomal enzymes dissolve the mineral component and break down the organic matrix, respectively. This process occurs for approximately 14 days.

3. Reversal: Over time, osteoblasts begin to replace osteoclasts cells at the site of bone turnover.

4. Formation: Osteoblasts cells begin to lay down new lamellar bone on top of old bone. In doing so, cement lines are created to mark the borders between the old and new bone matrix. This can take up to 70 days to complete.

AXIAL SKELETON

The axial skeleton is the subdivision of the human skeleton that mainly consists of the skull, rib cage, and vertebral column.

The axial skeleton consists of a total of 80 bones within it.

SKULL

The human skull consists of a total of 22 bones of which there are 8 cranium bones and 14 facial bones.

The cranium bones are present in the upper part of the skull that holds and protects the brain in a large space called the cranial vault. The cranium is formed from eight irregular flat plate-shaped bones which fit together.

The facial bones are present in the lower part of the skull. There is 14 facial bones like mandible, maxilla, zygomatic, etc.

Together the upper part of the skull and the lower part of the skull has 22 bones.

Humans are born with separate plates which later fuse to allow flexibility as the skull passes through the pelvis and birth canal during birth. During development, the eight separate plates of the immature bones fuse together into one single structure known as the Skull. The only bone that remains separate from the rest of the skull is the mandible.

VERTEBRAL COLUMN

The vertebral column, also known as the backbone or spine, is the main part of the axial skeleton. The number of vertebrae in a region can vary but overall the number remains the same. In a human's vertebral column, there are normally 33 vertebrae. The vertebrae in the human vertebral column are divided into 5 different regions, which correspond to the curves of the spinal column. Individual vertebrae are named according to their region and position. From top to bottom, the vertebrae are:
1. Cervical: This region has 7 vertebrae named from C1 to C7
2. Thoracic: This region has 12 vertebrae named from Th1 to Th12
3. Lumbar: This region has 5 vertebrae named from L1 to L5
4. Sacrum: This region has 5 vertebrae named from S1 to S5
5. Coccyx: This region has 4 vertebrae named from Cc1 to Cc4

(The sacrum region is a fused region formed by fusion of 5 vertebrae, each vertebra of sacrum region is known as “sacral bone” , after getting fused up they are collectively known as sacrum)

(The Coccyx region is a fused region formed by fusion of 4 vertebrae, each vertebra of the Coccyx region is known as “coccygeal bone” , after getting fused up they are collectively known as Coccyx)

At birth, the majority of humans have 33 separate vertebrae. However, during normal development several vertebrae fuse together, leaving a total of 24, in most cases. The confusion about whether or not there are 32-34 vertebrae stems from the fact that the two lowest vertebrae, the sacrum, and the coccyx, are single bones made up of several smaller bones which have fused together. This is how the vertebrae are counted: 24 separate vertebrae and the sacrum, formed from 5 fused vertebrae, and the coccyx, formed from 4 fused vertebrae. If you count the coccyx and sacrum each as one vertebra, then there are 26 vertebrae. If the fused vertebrae are all counted separately, then the total number of vertebrae comes to 33.

The vertebral column is curved in several places, these curves allow the human spine to better stabilize the body in the upright position.

The vertebral column surrounds the spinal cord which travels within the spinal canal, formed from a central hole within each vertebra. The spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body.


RIB CAGE

The rib cages are made up of a classic arrangement withholding 25 separate bones in which there are 12 pairs of ribs along with 1 sternum bone (also known as breastbone) attaching to the vertebral column.

These 25 bones of the rib cage together are also known as the thoracic cage or thoracic wall as most of them join the thoracic region of the vertebral column at the posterior side.

The rib cage is a semi-rigid bony and cartilaginous structure that protects the vital organs such as the heart and lungs and provides support to the shoulder girdle in order to form an integral part of the human skeleton system.

The rib cage provides attachments for many muscles of the neck, chest, abdomen, and back.

The rib cage also holds the main muscles of the respiratory system i.e diaphragm, intercostal muscle, etc.

The pairs of rib cage bones are classified into three types according to their structural arrangement:-

1. True Ribs

The uppermost seven pairs of ribs attach to the sternum with costal cartilage and are known as “true ribs.”

2. False Ribs

A rib is said to be false if it does not attach to the sternum directly. Here, the 8th, 9th, and 10th ribs get connected together, and then finally move to get joined sternum.

3. Floating Ribs:

The last two ribs are called “floating ribs” as they do not attach to the sternum or to other ribs and simply hang freely.

*The length of each rib increases from number one rib to rib number seven and then decreases until rib pair number 12. The first rib is the shortest, broadest, flattest, and most curved.

APPENDICULAR SKELETON

The APPENDICULAR skeleton is the subdivision of the human skeleton that mainly consists of the shoulder girdle, arms, hand, pelvis, legs, and feet.

The appendicular skeleton consists of a total of 126 bones within it.

1. SHOULDER GIRDLE (4 Bones)
Left and right Clavicle (2 Bones) and Scapula (2 Bones)

2. ARMS (6 Bones)
Left and right Humerus (2 Bones), Ulna (2 Bones), and Radius (2 Bones).

3. WRISTS & HANDS (54 bones)
Left and Right Carpals (16 Bones), Metacarpals (10 Bones), Phalanges (28 Bones).

4. PELVIS (2 Bones)
Left Hip Bone and Right Hip Bone (2 Bones).

5. LEGS (8 Bones)
Left and Right Femur (2 Bones), Patella (2 Bones), Tibia (2 Bones) and Fibula (2 Bones).

6. FEET & ANKLES (52 bones)
Left and Right Tarsals (14 Bones), Metatarsals (10 Bones), Phalanges (28 Bones).

SHOULDER GIRDLE

The shoulder girdle, also known as the pectoral girdle, is a set of bones that connects our upper limbs to the bones along the axis of our body. It provides the anatomical mechanism that is responsible for structural support and movement of our shoulder on the left and right sides of our body.

The shoulder girdle connects the muscles necessary for the shoulder and arm movement. The five muscles that include the function of the shoulder girdle are the trapezius, levator scapula, rhomboid, serratus anterior muscle, and pectoralis minor muscle.

The shoulder girdle on either side of our body isn’t joined together. This allows for our shoulders and arms to move and function independently.

The shoulder girdle consists of the total 4 bones that make up your shoulder:-

1. Clavicle: 2 Bones (Also Known as collarbone)
- It is also known as the collarbone.
- It is an s-shaped bone.
- It is located at the front of your body.
- It is the only long bone in the body that lies horizontally.
- It provides an attachment point for several muscles.
- It supports your shoulder, encourages a full range of motion, and protects your nerves and blood vessels that pass between the trunk of your body and your upper limbs.
- It provides the only direct connection between your pectoral girdle and axial skeleton.
- This bone is often shorter and less curved in women, while in men it’s longer and heavier with a more defined curve.

2. Scapula: 2 Bones (Also Known as shoulder blade)
- It is also known as the shoulder blade, shoulder bone, wing bone, speal bone, or blade bone.

- It is triangular in shape and connects our humerus with our clavicle bone.

- It is located at the back of your shoulder.

- It provides an attachment point for several muscles.

- It is divided into three borders:-
1. Medial border (vertebral border)
2. Lateral border (axillary border)
3. Superior border (the thinnest and shortest of the three borders)
- It has three angles:-
1. Inferior angle.
2. Superior angle (medial angle)
3. Lateral angle (head of the scapula is the thickest part of the scapula)


ARMS

The arms have total 6 bones in which there are left and right Humerus (2 Bones), Ulna (2 Bones), and Radius (2 Bones).

HUMERUS

The humerus is a long bone in the upper arms covering the length from shoulder to the elbow. It connects the forearm bones radius and ulna to the scapula.

RADIUS

The radius is one of the two long bones of the forearm. It extends from the outer to the lateral side of the elbow to the thumb side of the wrist when the body is in anatomical position.

ULNA
The ulna is also a long bone of the forearm along with the radius bone. It extends from the inner or medial side of the elbow to the smallest finger of the wrist, when the body is in anatomical position.

Both ulna and radius run parallel to each other when the body is in an anatomical position. The ulna is usually slightly longer than the radius, but the radius is thicker. Therefore the radius is considered to be the larger of the two.

WRISTS & HANDS

The wrist and hands contain a total of 54 bones of which there are 16 carpal bones, 10 metacarpal bones, and 28 phalange bones.

CARPAL BONES
The carpal bones are a group of eight small short sized bones that together form the wrist that connects the hand to the forearm. The main role of the carpal bones is to aid in effective positioning of the hand and better use of the muscles of the forearm.

METACARPAL BONES
The metacarpal bones are a group of five long bones present in the hands forming the middle part of the hand located between the phalanges of the fingers and the carpal bones of the wrist.

The metacarpal bones are also known as the metacarpus.

Each metacarpal bone consists of a body or shaft, and two extremities: the head at the distal end (near the fingers), and the base at the proximal or carpal end (close to the wrist).

