Answer

(c)

1 Fast muscle fibres contract spontaneously and reach anaerobic conditions in shorter time, so as to accumulate lactic acid faster in the muscles.

2 Slow muscle fibres have better ability to endure, as they are resistant to fatigue and contract slowly, it is because, of accumulation, of large amount, of myoglobin in them.

3 Actin filament form the isometric band in the muscle fibre because it is the only this actin protein which is present in that region.

4 Sarcomere is the contractile unit of skeletal muscle.

• Option (a) is incorrect because:

  • Fast muscle fibres (A) are matched with Myoglobin (1), but fast muscle fibres are associated with lactic acid accumulation, not myoglobin.
  • Slow muscle fibres (B) are matched with Lactic acid (2), but slow muscle fibres are associated with myoglobin, not lactic acid.
  • Actin filament (C) is correctly matched with I-band (4).
  • Sarcomere (D) is correctly matched with Contractile unit (3).

• Option (b) is incorrect because:

  • Fast muscle fibres (A) are correctly matched with Lactic acid (2).
  • Slow muscle fibres (B) are matched with Myoglobin (1), which is correct.
  • Actin filament (C) is matched with Contractile unit (3), but actin filaments are associated with the I-band, not the contractile unit.
  • Sarcomere (D) is matched with I-band (4), but sarcomere is the contractile unit, not the I-band.

• Option (d) is incorrect because:

  • Fast muscle fibres (A) are matched with Contractile unit (3), but fast muscle fibres are associated with lactic acid accumulation, not the contractile unit.
  • Slow muscle fibres (B) are matched with Lactic acid (2), but slow muscle fibres are associated with myoglobin, not lactic acid.
  • Actin filament (C) is correctly matched with I-band (4).
  • Sarcomere (D) is matched with Myoglobin (1), but sarcomere is the contractile unit, not associated with myoglobin.

Thinking Process

There are 12 pair of ribs which forms the bony lateral walls of the thoracic cage.

Answer

(b) Sternum This is a flat bone present just beneath the skin in the middle of the chest. It is about $15 \mathrm{~cm}$ long. It consist of three parts, i.e., manubrium the upper most part, body the middle portion and xiphoid process is the tip of the bone.

The true ribs ( 7 pairs) are attached to the sternum. While the, scapula and clavicle together constitute to form pectoral girdle, and llium is a part of pelvic girdle.

• Scapula: The scapula, also known as the shoulder blade, is not connected to the ribs. It is a part of the pectoral girdle and serves as an attachment point for several muscles that allow for the movement of the shoulder and arm.

• Clavicle: The clavicle, or collarbone, is also part of the pectoral girdle. It connects the arm to the body, but it does not attach to the ribs. Instead, it connects to the scapula and the sternum.

• Ilium: The ilium is a part of the pelvic girdle, not the thoracic cage. It is one of the three bones that make up the pelvis and is located in the lower part of the torso, far from the ribs.

Thinking Process

The structural arrangement of tissues by which bones are joined together are called joints.

Answer

(a) Pivot joint is the joints found between the atlas and axis and between the radius and ulna just below the elbow. This joint allows the movement in only one plane. In a pivot joint, rounded or pointed bone fits into a shallow depression of another bone.

Whereas, saddle joint provides free movement in two planes back-forth and side to side. The projection of one bone fits in saddle-shaped depression of another bone. The joint between the carpel and metacarpel of thumb in the hand is an example of saddle joint.

Hinge joint allows movement primarily in one plane. In a hinge joint reel (spoon) like surface of one bone fits into the concave surface of another bone for example the elbow, the knee, ankle etc.

Gliding joint also known as plane joint, it is a common type of synovial joint formed between bones that meet at flat or nearly flat articulating surface examples of gliding joint include carpel bones of the wrist and joint between the carpel and metacarpel of the palm.

• Saddle joint: This type of joint provides free movement in two planes, back-forth and side to side. The projection of one bone fits in a saddle-shaped depression of another bone. An example of a saddle joint is the joint between the carpal and metacarpal of the thumb in the hand. This is not the type of joint found between the atlas and axis.

• Hinge joint: This type of joint allows movement primarily in one plane. In a hinge joint, the reel (spoon)-like surface of one bone fits into the concave surface of another bone. Examples include the elbow, knee, and ankle. This is not the type of joint found between the atlas and axis.