PHALANGES
The phalanges are the set of long bones present in the hand & foot that make up the fingers of the hand & foot.
There is a total of 56 phalanges in the human body, with fourteen on each hand and foot. Three phalanges are present on each finger, with the exception of the thumb and large toe, which possess only two.
The phalanges of the hand are commonly known as the finger bones.
The phalanges of the foot differ from the hand in that they are often shorter and more compressed.

PELVIS

The pelvis skeleton is made up of two hip bone which is situated in the lower part of the human skeleton between the vertebral column and the femur bone of the thighs. The skeleton of the pelvis is a basin-shaped ring of bones.

Each hip bone consists of 3 bony sections named ilium, ischium, and pubis. During childhood, these sections are separate bones, joined by the cartilages. During puberty, they fuse together to form a single pair of hip bones. The entire pelvis region or pelvis girdle is formed by a pair of hip bones along with the sacrum and coccyx bone of the vertebral column.

The hip bones are connected to each other anteriorly at the pubic symphysis, and posteriorly to the sacrum at the sacroiliac joints to form the pelvic ring. This ring is very stable and allows very little mobility, it an essential for transmitting loads from the trunk to the lower limbs.

The pelvis serves three main functions which are:-

1. It bears the weight of the upper body when sitting and standing, transferring that weight from the axial skeleton to the lower appendicular skeleton when standing and walking.

2. It provides the attachments for and withstanding the forces of the powerful muscles of locomotion and posture.

3. It contains and protects the pelvic floor muscle along with the reproductive organs.

LEGS

The legs have a total of 8 bones which there are the left and right Femur (2 Bones), Patella (2 Bones), Tibia (2 Bones), and Fibula (2 Bones).

FEMUR
The femur is also known as the thigh bone as the femur is the only bone present in our thigh.

The femur serves as an attachment point for all the muscles that exert their force over the hip and knee joints. Here, a total of 23 muscles either originate from or insert onto the femur.

The femur is the strongest, largest, and thickest bone of the human body.

The femur length on average is 26.74% of a person's height in most cases.

The femur is a type of long bone having a large, thick, and nearly cylindrical body. It is a little broader above than in the center, broadest, and somewhat flattened from before backward below and two femurs converge medially toward the knee joint where they articulate with the proximal ends of the tibia bone.

The upper end of the femur contains the head, neck, the two trochanters, and adjacent structures. The head of the femur articulates with the acetabulum on the pelvic one forming the hip joint, while the distal end of the femur articulates with the tibia bone and patella, and forms a knee joint.

TIBIA

The tibia bone is also known as the shinbone or shankbone.

The tibia is the second largest long bone in the human skeleton after the femur bone of the thigh.

The tibia is situated on the inner side of the leg beside the fibula bone below the knee and it connects the knee with the ankle bones.

As compared to the fibula bone which sits below the knee joint beside the tibia bone, the tibia is the larger, stronger, and more frontal among these two bones below the knee.

The tibia bone is a part of four joints and they are:-
1. Knee Joint

2. Ankle Joint

3. Superior Tibiofibular Joint

4. Inferior Tibiofibular Joint.

FIBULA

The fibula bone is also known as calf bone.
The fibula bone is a long bone situated at the outer side of the tibia bone

The fibula bone is the smaller of the two bones present below the knees and in proportion to its length, the most slender of all the long bones.

Fibula’s upper extremity is small, placed toward the back of the head of the tibia, below the knee joint.
The fibula bone does not participate in the formation of the knee joint and it does not carry any significant weight of the body.

The fibula extends past the lower end of the tibia and forms the outer part of the ankle providing stability to the ankle joint.

The fibula's main functional role is to act as an attachment for muscles, as well as provide stability to the ankle joint.

FEET & ANKLES

The feet and ankles combined have a total of 52 bones in which there are left and right Tarsals (14 Bones), Metatarsals (10 Bones), and Phalanges (28 Bones).

TARSAL

The tarsal is a bunch of seven articulating short bones in each foot within the human skeleton making up a strong weight-bearing platform.

The seven bones in tarsals are calcaneus, talus, cuboid, navicular, medial cuneiform, intermediate cuneiform, and lateral cuneiform.

The tarsal is very much similar to the carpals in the wrist.

In humans, the tarsals, in combination with the metatarsal bones, form a longitudinal arch in the foot which is a shape well adapted for carrying and transferring weight while walking.

The calcaneus bone, also known as the heel bone, is the largest tarsal and forms the primacy at the back of the foot.

METATARSAL

The metatarsal bones are a bunch of five long bones in the foot.

The metatarsal bones are numbered from the medial side the first, second, third, fourth, and fifth metatarsal.

The metatarsal bones are similar to the metacarpal bones of the hand.

The lengths of the metatarsal bones in humans are, in descending order, second, third, fourth, fifth, and first. The first metatarsal is shorter and thicker than the others.

The base of each metatarsal bone articulates with one or more of the tarsal bones at the tarsometatarsal joints and the head with one of the first rows of phalanges at the Metatarsophalangeal joints. Their bases also articulate with each other at the inter-metatarsal joints.

The first metatarsal articulates with the medial cuneiform, and to a small extent to the intermediate cuneiform.
The second metatarsal articulates with all three cuneiforms.
The third metatarsal articulates with the lateral cuneiform.
The fourth metatarsal articulates with the lateral cuneiform and the cuboid.
The fifth metatarsal articulates with the cuboid.

PHALANGES

The phalanges are the types of long bones present in the hand & foot that make up the fingers of the hand & foot.

There are 14 phalanges in the foot which are arranged in a similar manner as in the hand.

The big toe has two phalanges (proximal and distal) whereas the remaining four fingers of the foot have three phalanges (proximal, intermediate, and distal).

There is a total of 56 phalanges in the human body, with fourteen on each hand and foot. Three phalanges are present on each finger, with the exception of the thumb and large toe, which possess only two.

Like the metatarsals, each phalange has a base (proximally), a shaft, and a head (distally). Like the 1st metatarsal, the first phalanges of the first digit are short and broad.

The phalanges of the foot differ from the hand in that they are often shorter and more compressed.

TYPES OF BONE

1. LONG BONE

The long bones are longer than they are wide, consisting of a shaft-like structure, and are a rounded head at each end.

Long bones function to support the weight of the body and with the help of muscles, long bones work as levers to facilitate movement.

Long bones are mostly located in the appendicular skeleton (mostly limbs).
Long bones in the lower limbs (tibia, fibula, femur, metatarsals, and phalanges)
Long bones in the upper limbs ( humerus, radius, ulna, metacarpals, and phalanges)

They are made up mostly of compact bone tissues, with lesser amounts of marrow tissues. (marrow is found mainly in the central skeleton, such as the pelvis, sternum, cranium, ribs, vertebrae, and scapulae, and variably found in the proximal epiphyseal ends of long bones such as the femur and humerus)

2. SHORT BONE

The short bones are small in size, about as long as they are wide with a rough cube-like structure along with an articular surface at each end of them to facilitate flexibility of structures.

The short bones consist only of a thin layer of compact bone surrounding a spongy interior.

The short bones provide flexibility, stability, and some smooth movement to our wrist and ankle. (The structure of short bones is the reason for you to make the fist where short bones of the wrist, roll up the finger into a fist)

The short bones are located in the wrist and ankle joints.
Short bones of wrists are scaphoid, lunate, triquetral, hamate, pisiform, capitate, trapezoid, and trapezium, which are collectively called carpels.
Short bones of ankles are calcaneus, talus, navicular, cuboid, lateral cuneiform, intermediate cuneiform, and medial cuneiform, which are collectively called tarsals.

3. FLAT BONES

Flat bones have a flattened and broad surface, this type of bone structure provides protection to the internal organs such as the brain, heart, and pelvic organs.

Flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone.

Inside the flat bones, most of the red blood cells are formed in adulthood. (Flat bones are very thin in structure and thus they consist of only red bone marrow, here yellow bone marrow does not present)

Flat bones can also provide large areas of attachment for muscles
(i.e scapula provides attachments to the numerous muscles)

The flat bones are present in the skull, scapula, and thoracic cage.

Flat bones of skull are occipital, parietal, frontal, nasal, lacrimal, and vomer,

The flat bones of the thoracic cage are the sternum and ribs.

4. SESAMOID BONES

Sesamoid bones are special types of bones that are embedded in tendons of the hands, knees, and feet in order to transmit muscular forces.

Sesamoid bones are small in size, and round in structure, they act like pulleys, providing a smooth surface for tendons to slide over.

Sesamoid bone's main function is to hold the tendon further away from the joint in order to increase the angle of the tendon and thus the leverage of the muscle is increased which effectively protects tendons from excessive wear and tear thus reducing the stress at the joints.

Sesamoid bones' best example is:-
1. Patella (it is the largest sesamoid bone in the body and is also known as the kneecap)
2. Pisiform {is a small rounded carpal bone situated where the palm of the hand meets the outer edge of the wrist and articulates with the triquetral. (triquetral is one of the eight carpal bones of the hand which forms the carpal arch)}

5. IRREGULAR BONES

Irregular bones are the types of bones that do not fit into the first four categories of bone (long, short, flat, and sesamoid) discussed above.

Irregular bones, as their name indicates, they have an irregular, complicated, and unusual shape.

Irregular bones consist of thin layers of compact bone surrounding
a spongy interior.
Irregular bone's main function is to protect internal organs, for example,
The vertebrae, irregular bones of the vertebral column, protect the spinal cord.
The irregular bones of the pelvis (pubis, ilium, and ischium) protect organs in the pelvic cavity.

*Sutural bones are extra bone pieces that can occur within the skull. These bones get fused together as we grow to form the solid skull which comes under the irregular bones. (Sutural bones are also known as Wormian bones)

FUNCTIONS OF BONE

As we well know, bones serve many functions but the three key functions of bone which are well known by most people, are:-
1. Locomotion (Long bones like the femur help us to move from one place to another)
2. Structure (Bone form the skeleton system which is responsible for our upright human posture)
3. Protection (Bones protect the vital organs i.e heart, brain, etc by acting as a protective shield covering them)

In addition, bones have three other very important functions, which might people don’t know, and it is:-
1. Blood Production (The bone contains bone marrow which produces blood cells i.e red blood cells, white blood cells, and platelets)
2. Mineral Storage (Bone stores minerals like calcium, phosphorus, etc inside it)
3. Energy Store (Fat cells, in form of adipose tissues, gets stored inside the bone in yellow bone marrow)

If we classify the functions of bone in-depth, we can classify bones function in three categories which are being:-

A) MECHANICAL
Bone provides a frame to keep the body supported.
Bone provides an attachment point for muscles and connective tissues.
Bones provide protection to internal organs i.e heart, brain, etc.
Bones provide a medium to transfer force and impacts.

B) SYNTHETIC
Bone contains bone marrow which produces blood cells (i.e red blood cells, white blood cells, and platelets) in a process called hematopoiesis.

Every day, over 2.5 billion red blood cells, white blood cells, and platelets are produced.
The bone marrow within the bones is also one of the major sites where defective or aged red blood cells are destroyed.

C) METABOLIC
Bones stores minerals
(i.e calcium, phosphorous, etc) inside them as reserves of important minerals for the body.

Bones store fats inside them as a storage reserve of fatty acids.

Bone maintains the acid-base balance of the blood by buffering the blood against excessive pH change by absorbing or releasing alkaline salts.

PARTS OF TYPICAL BONE

A typical bone can have many parts, each one for a performing particular function inside the body but let's study the four most important parts of the bone structure, which are Articular Cartilage, Epiphysis, Diaphysis, and Metaphysis. (Rest others i.e marrow, endosteum, periosteum, etc are already covered in the above sections)

ARTICULAR CARTILAGE

Articular cartilage is connective tissue, having a smooth surface, composed of white tissue that covers the ends of bones where they come together to form joints.

Articular cartilage has a layer of hyaline cartilage, which is present at the end surface of the bone to provide a low-friction gliding surface for bones to glide over each other during movement.

Articular cartilage connects one bone to the other for movement and minimizes peak pressures by absorbing shock at joints.


EPIPHYSIS

The epiphysis part is present at the top, extreme ends, and rounded portions of the bone where joints form. (simply epi means above)

The epiphysis is made of spongy cancellous bone enveloped by a thin layer of compact bone, and at the top, it is covered with articular cartilage to assist in movement at the bone extremities.

The epiphysis is connected to the bone shaft by the epiphyseal cartilage or growth plate which assists in the growth of bone length.

The epiphysis is present mostly in long bones i.e femur, humerus, etc, and contains red bone marrow, which is responsible for producing red blood cells.

The epiphysis is basically of four types:-

1. Pressure epiphysis: Assists in transmitting the weight.
2. Traction epiphysis: Supporting ligaments and tendons attach to these areas of the bone.
3. Atavistic epiphysis: Independent evolution but is now fused with another bone.
4. Aberrant epiphysis: These are deviations from the norm and are not always present.


METAPHYSIS

The metaphysis is the neck part of a long bone situated below the epiphysis and above the diaphysis, it is the place where the diaphysis meets the epiphysis.

The metaphysis contains a largely cancellous, or spongy, interior and it is the portion of bone where major bone growth occurs, as well as it has a relatively rich blood supply, it is the portion from where blood enters the bone.

The metaphysis contains a mixed community of cells i.e mesenchymal cells, hematopoietic cells, etc.
The metaphysis may be divided anatomically into three components based on tissue content: a bony component, an epiphyseal plate, and a fibrous component surrounding the periphery of the plate. Among these three, the epiphyseal plate plays a key role in our bone development. (Epiphyseal plate is a cartilaginous component also known as the growth plate, mainly it is a thin layer of hyaline cartilage enabling the bone to grow, that lies between the epiphyses and metaphysis. The plate is only found in children and adolescents; in adults, who have stopped growing, the plate is replaced by an epiphyseal line. This replacement is known as epiphyseal closure or growth plate fusion, and it occurs at approximate ages between 15 and 20 for girls and between 17 and 24 for boys, here the metaphysis stops growing altogether and completely ossifies into solid bone near the diaphysis, and the epiphyses.)


DIAPHYSIS

The diaphysis is the chief section of a long bone lying in the central region of the bone and has the most clearly tubular structure. Even the diaphysis itself means the “shaft of a long bone”.

The diaphysis part of the bone has the outer section formed from the cortical bone and a central portion formed from the cancellous bone, at one or commonly both ends, the diaphysis flares outward.

The diaphysis typically holds on to the bone marrow and adipose tissue inside it.
(In adults, this contains fat and is called yellow marrow)

It is formed by endochondral ossification before longitudinal growth continues with secondary ossification at the physis. (Endochondral ossification is one of the essential processes during fetal development of the human skeletal system by which bone tissues are created)

JOINTS

Joints are junction points where two or more bones connect together to move the body parts.

Joints form a structure that functions as a unit to hold the skeleton together and its structure depends upon its functions.

A joint is also known as an articulation and its study is called arthrology.


When it comes to the structure of joints, they are held together and supported by viscoelastic, tough bands of fibrous connective tissue called ligaments. Smooth articular cartilage prevents friction as the bones move against each other. In freely movable joints, the entire joint is enclosed inside a membrane filled with lubricating synovial fluid, which helps to provide extra cushioning against impact. Muscles are attached to bones with thick, tough bands of connective tissue called tendons. Where tendons lie close to the bone, tiny sacs called bursae sit between the tendon and the bone to reduce friction. A bursa is filled with synovial fluid.


FUNCTIONAL CLASSIFICATION OF JOINTS

1. SYNARTHROSIS

-It is an Immovable Joints.

-Here, two or more bones come together to form a joint but there is no movement between the bones. In most cases, the bones get fused together.

-Example:- sacrum, coccyx, cranium, etc.

The sacrum is five sacral vertebrae in the child, which fuse together in the adult into one bone the sacrum. Likewise, the coccyx lies below the sacrum. The cranium is made up of the frontal bone, two parietal bones, the occipital bone, and two temporal bones. The joints between these bones are called sutures.

2. AMPHIARTHROSIS

-It is a slightly movable joint.

- Here, two or more bones are held together with the help of cartilage so tightly that only limited movement is possible.

- Example:- Joints between vertebrae, ribs, pubic symphysis of the pelvis, etc.

Joints between vertebrae are the best example of this type of joint. Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc. Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them.

3. DIARTHROSIS

-It is a freely movable joint.

-Here, two or more bones come together to form a joint and the ligament stabilizes the joint.

-These types of joints include all synovial joints of the body, which provide the majority of body movements.

-Example:- Joints of the elbow, shoulder, ankle, etc.

Because of its freedom of movement, it is often referred to as a true joint. Most joints within the human body are this type. Motion is the purpose of these joints.

HINGE JOINT

- It is also known as ginglymus.

- It is a type of joint that permits movement in one direction like the hinge on a door. (Has only one degree of freedom)

- Typically hinge joints help us to perform flexion & extension actions in the human body.

- Examples:- The interphalangeal joints of the fingers, elbow joint, etc.

In hinge joints, the slightly rounded end of one bone fits into the slightly hollow end of the other bone. In this way, one bone moves while the other remains stationary, like the hinge of a door. As we well know the purpose of a hinge on a cabinet door, which is to keep the door aligned in order to properly open and close.

GLIDING JOINT

- It is also known as Plane Joint or Arthrodia.

- These joints allow only gliding or sliding movements and are multi-axial such as the articulation between vertebrae.

- It involves bones that lie next to one another and as movement takes place, they glide (or slide or rub) together.

- Example:- Joint formed by Carpal bones of the wrist, and Tarsal bones of the foot, Acromioclavicular joint (Between the acromion of the scapula and the clavicle), etc.

When the wrist or ankle moves, the Carpal bones of the wrist, and the Tarsal bones of the foot move slightly against one another. This movement involves the intercarpal and intertarsal joints, respectively.

PIVOT JOINT

- It is also known as trochoid or rotary joint.