• Gliding joint: Also known as a plane joint, it is a common type of synovial joint formed between bones that meet at flat or nearly flat articulating surfaces. Examples of gliding joints include the carpal bones of the wrist and the joint between the carpal and metacarpal of the palm. This is not the type of joint found between the atlas and axis.

Answer

(c) The globular head of myosin in muscle is an active ATPase enzyme having binding sites for ATP and active site for actin.

While the ATPase is not found in actinin, troponin or actin of muscle fibre.

• Actinin: Actinin is a protein that primarily functions to anchor actin filaments to the Z-line in muscle cells. It does not possess ATPase activity.

• Troponin: Troponin is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal and cardiac muscle. It does not have ATPase activity.

• Actin: Actin is a protein that forms thin filaments in muscle fibers and plays a crucial role in muscle contraction. However, it does not have intrinsic ATPase activity; instead, it interacts with myosin, which has the ATPase activity necessary for muscle contraction.

Answer

(c) Intervertebral disc is found in the vertebral column of mammals. These are present between the bodies of adjacent vertebrae from second cervical vertebra to the sacrum. Each disc consist of an outer fibrous ring made of fibrocartilage and an inner soft pulpy, highly elastic substance.

This disc are majorly involved in formation of strong joints that permits various movements of the vertebral column and absorb vertical shockes.

Intervertebral disc is not found in the vertebral column of birds reptiles or ambhibians

• Birds: Birds have a different vertebral structure compared to mammals. They possess a series of fused vertebrae, particularly in the thoracic region, which provides rigidity necessary for flight. They do not have intervertebral discs as seen in mammals.

• Reptiles: Reptiles have a more rigid and less flexible vertebral column compared to mammals. Their vertebrae are often connected by more rigid cartilaginous or bony structures, and they lack the intervertebral discs that provide cushioning and flexibility in mammals.

• Amphibians: Amphibians have a simpler vertebral structure compared to mammals. Their vertebrae are often connected by cartilaginous pads rather than the complex intervertebral discs found in mammals. This simpler structure supports their less demanding locomotion needs.

Thinking Process

Our vertebral column is formed by 26 serially arranged units called vertebrae, there are dorsally placed, extending from the base of the skull and constitutes the main frame work of the trunk.

Answer

(d) The correct sequence showing the vertebral column of human being is Cervical — thoracic — lumbar — sacrals — coccygeal

Whereas, the other sequential order of vertebrae are wrong.

• Option (a) is incorrect because it places the lumbar vertebrae before the thoracic vertebrae. The correct order is cervical, thoracic, lumbar, sacral, and coccygeal.

• Option (b) is incorrect because it places the sacral vertebrae before the lumbar vertebrae. The correct order is cervical, thoracic, lumbar, sacral, and coccygeal.

• Option (c) is incorrect because it places the sacral vertebrae before the thoracic and lumbar vertebrae. The correct order is cervical, thoracic, lumbar, sacral, and coccygeal.

Thinking Process

Joints are points of contact between two bones or between bones and cartilages.

Answer

(a) The joint present between humerus and pectoral girdle is ball and socket joint. Hinge joint is present between atlas and axis not between humerus and pectoral girdle. The elbow, knee, ankle and interphalangeal joints are the examples of hinge joints.

(Also, refer to Q. 3)

Rest of the pair are correct regarding to the presence of joints.

• (b) Pivot joint - between atlas, axis and occipital condyle: The pivot joint is actually between the atlas and axis, but not involving the occipital condyle. The occipital condyle forms a condyloid joint with the atlas.

• (c) Gliding joint - between the carpals: This option is correct. There is no incorrect information here.

• (d) Saddle joint - between carpel and metacarpals of thumb: This option is also correct. There is no incorrect information here.

Answer

(d) Knee joint and elbow joints are examples of hinge joints. (Also, refer to Q. 3 and 7)

• Saddle joint: Saddle joints allow for a wide range of movement, including back-and-forth and side-to-side motions, but they do not allow for rotation. Examples include the thumb joint (carpometacarpal joint of the thumb). The knee and elbow joints do not exhibit this type of movement.

• Ball and socket joint: Ball and socket joints allow for the most extensive range of movement, including rotation, as seen in the shoulder and hip joints. The knee and elbow joints do not allow for such extensive movement and rotation.