- In the pivot joint, the movement is primarily rotation with one bone rotating in a ring of another. It consists of the rounded end of one bone fitting into a ring formed by the other bone.

- The pivot joint’s structure allows rotational movement, as the rounded bone moves around its own axis.

- Examples:- Median atlanto-axial joint, proximal radioulnar joint, distal radioulnar joint, etc.
In the median atlanto-axial joint, which is a joint in the upper part of the neck between the atlas bone and the axis bone, the first and second vertebrae of the neck allow the head to move back and forth forms a pivot joint.

BALL & SOCKET JOINT

- It is also known as spheroidea or enarthrosis.

- In the ball and socket joint, the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone.

- The ball and socket configuration allows for movement with 3 degrees of freedom, which is more than any other type of synovial joint.

- It can perform many action i.e flexion / extension, abduction / adduction, rotation, and circumduction.

- Example:- Shoulder joint and hip joint.

In the hip joint, the round head of the femur fits in the cup-like socket called as acetabulum (part of the pelvis bone), similarly in the shoulder joint, the round head of the humerus bone fits in the cup-like socket called a glenoid cavity (part of the scapula bone).

SADDLE JOINT

- It is also known as sellar Joint.

- In this joint, the opposing bones that come together are convex and concave.

-The surface of the joint (bone) is saddle-shaped. (like a western saddle used in horse riding)

- Human skeleton only has a few saddle joints. It does not allow rotational movement rest all other movements i.e flexion, extension, abduction, adduction, and circumduction are possible.

- Example:- Carpometacarpal joint of the thumb (Junction between the first metacarpal and the greater trapezium carpal), Sternoclavicular joint of the thorax, Calcaneocuboid Joint of the heel, etc.

CONYLOID JOINT

- It is also known as ellipsoidal, or bicondylar.

- This joint is similar to the saddle joint in that a condyle of one bone fits into an elliptical cavity of another bone, allowing all movements except rotation.

- Functionally, condyloid joints are biaxial joints that allow for two planes of movement, they have two degrees of freedom. The axial rotation is not possible in the condyloid joints but they can perform other movements like abduction/adduction, flexion/extension, and circumduction.

Example:- Wrist Joint, Metacarpophalangeal joint, Metatarsophalangeal joints, atlanto-occipital joints.

COMPOUND JOINT

Despite above mentioned six types of diarthrosis joint (synovial joint), there is one special type of joint called a compound joint. A joint is said to be a compound joint when they possess some extra components or shows some modified qualities among above mentioned six types of diarthrosis joint (synovial joint).

In our body, the knee joints are a perfect example of a compound joint, as the knee joint has extra components (the knee joint is made up of three bones which are the femur, tibia, and patella) and it is a modified version of hinge joint (as the knee permits flexion and extension actions about a virtual transverse axis, as well as a slight medial and lateral rotation about the axis of the lower leg in the flexed position)

The knee joint is the largest joint in the human body. As we well know that our knee joins the thigh with the leg and consists of two joints which are:-
1. Tibiofemoral joint (Between the femur and tibia)

2. Patellofemoral joint (Between the femur and patella)

MUSCLE COMPOSITION

Here in this section of our course, we will discuss the ingredients from which our skeletal muscles are made up of. Basically, there are five nutrients from which our skeletal muscles are made up and these are present in varying proportions, which is:-
WATER: 70 - 75 %
PROTEIN: 18 - 23%
LIPIDS: 1 – 5%
GLYCOGEN: 1 - 2%
MINERAL SALTS: 1%

LIPIDS
The “lipid" is occasionally used as a synonym for fats. Fats are a subgroup of lipids called triglycerides. Lipids also confine molecules such as fatty acids and their derivatives like triglycerides, diglycerides, monoglycerides, and phospholipids, as well as other sterol-containing metabolites such as cholesterol.

GLYCOGEN
Glycogen is the storage form of carbohydrates in mammals. In humans, the majority of glycogen is stored in skeletal muscles and the liver. (Approximately 400 to 500 g of glycogen is stored in skeletal muscles and the liver 80 to 100 g of glycogen is stored in the liver.)

The body breaks down most carbohydrates from the foods we eat and converts them to a type of sugar called glucose. Glucose is the main source of fuel for our cells. When the body doesn't need to use glucose for energy, it stores it in the liver and muscles. When the body doesn't need to use glucose for energy, it stores it in the liver and muscles.

This stored form of glucose is made up of many connected glucose molecules and is called glycogen. Glycogen is a multi-branched polysaccharide of glucose that serves as a form of energy storage in mammals. When the body needs a quick boost of energy or when the body isn't getting glucose from food, glycogen is broken down to release glucose into the bloodstream to be used as fuel for the cells.

Food is supplied in larger meals, but the blood glucose concentration has to be kept within narrow limits to survive and stay healthy.

Glycogen functions as one of two forms of energy reserves, glycogen being for short-term and the other form being triglyceride stored in adipose tissue (Also called body fat) for long-term storage.

MINERAL SALT
Mineral salts are crucial to the organism as they make up around 4% of our body mass. In combination with other nutrients, they ensure that the organism functions optimally by doing the following works, which are:-
1. Control the Osmotic Pressure (hydrous balance)
2. Regulate the acid-alkali balance
3. Play key part in the formation of certain structures i.e bones, teeth, etc.
4. Involved in the structure of various enzymes and hormones.
5. Catalyse many of the metabolism’s reactions.
Among minerals, there are few mineral salts like sodium, potassium, etc that are needed by the organism in moderate to large quantities (0.2 g to 10 g/day), and few mineral salts like iodine, fluoride, etc that are needed by the organism in small quantities (1 to 100 mg/day.)

MUSCLE TISSUE CLASSIFICATION

Scientists are great classifiers, they believe that to understand something better we need to segregate them and group them in as many ways as possible. In human physiology biologists have classified most parts of the human body i.e bones are classified, nerves are classified, cells are classified, and thus muscles are also classified.

Classification is necessary because the body has several kinds of muscle, situated at different locations within the body, made up of different types of muscle fibers, have varying appearances, and show differences in nerve control, etc.

SKELETAL MUSCLES

The skeleton muscles are those muscles that are present over our bone structure (skeleton). There are more than 600 skeletal muscles in the human body, making up around 40% to 50% of body weight in which most muscles occur in bilaterally-placed pairs to serve both sides of the body like muscles of the chest, back, biceps, triceps, legs, etc and the skeletal muscles can be developed with proper weight training.

The skeleton muscles are attached to the bones with the help of connective tissue i.e tendon and are involved in the functioning of different parts of the body like movements of body parts in relation to each other. (The tendons are a non-contractile part of the muscle made up of dense fibrous connective tissue that attaches the muscles to bones to give skeletal movement, here the length of a muscle includes the tendon's lengths too).

Skeletal muscles are soft in nature and have a plentiful supply of blood vessels and nerves in order to aid in their primary function which is muscular contraction. (Skeletal muscle has the arrangement of two contractile proteins myosin and actin. Both myosin and actin are two of the myofilament in the myofibrils of the muscle tissue. The myosin forms the thick filaments while the actin forms the thin filaments, and is arranged in repeating units called sarcomeres. The interaction/sliding of both proteins myosin and actin over each other results in muscle contraction.)

The skeleton muscles are also called voluntary muscles as they come under the control of the nervous system more specifically it is controlled by the body’s somatic nervous system.

The muscle cells of skeletal muscles are much longer than in the other types of muscle tissues.

The skeletal muscle has a striated appearance, its long, thin, multinucleated fibres are crossed with a regular pattern of fine red and white lines, giving the muscle a distinctive striped appearance (also known as striation).


CARDIAC MUSCLE

The Cardiac muscle is a specialized type of muscle tissue that forms the heart. It contains cardiac muscle cells, which perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body. Doing regular cardiovascular exercises like running, jogging, walking, cycling, stair climbing, etc can help strengthen the cardiac muscle tissue and keep the heart and lungs healthy.

The heart wall is a three-layered structure with a thick layer of Cardiac muscle sandwiched between the inner endocardium and the outer epicardium of the heart.

The Cardiac muscle is also known as Myocardium and it only exists in the heart.

The Cardiac muscle produces involuntary movements. This means that they are automatic and that a person cannot control them.

The Cardiac muscle is highly organized and contains many types of cells, including fibroblasts, smooth muscle cells, and cardiomyocytes. Cardiac muscle cells are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighboring cells - known as a functional syncytium - working to efficiently pump blood from the heart.

The heart also contains specialized types of cardiac tissue containing “pacemaker” cells. These contract and expand in response to electrical impulses from the nervous system. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store (from the sarcoplasmic reticulum). The rise in calcium causes the cell's myofilaments to slide past each other resulting in the heart’s contraction and relaxations. (Pacemaker cells generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax.)

The pacemaker cells control heart rate and determine how fast the heart pumps blood. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart’s pacemaker.

Contracting heart muscle uses a lot of energy and therefore requires a constant flow of blood to provide oxygen and nutrients.

The cardiac muscle also has a striated appearance like skeletal muscles.

SMOOTH MUSCLE

The smooth muscle is found throughout the body around various:-

1. Hollow organs like the stomach, intestines, bladder, uterus, etc.

2. Tracts of the respiratory, urinary, reproductive system, etc.

3. Passageway walls like blood, and lymph vessels.

4. It is present in the eyes, where it functions to change the size of the iris and alter the shape of the lens.

5. It is also present in the skin where it causes hair to stand erect in response to cold temperature or fear.

The smooth muscle is not under voluntary control thus it is called involuntary muscle.

The smooth muscle has a non-striated appearance because it has no sarcomeres and therefore no striations (bands or stripes).

The smooth muscle is divided into two subgroups, single-unit, and multi-unit smooth muscle. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. (The process in cell division by which the nucleus divides, typically consisting of four stages, prophase, metaphase, anaphase, and telophase, and normally results in two new nuclei, each of which contains a complete copy of the parental chromosomes)

The smooth muscle differs from skeletal and cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than skeletal and cardiac muscle.

Smooth muscle may contract spontaneously or as in the gut special pacemakers cells, interstitial cells of Cajal produce rhythmic contractions. Also, contraction, as well as relaxation, can be induced by a number of physiochemical agents like hormones, drugs, and neurotransmitters, especially from the autonomic nervous system. (Autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal).

Smooth muscle contracts slowly and may maintain the contraction for prolonged periods in blood vessels. In the digestive tract, smooth muscle contracts in a rhythmic peristaltic manner, rhythmically forcing foodstuffs through the digestive tract as the result of phasic contraction.

The smooth muscle plays an important role in the ducts of exocrine glands. It fulfills various tasks such as sealing orifices like pylorus (pylorus is the opening from the stomach into the duodenum), uterine, etc, or the transport of the chyme (chyme is the pulpy acidic fluid that passes from the stomach to the small intestine, consisting of gastric juices and partly digested food) through wavelike contractions of the intestinal tube. On the one hand, smooth muscle cells contract slower than skeletal muscle cells, on the other hand, they are stronger, more sustained, and require less energy.

The smooth muscle contraction relies on the presence of Ca++ ions, smooth muscle fibers have a much smaller diameter than skeletal muscle cells. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma (similar to cardiac muscle fibers) that open during the action potential along the sarcolemma.

The most smooth muscles must-have function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy.

EXCITABILITY

In muscle anatomy, the excitability term is used to showcase the ability of the cell to respond to stimuli. Here the stimuli can either be by a motor neuron or by a hormone. Neurons are identified as excitable cells because they have the ability to be electrically excited resulting in the generation of action potentials.

Muscle and nerve cells are considered to be "excitable" because they're capable of transmitting action potentials after electrical or chemical stimulation sufficient to depolarize the plasma membrane. The excitability of neurons, the ability to generate a large, rapid change of membrane voltage in response to a very small stimulus, is based on the action potential.

Thus excitability is a property of a cell, allowing it to respond to stimulation by rapid changes in membrane potential produced by ion fluxes across the plasma membrane.

In simple words, it describes the ease with which the cell i.e muscle cells responds to a stimulus with a regenerative action potential.

CONTRACTIBILITY

Contractibility or Contractility is a unique property of a muscle by which it can be shortened when force is applied. It allows muscle tissue to pull on its attachment points and shorten with force. For example, in order to flex (decrease the angle of a joint) your elbow, you need to contract (shorten) the biceps brachii and other elbow flexor muscles in the anterior arm.

Muscle contractility plays a vital role in various physiological processes of our day-to-day life. The muscle contractibility is predominately dependent upon two factors which are:-

1. Calcium Concentration
Calcium triggers contraction in skeletal muscle. During muscle contraction, the thin filaments i.e actin slide over the thick filaments i.e myosin. A signal sent by the central nervous system via motor neurons initiates muscle contraction. Acetylcholine is released when a neural signal reaches a neuromuscular junction and an action potential is generated in the sarcolemma. When this spreads through the muscle fibre, calcium ion is released in the sarcoplasm. Calcium then binds to troponin on actin filaments and exposes the active sites for myosin.

2. Temperature
Temperature can greatly affect the ability of your muscle to contract. It is more difficult for muscle to contract in cold temperatures than in warmer conditions. Temperature affects the ease with which oxygen is released from hemoglobin.

Muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

Muscle contractions can be described based on two variables: length and tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remain the same. In contrast, a muscle contraction is an isotonic contraction if muscle tension remains the same throughout the contraction. If the muscle length shortens, the contraction is in the concentric phase if the muscle length lengthens, the contraction is in the eccentric phase.

STRUCTURE OF SKELTON MUSCLE CELL

Each skeletal muscle fiber is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers, bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium.

Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The connective tissue covering furnishes support and protection for the delicate cells and allows them to withstand the forces of contraction. The coverings also provide pathways for the passage of blood vessels and nerves.

Commonly, the epimysium, perimysium, and endomysium extend beyond the fleshy part of the muscle, the belly or gaster, to form a thick rope-like tendon or a broad, flat sheet-like aponeurosis. The tendon and aponeurosis form indirect attachments from muscles to the periosteum of bones or to the connective tissue of other muscles. Typically a muscle spans a joint and is attached to bones by tendons at both ends. One of the bones remains relatively fixed or stable while the other end moves as a result of muscle contraction.

Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle, contraction. Before a skeletal muscle fiber can contract, it has to receive an impulse from nerve cells. Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called Capillaries.

The contractile filaments that perform the work of the muscle are called myofilaments. Two basic types of myofilaments perform the work of the muscle. One type is the thick myosin filament the other is the thin actin filament. The myosin filament has molecular "heads" that extend to attractor sites on the adjacent actin filament and bend to bring about contraction. These myosin and actin filaments lie parallel to each other in an overlapping pattern that produces the characteristic striped (striated) appearance of skeletal muscle. Several of these myofilaments together form a sarcomere, which is considered the "unit" of contraction in a muscle cell.

A string of sarcomeres lined up in sequence form a myofibril. Surrounding and penetrating the myofibrils is a system of microscopic tubes called transverse tubules and the sarcoplasmic reticulum. These tubules carry the chemical trigger, calcium, necessary to initiate contraction at the molecular level. A muscle cell is composed of several myofibrils.

The expression "muscle cell" is equivalent to the expression "muscle fiber."

The number of muscle cells in the body is believed to remain constant; when we strengthen muscles or increase their size and bulk, it is the contractile content, not the number, of the cells or fibers that is changed. Unlike most cells, muscle cells contain many nuclei scattered along the length of the cell. Multiple nuclei are necessary because muscle cells can be quite long, and their internal needs, which must be assessed and met by the nuclei, vary from one part of the cell to the next. Muscle cells are second only to nerve cells in length and can be over 11 inches long in some muscles.

SLIDING FILAMENT THEORY (Cross-Bridge Theory)

The most commonly accepted theory of muscle function is the cross-bridge theory. It attempts to explain the contractile action of muscle tissue— that is, how muscle tissue shortens when stimulated by a motor neuron.
When a nerve impulse excites the neuromuscular junction, calcium is released from the sarcoplasmic reticulum into the fluid surrounding the myofilaments. This causes a molecular response in which attractor sites on the actin filaments are exposed, attracting "heads" from the myosin filaments, which cross the gap between the filaments, attach themselves to their sites on the actin filaments, and bend, propelling the actin filaments into a more deeply overlapped and interlocked position in relation to the myosin filaments. This shortens the sarcomere and, as all the sarcomeres in many muscle cells shorten, muscle contraction occurs. Muscle tissue is capable of shortening by about 40% of its length. When nerve stimulation ceases, the calcium is actively transported back into the transverse tubules, the myosin heads release, and contraction stops. The muscle, however, cannot lengthen on its own. The contractile units (sarcomeres) must be stretched back to their starting position by an outside force, such as the pull of gravity or an opposing muscle before it can again shorten in contraction.

ELASTICITY

In science, elasticity means an ability of a substance to return to its normal shape when the applied force is removed. Our muscle also possesses such property, it’s a natural ability of a muscle to recover to its original form upon the removal of the applied force.

In more simple words, elasticity is the ability to return to normal length after a stretch. The muscle's elasticity returns it to normal resting length following a stretch and provides for the smooth transmission of tension from muscle to bone.

Muscle force production occurs within an environment of tissues that exhibit spring-like behavior, and this elasticity is a critical determinant of muscle performance during movements.

By influencing the speed of contractile elements i.e actin and myosin, elastic structures can have a profound effect on muscle force, power, and work. In very rapid movements, elastic mechanisms can amplify muscle power by storing the work of muscle contraction slowly and releasing it rapidly.

EXTENSIBILITY

Extensibility is defined as the ability of a muscle to be stretched without tearing. In our body, most of the cells lack the capacity to stretch or lengthen, if they attempt to do so they only damage or destroy them. Muscle cells contract, and in order for them to retain this ability, they must accordingly possess extensibility, or the capacity to lengthen. Your muscle cells can be stretched to about three times their contracted length without rupturing. This is important because, in a lot of coordinated movements, so-called antagonistic muscles operate such that one is lengthening while the other is contracting. Lack of extensibility is known as spasticity.