• Pivot joint: Pivot joints allow for rotational movement around a single axis, such as the joint between the first and second cervical vertebrae (the atlas and axis) in the neck. The knee and elbow joints do not primarily allow for rotational movement.

Thinking Process

In some cells microfilaments are involved in showcasing amoeboid movement.

Answer

(c) Some specialised cells in blood like macrophages and leucocytes exhibit amoeboid movement. They have the ability to reach the interstitial fluid by squeezing through the thin walls of blood vessels, while ciliary movement flageller movement or gliding movement are not shown by macrophages and leucocytes.

• Ciliary movement: This type of movement is typically exhibited by cells that have cilia, which are hair-like structures that beat in a coordinated manner to move the cell or to move fluid over the cell surface. Macrophages and leucocytes do not have cilia and therefore do not exhibit ciliary movement.

• Flagellar movement: This type of movement is characteristic of cells that have flagella, which are long, whip-like structures that propel the cell. Examples include sperm cells and certain bacteria. Macrophages and leucocytes do not have flagella and thus do not exhibit flagellar movement.

• Gliding movement: This type of movement is seen in certain types of bacteria and protozoa, where the cell moves smoothly over a surface without the use of cilia or flagella. Macrophages and leucocytes do not exhibit gliding movement as they move by extending and retracting their cell membrane in an amoeboid fashion.

Answer

(d) Atherosclerosis also known as ateriosclerotic vascular disease, where arteries wall get thickens as a result of invasion and accumulation of WBC, containing both livings active WBCs (white blood cells) and remnants of dead WBC’s along with cholesterol and triglycerides. Remaining diseases, i.e., arthritis, osteoporosis and rickets are bone disorders.

• Arthritis: Arthritis is a disorder that affects the joints, causing inflammation, pain, and stiffness. It is not limited to bones but involves the entire joint structure, including cartilage, synovium, and surrounding tissues.

• Osteoporosis: Osteoporosis is a bone disorder characterized by decreased bone density and mass, leading to fragile bones that are more prone to fractures. It directly affects the bone structure and strength.

• Rickets: Rickets is a bone disorder that occurs in children due to a deficiency of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones, causing deformities and growth disturbances.

Thinking Process

The walls of internal organs such as the blood vessels, stomach and intestine contain smooth muscles.

Answer

(c) Smooth muscles are ‘involuntary’ and non-striated muscles as they cannot be controlled directly like that of skeletal muscles which are voluntary, controlled and possess striations. They, inner walls of alimentary canal are non-striated and involuntary muscles. Rest other statements are correct.

• (a) Heart muscles are striated and involuntary: This statement is correct. Heart muscles, also known as cardiac muscles, are indeed striated and involuntary.

• (b) The muscles of hands and legs are striated and voluntary: This statement is correct. The muscles in the hands and legs are skeletal muscles, which are striated and under voluntary control.

• (d) Muscles located in the reproductive tracts are unstriated and involuntary: This statement is correct. The muscles in the reproductive tracts are smooth muscles, which are unstriated and involuntary.

Thinking Process

Skull, vertebral column, ribs and sternum constitute the axial skeleton. Limb, bones and girdles form the appendicular skeleton.

Answer

(b) Head of humerus bone articulates with the glenoid cavity of pectoral girdle. This articulation results in the formation of ball and socket joints, e.g., ball and socket joints present in shoulder while other statement are incorrect.

• (a) The head of the humerus bone does not articulate with the acetabulum of the pectoral girdle. The acetabulum is a part of the pelvic girdle, not the pectoral girdle.

• (c) The head of the humerus bone does not articulate with a cavity called the acetabulum of the pelvic girdle. The acetabulum is the socket in the pelvic girdle that articulates with the head of the femur, not the humerus.

• (d) The head of the humerus bone does not articulate with a glenoid cavity of the pelvic girdle. The glenoid cavity is part of the pectoral girdle, not the pelvic girdle.

Answer

(b) Cardiac muscle fibres are supplied with both central and autonomic nervous system and are not under the control of the will of the animal, i.e., they are involuntary. These muscles possess striation but they never get fatigued as the myofibrils of heart have transverse faint dark and light bands which alternate with each other giving them striped appearence.

While muscles of the wall of alimentary canal are smooth muscle, i.e., non-striated and involuntary, muscle assisting locomotion, i.e., skeletal muscles are striated and voluntary and muscles of eyelid are involuntary but striations muscles this the other options are wrong.