TONICITY

Tonicity is the ability to become hard. Our muscle also has this property to save itself because if a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex that helps guard against danger as well as helps to maintain balance by making our muscle hard(or tensed).

When a muscle is exercised, more muscle fibers become active, and the muscle becomes firmer. This factor is called muscle tone.

FUNCTIONS OF MUSCLES

Our muscles perform numerous essential functions, and these muscle functions are controlled by the nervous system. The nervous system stimulates portions of contractile tissue to contract in patterns that will produce the desired effect, and this activation usually involves parts of several muscles acting in fine coordination.

MOVEMENT

The muscles create all the movement in our body. Muscles pull on the joints, allowing us to move. The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.

When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.

Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to their original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contract. Then the biceps relaxes and the triceps contract to straighten the elbow.

HOW DO OUR MUSCLES PRODUCE HEAT?

Our skeletal muscle helps our body to regulate its bodily core temperature. Whenever our body's core temperature goes down then our skeletal muscles start producing heat in order to maintain our body's core temperature. The human body has an ideal body core of 37.7 °C, it is also called normothermic temperature. The ideal body core temperature may vary from person to person because the physiological factors i.e muscle mass, fat mass, BMR, age, etc varies person-to-person. But our body is designed with such amazing mechanisms that our ideal body core temperature always remains between 36 °C and 39 °C. At this mentioned range all bodily organ functions are at their best.

Our environment has huge impact on our body's core temperature for example in the winter season our bodily core temperature starts coming down below 36 °C. When this happens our body comes in alert mode and start taking some corrective measure to prevent the fall of body core temperature and tries to elevate the body core temperature to bring it back in between the range of ideal body core temperature. To achieve this our muscles play a key role by producing the heat inside our body and regulating its temperature. Our muscles can produce heat in two ways which are:-

1. Involuntary Mechanism

2. Voluntary Mechanism

INVOLUNTARY MECHANISM
In the involuntary mechanism, you don’t have to do anything, it is not in our control, and our body does all tasks on its own. Here the body starts shivering to elevate the body’s core temperature. You have also observed that when you feel too cold, your body automatically starts shivering. By shivering, our body tries to elevate the body core temperature which has fallen down due to the coldness felt by our body. Thus to bring back the bodily core temperature our body triggers the shivering refluxes resulting in vibration in vibration (shivering) of our skeletal muscle which leads to heat production.

VOLUNTARY MECHANISM
In a voluntary mechanism, we can control the process. Here, whenever you perform any actions i.e exercises your skeletal muscle starts contracting. Muscle contraction produces heat inside our bodies. You may have also observed that when you do intense exercise you feel the warmth.

Thus by utilizing the above two mechanism our muscles produce heat inside our body.

JOINT STABILIZATION

As we well know that joints are formed when two or more bones come together at a point, here the muscles and connective tissues like ligaments that cross or attach with the bone-forming joint, helps to stabilize the joint.

If the muscle surrounding your joint or muscle that crosses the joint is strong and intact then it leads to strong and healthy joint. Most probably, you never have to face the problems like joint pain, joint inflammation, etc if the surrounding muscle of a joint is at its peak health condition.

In 80% cases the joint problem arises due to the weakness of muscles surrounding the joints, and here automatically if the muscle surrounding the joints are weak then the strength of the joint will decrease, and they will become weak and unstable. If you have not strengthened the muscles surrounding the joint then eventually the joint start experiencing more stress over them due to load-bearing which ultimately leads to serious concern related to joints.

STRONGER MUSCLES LEAD TO STRONGER JOINTS

POSTURE IMPROVEMENT

When you have a strong skeletal muscle structure then it helps in improving your body posture as strong muscles lead to erect and upright bodily posture, whereas weak muscles lead to weak and bent body posture.

Weak & bent posture can result in several postural abnormalities like kyphosis (round upper back), lordosis (increased inward spine curvature), scoliosis (lateral bend in the spine), pelvic tilt, etc. Thus poor posture results in disturbance in the body’s alignment and it’s overall functionality.

If the postural abnormalities caused by weak musculature are not corrected within certain time period then it can lead to joint and muscle pain.

STRONG & FLEXIBLE MUSCLES LEADS TO GOOD POSTURE WHEREAS WEAK, STIFF, AND TIGHT MUSCLE LEADS TO POOR POSTURE.


BLOOD CIRCULATION

Our cardiac muscle(heart muscle) helps in blood circulation. As we well know that our heart is like a pump, it pushes the blood throughout the body for optimal circulation. Our heart is the hardest working muscle it pumps approximately 2000 gallons of blood per day (7570 liters of blood per day).


DIGESTION

Our smooth muscles like muscles present in our gastrointestinal tract (GI Passage) control our digestion process. The GI tract is a long hollow muscle starting from the mouth and ending up in the anus. The GI tract is also called the alimentary canal, and it is approximate 9 m in length.

MUSCLE FIBER CLASSIFICATION

There are numerous methods are used for muscle fiber classification. This range of muscle fiber types allows for the wide variety of capabilities that human muscles display. Human skeletal muscle is composed of diverse content in a collection of muscle fiber types. Muscle fibers grow when exercised and shrink when not in use. This is due to the fact that exercise stimulates the increase in myofibrils which increase the overall size of muscle cells. Well-exercised muscles can not only add more size but can also develop more mitochondria, myoglobin, glycogen, and a higher density of capillaries. However muscle cells cannot divide to produce new cells, and as a result, there are fewer muscle cells in an adult than in a newborn. Our skeletal muscle cells are the individual contractile cells within a muscle and are often termed as muscle fibers. A single muscle such as the biceps in a young adult male contains around 253,000 muscle fibers. Skeletal muscle fibers are the only muscle cells that are multinucleated with nuclei often referred to as myonuclei. Many nuclei are needed by the skeletal muscle cell for the large amounts of proteins and enzymes needed to be produced for the cell's normal functioning. A single muscle fiber can contain hundreds to thousands of nuclei. A muscle fiber for example in the human biceps with a length of 10 cm can have as many as 3000 nuclei.

Two commonly used methods for muscle fiber classifications are micro-anatomy staining for myosin ATPase activity and immuno micro-anatomy staining for myosin heavy chain (MHC) type.

Myosin ATPase activity is commonly referred to as simply "fiber type", and results from the direct assaying of ATPase activity under various conditions. Myosin heavy chain staining is commonly referred to as simply "MHC fiber type”. These methods are closely related physiologically, as the MHC type is the primary determinant of ATPase activity.

Mostly, muscle fiber classifications by Myosin ATPase fiber types are preferred over Myosin heavy chain fiber types. According to Myosin ATPase fiber type, there are two types of muscle fiber: Type I, which is slow (also called slow oxidative (SO) or slow twitch muscle fiber), and Type II (also called fast twitch muscle fiber) which is fast. Type II has two divisions of type II A (fast oxidative (FO)), and type II B (fast glycolytic (FG)), giving three main fiber types. These fibers have relatively distinct metabolic, contractile, and motor unit properties. Most skeletal muscles in a human contain(s) all three types, although in varying proportions.

In order to understand muscle fiber classification, we use various parameters so that we can differentiate between them. Let’s discuss these parameters first.

TWITCH
A twitch is defined as when one muscle fiber contracts in response to a command (stimulus) by the nervous system. In other words, the twitch refers to how quickly and often the muscle moves. Thus twitch is the small, quick, sudden movement and in the simplest word, we can call it the reaction time. Type 1 muscle fibers are slow-twitch muscle fibers, which move more slowly but help to keep you moving longer while Type 2 muscle fibers are fast-twitch muscle fibers, which help you move faster, but for shorter periods.


CAPILLARIES
In human physiology, a capillary is any of the small, delicate blood vessels that form networks throughout the bodily tissues. The capillary wall performs an important function by allowing nutrients and waste substances to pass across it. The capillaries are about 8 to 10 microns in diameter, just large enough for red blood cells to pass through them in a single file. It is through the capillaries that oxygen, nutrients, and wastes are exchanged between the blood and the tissues. Here, in Type 1 Muscle fibers high amount of blood capillaries are present whereas in Type 2 muscle fibers less amount of blood capillaries are present.


MITOCHONDRIA
Mitochondria is a rod-shaped, double membrane-bound organelle found in most eukaryotic organisms. They are found inside the cytoplasm and essentially function as the cell’s “digestive system.” They play a major role in breaking down nutrients and generating energy-rich molecules for the cell. Many of the biochemical reactions involved in cellular respiration take place within the mitochondria. The mitochondrial matrix is a viscous fluid that contains a mixture of enzymes and proteins. The enzymes present in the matrix play an important role in the synthesis of ATP molecules.

Mitochondria are also called the powerhouse of the cell because they are responsible for producing ATP, the energy currency of the cell. Their key function is to produce energy through the process of oxidative phosphorylation. Besides this, it is responsible for regulating the metabolic activity of the cell. It also promotes cell multiplication and cell growth. Mitochondria also detox ammonia in the liver cells. Responsible for building certain parts of the blood and various hormones like testosterone and estrogen. It also helps in maintaining an adequate concentration of calcium ions within the compartments of the cell. Here, in Type 1 Muscle fibers high amount of mitochondria is present whereas in Type 2 muscle fibers less amount of mitochondria are present.