• Muscles in the wall of the alimentary canal are smooth muscles, which are non-striated and involuntary.
• Muscles assisting locomotion are skeletal muscles, which are striated but voluntary.
• Muscles of the eyelids are involuntary but striated, making them different from the cardiac muscles in terms of control and function.

Answer

(b) A. $\rightarrow$ (4) B. $\rightarrow$ (3) C. $\rightarrow(1)$ D. $\rightarrow$ (2).

Sternum is a flat bone present just under beneath the skin in the middle of the front of the chest.

Glenoid Cavity is the depression which articulates with the head of the humerus to form the ball and socket joint in pectoral girdle.

Freely Movable Joints are characterised by the presence of a fluid filled synovial cavity between the articulating surface of the two bones. This fluids represents the synovial fluid, e.g., in gliding and hinge joints.

Cartilagenous Joints are present between the adjacent vertebrae in the vertebral column.

• Option (a):

  • A. Sternum $\rightarrow$ (2): Incorrect because the sternum is a flat bone, not associated with vertebrae.
  • B. Glenoid cavity $\rightarrow$ (1): Incorrect because the glenoid cavity is part of the pectoral girdle, not related to synovial fluid.
  • C. Freely movable joint $\rightarrow$ (3): Incorrect because freely movable joints are characterized by synovial fluid, not the pectoral girdle.
  • D. Cartilagenous joint $\rightarrow$ (4): Incorrect because cartilaginous joints are found between vertebrae, not flat bones.

• Option (c):

  • A. Sternum $\rightarrow$ (2): Incorrect because the sternum is a flat bone, not associated with vertebrae.
  • B. Glenoid cavity $\rightarrow$ (1): Incorrect because the glenoid cavity is part of the pectoral girdle, not related to synovial fluid.
  • C. Freely movable joint $\rightarrow$ (4): Incorrect because freely movable joints are characterized by synovial fluid, not flat bones.
  • D. Cartilagenous joint $\rightarrow$ (3): Incorrect because cartilaginous joints are found between vertebrae, not the pectoral girdle.

• Option (d):

  • A. Sternum $\rightarrow$ (4): Correct.
  • B. Glenoid cavity $\rightarrow$ (1): Incorrect because the glenoid cavity is part of the pectoral girdle, not related to synovial fluid.
  • C. Freely movable joint $\rightarrow$ (2): Incorrect because freely movable joints are characterized by synovial fluid, not vertebrae.
  • D. Cartilagenous joint $\rightarrow$ (4): Incorrect because cartilaginous joints are found between vertebrae, not flat bones.

Thinking Process

Cells of the human body exhibit three main types of movements, namely amoeboid, ciliary and muscular.

Answer

(a) Macrophages and leucocytes in blood exhibit amoeboid movement. Cytoskeletal elements like microfilaments are also involved in amoeboid movement.

(b) Ciliary Movement These types of movements occurs mostly in the internal organs, which are lined by the ciliated epithelium. e.g., cilia in trachea helps in removing dust particle and foreign substances inhaled along with atmospheric air.

Passage of ova through the female reproductive tract is also facilitated by the ciliary movement. This is due to the presence of ciliated epithelium in the Fallopian tube.

Answer

Locomotion requires a prefect coordinated activity of muscular, skeletal and neural systems.

Thinking Process

Mechanism of contraction in our body occurs through skeletal muscles made of a number of muscle bundles. Each muscle bundle contains a number of muscle fibres.

Answer

There parts belongs to the muscle fibre, which is lined by the plasma membrane called sarcolemma. Muscle fibre is a syncitium because sarcoplasm (the cytoplasm) of muscle fibre contains number of nuclei and sarcoplasmic reticulum is the endoplasmic reticulum of the muscle fibre and is the store house of calcium ions.

Thinking Process

Each actin filament is made of two ’ $F$ ’ (filamentous) actins helically wound to each other and each ’ $F$ ’ actin is a polymer of monomeric ’ $G$ ’ (globular) actins..

Answer

Representing different component of actin filament

Answer

Each middle ear contains three tiny bones, named, i.e., malleus, incus and stapes which are collectively called as ear ossicles.

Answer

Difference between the matrix of bones and cartilage

Matrix of Bones	Matrix of Cartilage
Matrix of bones has an inflexible material, the ossein.	Matrix of cartilage has a flexible material, the chondrin.
Matrix of bones contains calcium salts.	Calcium salts may or may not be present in matrix.

Answer

Myasthenia gravis is an autoimmune disorder of skeletal muscle, affecting neuromuscular junction, that leads to fatigue, weakening and paralysis of the skeletal muscle.

Answer

The presence of synovial fluid, between articulating surface of the two bones enclosed within synovial cavity of synovial joints to makes our joints to function without grinding noise and pain.

Answer

Ball and socket joint are present between humerus and pectoral girdle. These joints allows free movement of bone in all direction. e.g., shoulder jointds (humerus bone in socket of pectoral girdle) and hip joints femur bone in socket pelvic girdle.

Answer

Our forearm is made of three different bones, i.e., humerus, radius and ulna.These bones can be seen in following figure

Thinking Process

There are 12 pairs of ribs. Each rib consist of a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum.

Answer

(a) Bicephalic ribs, each ribs has two articulating surfaces on its dorsal end hence, are called as bicephalic ribs.

(b) True ribs are the first seven pairs of ribs Dorsally these ribs are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.

(c) Floating ribs are the last two pair (11th and 12th) of ribs and are not connected ventrally to the sternum therefore, called as floating ribs.

Answer

The condition of stiff and inflammed joints is called as osteoporosis. It is an age-related disorder characterised by decreased bone mass and increased chances of fractures. Decreased levels of estrogen is a common cause for osteoporosis in females after menopause, in old aged females.

Thinking Process

Parathyroid and thyroid glands, functions under the feed back control of blood calcium level.

Answer

(a) More $\mathrm{Ca}^{2+}$ concentration in extracellular fluid is associated with hyperparathyroidism. It causes **demineralisation, resulting in softening and bending **of the bones. This condition leads to osteoporosis.

(b) Very less amount of $\mathrm{Ca}^{2+}$ in extracellular fluid is associated with hypoparathyroidism. This increases the excitability of nerves and muscles, causing cramps, sustained contraction of the muscles of larynx, face, hands and feet. This disorder is called parathyroidtetany or hypercalcemictetany.

Answer

Parathyroid hormone and calcitonin results in the fluctuation of $\mathrm{Ca}^{2+}$ level. Parathyroid Hormone (PTH) increases the $\mathrm{Ca}^{2+}$ levels in the blood. PTH acts on the bones and stimulates the process of bone resorption (dissolution/demineralisation).

PTH also stimulates reabsorption of $\mathrm{Ca}^{2+}$ by the renal tubules and increases $\mathrm{Ca}^{2+}$ absorption from the digested food.

Calcitonin is a 32-amino acid linear polypeptide hormone, that is produced in humans primarily by the parafollicular cells of the thyroid. It acts by reducing blood calcium $\left(\mathrm{Ca}^{2+}\right)$, levels opposing the effect of Parathyroid Hormone (PTH).

Answer

(b) Rahul has gained weight because his muscle shape has changed. Regular exercise increase the body muscle as there is an enlargement of muscles due to increase in the amount of sarcoplasm and mitochondria and the strength he to developed led him gain the mass and size of body muscle and reduction in fat content.

Answer

(a) Due, to continuous exercise her muscles got fatigues because of the accumulation of lactic acid within skeletal muscles. Pain is also oftenly experienced in the fatigued muscles

(b) Her breathing rate changes from normal to high as during exercise, her body muscle require more oxygen for the ATP production, than the normal value, hence her breathing enhances, to lake most oxygen from the atmosphere.

Answer

Gout is a disease, caused due to defect in purine metabolism. It causes accumulation of excess of uric acid and its crystals in the joints. The level of uric acid and crystals of its salts get raised in blood causing their accumulation in the joints causes gouty arthritis. The excess of urates in blood can also lead to the formation stones in the kidneys.

Answer

ATP (Adenosin Triphosphate) is the source of energy for muscle contraction. The head of each myosin molecule contains an enzyme called myosin ATPase.

In the presence of this enzyme along with $\mathrm{Ca}^{2+}$ then, and $\mathrm{Mg}^{2+}$ ions the ATP molecule breaks down into ADP and inorganic phosphate, thus releasing energy in the head of myosin.

$$ \text { ATP } \rightarrow \text { ADP }+P_{i}+\text { Energy } $$

Energy from ATP causes energised myosin to cross bridges and to bind with actin and in this way initiates muscle contraction.