MYOGLOBIN
Myoglobin is a type of protein found in the cardiac and skeletal muscles of humans. It is an oxygen-carving molecule whose main function is to bind iron and oxygen.

Myoglobin has similar functioning as hemoglobin. Compared to hemoglobin, myoglobin has a higher affinity for oxygen and does not have cooperative binding with oxygen as hemoglobin does. This difference is related to its different role: whereas hemoglobin transports oxygen, myoglobin's function is to store oxygen. Myoglobin contains hemes, pigments responsible for the color of red colour of muscles.

The muscle fibers were categorized depending on their varying colour, which is due to the myoglobin content. Type I fibers appear red due to the high levels of myoglobin. Red muscle fibers tend to have more mitochondria and greater local capillary density. These fibers are more suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP.

Thus the high concentration of myoglobin in muscle cells allows organisms to hold their breath for a longer period of time. The Type II fibers are white due to relatively low myoglobin content and reliance on glycolytic enzymes.


CONTRACTION PERIOD
The contraction period is the time frame that tells us how long a muscle can remain in its shortest length when the load is applied. The more the contraction period a muscle has the more suitable it becomes for cardiovascular activity and when a muscle has the less contraction period it is well suited for the intense weight lifting activity. Here, in Type 1 Muscle fibers has a long contraction period whereas in Type 2 muscle fibers have a short contraction period.


RESISTANCE TO FATIGUE
As we are well aware of the word “fatigue” which simply means “to feel exhausted”, the resistance to fatigue parameters tells us how long a muscle can perform the same activity without getting exhausted or tired. Here, the Type 1 Muscle fibers have a high resistance to fatigue whereas the Type 2 muscle fibers have a low resistance to fatigue and it is due to the fact that Type 1 Muscle fibers have more mitochondria which makes the muscle fiber suitable to perform the contraction for the longer duration.
(With more mitochondria more ATP production is possible)


OXIDATION
The oxidation of fat and carbohydrate provides energy for the contracting muscles. Oxidation is the term for oxygen transfer which indicates the gain of oxygen molecules during a reaction. Biological oxidation is an energy-producing reaction in living cells. Type I Muscle fibers gain their energy by the aerobic energy production mechanism (oxygen needed to produce the energy). Here, the Type 1 muscle fibers perform slow oxidation whereas the Type II muscle fibers perform fast oxidation. {Type II has two divisions which are type II A (fast oxidative (FO)) and type II B (fast glycolytic (FG)}


FORCE GENERATION & CONTRIBUTION TO MUSCLE STRENGTH
Type I muscle fibers generate less force whereas type II muscle fibers generate more force because type I muscle fibers mainly contain slow-twitch muscle fibers and get their energy from slow oxidation whereas type II muscle fibers are able to generate more force because type II muscle fibers have fast-twitch muscle fibers and get their energy from fast oxidation & fast glycolytic mechanism. Type I muscle fibers attain less hypertrophy after intense training whereas type II muscle fibers attain more hypertrophy (ability to gain muscle size) after intense weight training. Thus Type I muscle fibers contribute less to muscle strength whereas type II muscle fibers contribute more to muscle strength.

Type I fibers
Type I fibers are also called slow-twitch fibers, these fibers have rich in capillary mitochondria, myoglobin, etc. They have long contraction periods and high resistance to fatigue which make them suitable for doing cardiovascular exercises, marathons, maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. Because of their high myoglobin content, slow-twitch fibers are also called red fibers.


Type II A fibers
Type II A fibers are one of the types of fast-twitch fibers sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than Type I fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, Type II A fibers do not possess significant myoglobin, giving them a lighter colour than the red Type I fibers. Type II A fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement such as sprinting. Type II A fibers are useful for this type of movement because they produce more tension than Type I fibers but they are more fatigue-resistant than Type II B.

Type II B fibers
Type II B fibers are one of the types of fast-twitch fibers, that primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. Type II B fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.

Type II A fibers

Type II A fibers are one of the types of fast-twitch fibers sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than Type I fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, Type II A fibers do not possess significant myoglobin, giving them a lighter colour than the red Type I fibers. Type II A fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement such as sprinting. Type II A fibers are useful for this type of movement because they produce more tension than Type I fibers but they are more fatigue-resistant than Type II B.

Type II B fibers

Type II B fibers are one of the types of fast-twitch fibers, that primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. Type II B fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.

ISOTONIC CONTRACTION

In isotonic contraction, muscles generate force in which the length of the muscle changes but the tension in the muscle remains constant and it can be a concentric contraction or eccentric contraction.

1. CONCENTRIC CONTRACTION

A concentric contraction is an isotonic contraction where the muscle shortens. In this, the muscle tension rises to meet the resistance, then remains the same as the muscle shortens. In a concentric contraction, the muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. This occurs when the force generated by the muscle exceeds the load opposing its contraction.
During a concentric contraction, a muscle is stimulated to contract according to the sliding filament theory. This occurs throughout the length of the muscle, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint.

A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force and overcoming resistance. For example, when lifting a heavy weight while doing a biceps curl exercise a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder.

2. ECCENTRIC CONTRACTION

Eccentric contractions cause muscles to elongate (muscle lengthen) in response to a greater opposing force. Here, the muscle lengthens due to the resistance being greater than the force the muscle is producing. Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load.

This can occur involuntarily (i.e when attempting to move a weight too heavy for the muscle to lift) or voluntarily (i.e when the muscle is 'smoothing out' a movement or resisting gravity such as during downhill walking). Over the short-term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone. During an eccentric contraction of the biceps muscle, the elbow starts the movement while bent and then straightens as the hand moves away from the shoulder.

Muscles undergoing heavy eccentric loading suffer greater damage when overloaded as compared to concentric loading. When eccentric contractions are used in weight training, they are normally called negatives. Exercise featuring a heavy eccentric load can actually support a greater weight (muscles are approximately 40% stronger during eccentric contractions than during concentric contractions) and also results in greater muscular damage. Exercise that incorporates both eccentric and concentric muscular contractions (i.e. involving a strong contraction and a controlled lowering of the weight) can produce greater gains in strength than concentric contractions alone.

ISOMETRIC CONTRACTION

Isometric contractions generate force without changing the length of the muscle. Force itself can be differentiated as either tension or load. Muscle tension is the force exerted by the muscle on an object whereas a load is a force exerted by an object on the muscle. When muscle tension changes without any corresponding changes in muscle length, the muscle contraction is described as isometric.

An isometric exercise is a form of exercise involving the static contraction of a muscle without any visible movement in the angle of the joint. In other words, the joint is static; there is no lengthening or shortening of the muscle fibers and the limbs don't move. Examples:- Plank, wall sit, boat pose, etc.

Isometric exercises have numerous benefits, a few are listed below:-

1. Used for rehabilitation
This is an important aspect of isometric exercises because exercises that require joint movement can place a lot of stress on individual joints, especially over time with repeated usage thus isometric exercises serves the best option in this case for general strengthening without placing stress on the joints.

2. No equipment is needed
They can be done anywhere with no equipment.

3. Helps in developing static muscle strength.
Some sports require a high level of static muscle strength. Gymnastics, yoga, rock climbing, and downhill skiing, for example, all have static strength requirements. These exercises require a lot of strength, if not a lot of joint movement.

ISOKINETIC CONTRACTION

It is a type of contraction in which movement is present, here our muscle changes its length and generate force, similar to isotonic contraction it also has both concentric and eccentric phase but here the movement during both concentric and eccentric contraction is controlled by any person or gadget or machine in order to provide a consistent rate of speed.

Thus when we oppose or control our concentric and eccentric phase of movement by external means i.e person or gadget or machine then the contraction felt by our muscle is called isokinetic contraction. It is the rate of speed, in fact, that separates it from other types of muscle contraction and requires a specialized piece of equipment i.e isokinetic dynamometer or any person or some gadget. When used for exercise, isokinetic movements allow muscles to exert maximum force within the range of joint movements at a constant speed.

Benefits of isokinetic exercise Isokinetic exercises are often used for rehabilitation and recovery since it’s a controlled form of exercise.

Physical therapists and occupational therapists use isokinetic machines to help people recover from a stroke, an injury, or a medical issue.

Isokinetic machines can also be used to treat imbalances in the body that have the potential to cause injury.

Thus summarizing isokinetic contraction, being able to control the resistance and speed helps to
1. Prevent Injury
2. Increase Muscle Flexibility
3. Control Muscle Development
4. Treat Muscle Imbalance
5. Faster Rehabilitation

CONNECTIVE TISSUES

As you can guess from the name itself that, connective tissue serves a "connecting" function. Connective tissue is a type of tissue made up of mainly fibers forming a framework and support structure for body tissues and organs. It is the material between the cells of the body that gives tissues form and strength. Many organs, joints, etc are surrounded by various connective tissues, they protect them from sudden tearing external forces, excessive stretching, etc. Cartilage, ligament, tendon, etc are a few examples of connective tissue.

KeyNote
There are two types of connective tissue
1. Fibrous Connective Tissue
2. Non-Fibrous Connective Tissues
(For Example, The adipose tissue is the Non-Fibrous connective tissue, its main function is to store energy in the form of lipids (fat) and it gives mechanical cushioning to the body)

COMPONENTS OF CONNECTIVE TISSUES
Connective tissues have three main components which are:- 1. Fiber 2. Ground Substance 3. Cells

There are mainly three types of fibers which are:-

1. Collagenous Fiber
Collagen fiber is the fiber in the extracellular matrix of connective tissues characterized by being elongated. It is typically arranged in branching bundles of indefinite length. It is a strong, flexible, and insoluble fiber. It occurs in the tendon, ligaments, bone, and cartilage, etc.

2. Elastic Fiber
Elastic fibers are an essential component of the extracellular matrix composed of elastin (a special kind of protein). These fibers are able to stretch many times their length and snap back to their original length when relaxed without loss of energy. They give a recoil effect specially to your arteries and lungs. They are also known as yellow fibers.

3. Reticular Fiber
Reticular fiber forms a platform for other cells and acts as a supporting mesh in soft tissues. It is made up of type III collagen fiber and is found in our liver, bone marrow, lymphatic organs, etc.

GROUND SUBSCTANCE

Ground substances are the shapeless, formless, gel-like, noncellular component of the extracellular matrix where the fibers and cells of connective tissue are fixed firmly.

It is a transparent material with the properties of a viscous solution or a highly hydrated thin gel, thus due to this property it provides lubrication for collagen fibers and it fills the spaces between fibers and cells.

Our body cells utilize these ground substance for water storage, support system, binding agent, and as a medium which can perform intercellular exchange (Intercellular exchange means it transfer information from one cell to another one, mainly it carry the information from between blood cells and other types of cells).

Dense connective tissue has a smaller amount of ground substance compared to the fibrous material. Ground substance is primarily composed of water and large organic molecules, such as glycosaminoglycans, proteoglycans, and glycoproteins.

CELLS

The cells are the basic structural, and functional unit of a living organism. There are many types of cells like fibroblast cells, adipose cells, adipocytes, macrophage cells, etc. Cells can acquire specified functions and carry out various tasks within the cell such as replication, DNA repair, protein synthesis, and motility.

Cells are of two types:
1. Eukaryotic, which contains a nucleus
2. Prokaryotic cells, which do not have a nucleus, but a nucleoid region is still present.
Prokaryotes are single-celled organisms, while eukaryotes may be either single-celled or multicellular.

A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. Although cells are much larger than atoms, they are still very small. Most cells are measured in micrometers due to their small size.

The cells of connective tissue include two types that are relatively stationary 1. fibroblasts 2. Adipose cells. There are also several types of motile migrating cells mast cells, macrophages, monocytes, lymphocytes, plasma cells, and eosinophils. Fibroblasts are the most common cell type of connective tissue. They produce both fibers and amorphous ground substances.

CARTILAGE

Cartilage is a tough but flexible tissue that provides a flexible rigidity to the structures it supports. It may constitute a much greater proportion of the skeleton. It is not as hard and rigid as bone, but it is much stiffer and much less flexible than muscle.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, a plentiful ground substance that is rich in proteoglycan and elastin fibers.

Cartilage does not contain blood vessels or nerves. Nutrition is supplied to the chondrocytes by diffusion (Diffusion is the movement of anything through a semipermeable barrier from an area of higher concentration to an area of lower concentration).

Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae (a cavity or depression, especially in bone) they cannot migrate to damaged areas. Therefore, damaged cartilage is difficult to heal. Also, because cartilage does not have a blood supply, the deposition of the new matrix is very slow.

Cartilage is classified into three types, which are:-
1. ELASTIC
2. FIBRO
3. HYALINE

ELASTIC CARTILAGE

The elastic cartilage is soft & flexible cartilage. It provides strength, support, elasticity and allows flexibility while maintaining the shape of the structure. Elastic cartilage is quite similar to hyaline cartilage but its matrix contains many elastic fibers. The abundance of elastic fibers makes elastic cartilage more flexible.

The elastic cartilage is found in the epiglottis of the larynx, the auricle of the outer ear, and the (Eustachian) auditory tubes. It has a yellowish color and is surrounded by a perichondrium. Here, the chondrocytes cells are located between a network of threadlike elastic fibers.

FIBRO CARTILAGE

Fibrocartilage is the toughest of the three types of cartilage. It has no perichondrium and has a matrix that contains dense bundles of collagen fibers embedded with chondrocytes, making it durable and tough. Because of the presence of these extra collagen fibers, not only can fibrocartilage resist compression but it can also resist pulling. This makes it perfect to provide support and rigidity.

Fibrocartilage is will be found where strong support and the ability to withstand heavy pressure are required, such as in the pubic symphysis (the point where hipbones join anteriorly), and intervertebral discs (discs between vertebrae), and the menisci of knees (disc of knee joint).

HYALINE CARTILAGE

Hyaline Cartilage is tough cartilage but has a smooth surface and is the most common of the three types of cartilage. It has a matrix that contains closely packed collagen fibers, making it tough but slightly flexible. Because of its smooth surfaces, it allows tissues to slide/glide more easily, as well as provides flexibility and support.

The hyaline cartilage contains a gel-like amorphous matrix. It has a flexible cushioning properties and resists compressive stress. Its primary function is to provide some cushioning, reduce friction between the bone ends, and absorb shocks in joints.

It is the most abundant type of cartilage: It is found at the ends of long bones (as articular cartilage). It supports the tip of the nose, the trachea, and the bronchi. It forms most of the larynx and connects the ribs to the sternum. It is also found in epiphyseal plates of children.

Hyaline cartilage is pearl-grey in color, with a firm consistency, and has a considerable amount of collagen. The presence of collagen fibers makes such structures and joints strong, but with limited mobility and flexibility. It contains no nerves or blood vessels.

LIGAMENT

A ligament is a short band of tough dense fibrous connective tissue composed mainly of stringy, bundled collagen fibers. Ligaments connect bones to other bones to form a joint. (They do not connect muscles to bones; that is the function of tendons.) Some ligaments limit the mobility of articulations or prevent certain movements altogether.

Ligaments act as mechanical reinforcements for example the capsular ligaments are part of the articular capsule that surrounds synovial joints. Extra-capsular ligaments join bones together and provide joint stability.

Ligaments are viscoelastic. They gradually strain when under tension and return to their original shape when the tension is removed. However, they cannot retain their original shape when extended past a certain point or for a prolonged period of time. This is one reason why dislocated joints must be set as quickly as possible, if the ligaments lengthen too much, then the joint will be weakened, becoming prone to future dislocations. Ligaments cannot usually be regenerated naturally. Athletes, gymnasts, and martial artists perform stretching exercises to lengthen their ligaments, making their joints more supple. The term double-jointed refers to people who have more elastic ligaments, allowing their joints to stretch further.

The ligament consists of spindle-shaped cells known as fibrocytes, with a little ground substance. The branch of anatomy which studies ligaments is called desmology.

TENDON

A tendon is a strong, tough band of fibrous connective tissue that connects muscle to bone in order to transmit the force generated by muscle into the bone to perform actions.

A tendon is similar to ligaments but they serve a different function as the ligaments join one bone to another while typically tendons connect muscles to bones. Together a combination of tendons and muscles can only exert a pulling force.

Tendons are made up of specialized cells called tenocytes (tendon-specific fibroblasts) and their main task is to synthesize the extracellular matrix. The extracellular matrix of the tendon is very well organized thus forming a long triple helix which makes the tendon very strong. Here it is made up of more than 86% collagen fiber mainly type I collagen fiber which imparts tendon a unique property to resist a high level of tensile force, they can easily bear the stretches imposed over them. The unique thing about tendons is that their length varies person-to-person, it is not necessary that two-person has the same tendon length.

The tendon length is the ultimate factor that decides how much muscle you can make. People with short-length tendons are more likely to gain and build more muscle as compared to people who have a long-length of a tendon.

It is widely observed that huge bodybuilders who have more muscle mass have short tendon lengths while slim elite runners and sprinters have long tendon lengths. Here, the tendon length is purely dependent upon genetics, you can not increase or decrease your tendon length.

Tendons are also viscoelastic similar to ligaments. They gradually strain when under tension and return to their original shape when the tension is removed. However, they cannot retain their original shape when extended past a certain point or for a prolonged period of time. This is one reason why dislocated joints must be set as quickly as possible, if the tendon lengthens too much, then the joint will be weakened, becoming prone to future dislocations. Similar to ligaments tendon also cannot usually be regenerated naturally because they have a very limited supply of blood thus nutrient are not reaching them at a fast rate which makes their healing very slow. And the cells of the tendon, tenocytes (tendon-specific fibroblasts) are also not capable on itself to repair the damaged tendons.