Thinking Process

Pectoral and pelvic girdle bones help in the articulation of the upper and the lower limbs respectively with the axial skeleton.

Answer

Pectoral girdle Each half of the pectoral girdle consist of a clavicle and a scapula. The dorsal flat, triangular body of scapula has a slightly elevated ridge called the spine that, projects a flat expanded process called the acromion and the clavicle articulating with it.

Below the acromion their is a depression called the glenoid cavity which articulates with the head of the humerous to form the shoulder joint. Pelvic girdle consist of two coxal bones. each formed by the fusion of three bones, ilium, ischium and pubis. It articulates with femur through a cavity called acetabulum forming thigh joint.

Thinking Process

Concentration of calcium ion $\left(\mathrm{Ca}^{2+}\right)$ in body fluid mainly affects the muscle contraction, as the binding of contractile proteins actin and myosin depends on it.

Answer

For the muscle fibre to contract, the binding site on thin filaments must be uncovered. This occurs when $\mathrm{Ca}^{2+}$ bind to another set of regulatory proteins, called troponin complex which control the position of tropomyosin on the thin filament.

The calcium binding rearranges the tropomyosin, troponin complex, exposing the myosin-binding sites on the thin filament. When $\mathrm{Ca}^{2+}$ is present in the cytosol, the thin and thick filament slide part each other resulting in muscle contraction.

Similarly, when the $\mathrm{Ca}^{2+}$ concentration falls, the binding sites get covered and contraction stops.

In case of tetany there occur low calcium levels in body fluid due to diminished function of parathyroid gland. This gland is mainly involved in the secretion of parathyroid hormone which is associated in regulating calcium levels in blood. Tetany results in periodic painful muscular spasm (wild contraction) and tremors.

Answer

Slipped disc is a medical condition in which spine is affected due to wear and tear in the outer fibrous ring (anulus fibrosus) of an intervertebral disc, allowing the soft, central portion to bulge out beyond the damaged outer rings.

These intervertebral disc are present between the bodies of adjacent vertebrae from the second cervical vertebra to the sacrum. This discs form strong joints that permit various movements of vertebral column and absorbs vertical shock.

The cause of slip disc can be due to general wear and tear of intervertebral disc during performing various jobs, that require constant sitting and squatting. The slipped disc or herniation occurs in two regions of body, i.e., cervical disc and lumber disc.

Slip disc in lower back lumber disc, lead to sharp pain in one part of leg due to sciatica, (disturbance in sciatic nerve), hip and cause numbness in other lower parts of the body.

Slip disc in neck region (cervical disc) leads to pain while moving neck near or over the shoulder bone or pain occurs while moving forearm, or fingers. It also causes numbness in shoulder, elbow, forearm and finger area.

Hence, slipped disc affect the upper and lower body parts, thus, influencing life style and health.

Thinking Process

The contraction of muscle fibre takes place by the sliding of the thin filaments over thick filaments.

Answer

Sliding filament theory

This theory is applicable to smooth, cardiac and skeletal muscles. The essential features of this theory are as follows

(i) During muscle contraction, thin myofilaments slide inward towards the $\mathrm{H}$-zone.

(ii) The sarcomere, the basic unit of muscle contraction, shortens, without changing the length of thin and thick myofilaments.

(iii) The cross-bridge of the thick myofilaments connect with the portions of actin of the thin myofilaments. These cross-bridge move on the surface of the thin myofilaments, resulting in the sliding of thin and thick myofilaments over each other. (iv) The length of the thick and thin myofilaments do not change during muscle contraction.

(v) A muscle fibre maintains a resting potential under resting conditions just like a nerve fibre. As soon as a nerve impulse reaches the terminal end of the axon, small sacs called synaptic vesicles fuse with the axon membrane and release a chemical transmitter,called acetylcholine.

It diffuses across the synaptic cleft (the space between the axon membrane and the motor end plate) and binds to the receptor sites of the motor end plate.

(vi) As soon as depolarisation of the motor end plate reaches a certain level, it creates an action potential. After this, an enzyme cholinesterase present along with the receptor sites for acetylcholine breaks down acetylcholine into acetate and choline.

A portion of the choline diffuses back to the axon and is reused to synthesise more acetylcholine for the transmission of subsequent impulses.

(vii) Calcium plays a key regulatory role in muscle contraction. The $\mathrm{Ca}^{+}$ions bind to troponin causing a change in its shape and position. This in turn alters the shape and position of tropomyosin.

This shift exposes the active sites on the F-actin molecules and myosin cross-bridges are then able to bind to these active sites.

(viii) The head of each myosin molecule contains an enzyme myosin ATPase. In the presence of myosin ATPase, $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ ions, ATP breaks down into ADP and inorganic phosphate as

$$ \text { ATP } \xrightarrow[\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}]{\text { Myosin ATPase }} \text { ADP }+\mathrm{P}_{\mathrm{i}}+\text { Energy } $$

(ix) Energy from ATP causes energised myosin cross-bridges to bind to actin. The energised cross-bridge move, causing the thin myofilaments to slide along the thick myofilaments. This movement is like the movement of the oars of a boat.

(x) As stated earlier in theory, there is no shortening of thin and thick myofilaments. However, the sarcomere shortens, because of the sliding of the thin myofilaments produced by cross-bridge movements. The $\mathrm{H}$-zone and $\mathrm{I}$-band shorten, but the width of the A-band remains constant.

Answer

Formation of cross-bridge between the actin and myosin filament help muscle to contract.

(i) An ATP molecule joins the active site on myosin head of myosin myofilament. These heads contains an enzyme, myosin ATPase that along with $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ ions catalyses the breakdown of ATP.

$$ \text { ATP } \xrightarrow[\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}]{\text { Myosin ATPase }} \text { ADP }+\mathrm{P}_{\mathrm{i}}+\text { Energy } $$

(ii) The energy is transferred to myosin head which energises and straightens to join an active site on actin myofilament, forming a cross-bridge.

(iii) The energised cross-bridges move, causing the attached actin filaments to move towards the centre of A-band. The Z-line is also pulled inwards causing shortening of sarcomere, i.e., contraction. It is clear from the above explanation that during contraction A-bands retain the length, while I-bands get reduced.

(iv) The myosin head releases $A D P$ and $\mathrm{Pi}$, relaxes to its low energy state. The head detaches from actin myofilaments when new ATP molecule joins it and cross-bridge are broken.

(v) In repeating cycle, the free head cleaves the new ATP. The cycles of cross-bridge formation and breakage is repeated causing further sliding.

Showing movement of the thin filaments and the relative size of the I-band and $\mathrm{H}$-zones

(vi) Muscle relaxation occurs after contraction when the calcium ions are pumped back to the sarcoplasmic cisternae, thus, blocking the active sites on actin myofilaments. The $Z$-line returns to original position, i.e., relaxation.

Answer

Calcium plays a key regulatory role in muscle contraction. These ions bind to troponin causing change in its shape and position. This in turn alters the shape and position of tropomyosin. This shift exposes the active sites on the F-actin molecules and myosin cross-bridges able to bind to these active sites.

The complete process is outlined in the figure below

Role of calcium ion, is the contraction and relaxation process. The head of each myosin molecule contains an enzyme myosin ATPase. In the presence of myosin ATPase, $\mathrm{Ca}^{2+}$ and $\mathrm{Mg}^{2+}$ ions, ATP breaks down into ADP and inorganic phosphate as

$$ \text { ATP } \xrightarrow[\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}]{\text { Myosin ATPase }} \text { ADP }+\mathrm{P}_{\mathrm{i}}+\text { Energy } $$

Energy from ATP causes energised myosin cross-bridges to bind with actin.

Answer

The pectoral and pelvic girdle are responsible in providing support to the upper and lower body portions

Pectoral Girdle	Pelvic Girdle
It occurs in the shoulder region, hence also called as shoulder girdle.	It occurs in the hip region, hence also called as hip girdle.
Pectoral girdles are divided into two parts, i.e., one clavicle and one scapula.	There is one pelvic girdle, which is formed by two, innominate bones. Each bone consist of three parts. i.e., ilium, ischium and pubis.
Clavicle and scapula helps in articulation of the upper limb with axial skeleton.	The innominate at the middle of its lateral surface has a deep, cup shaped acetabulum. where head of the femur articulates the two halves of the pelvic girdle and meet ventrally to form public symphysis.
It has no articulation with the vertebral column.	It has articulation with vertebral column.
Bones associated with pectoral girdle are light, as they are not subjected to much stress.	Bones associated with pelvic girdle are hard as they are subjected to much stress
There perform like holding, lifting	There function like running, standing, jumping.