Locomotion and Movement

Given: We need to represent the structural unit of a myofibril — the sarcomere.

Description of the diagram:

A sarcomere is the functional unit of a myofibril, bounded on either side by Z lines (Z discs).

$$\underbrace{|\!\!\!\underbrace{\quad I\text{-band}\quad|\underbrace{\quad\quad A\text{-band}\quad\quad}_{\text{H-zone in centre}}|\quad I\text{-band}\quad}_{\text{Sarcomere}}\!\!\!|}_{Z\text{ line}\qquad\qquad\qquad\qquad\qquad\qquad Z\text{ line}}$$

Regions of a sarcomere:

1. Z line (Z disc): Boundary of each sarcomere. Thin (actin) filaments are anchored here.
2. I band (Isotropic band): Light band on either side of the Z line. Contains only thin filaments (actin). It is bisected by the Z line.
3. A band (Anisotropic band): Dark central band. Contains thick filaments (myosin) throughout, and thin filaments overlap at the periphery of this band.
4. H zone: Central lighter region within the A band where only thick filaments (myosin) are present (no overlap with actin).
5. M line: A thin line at the centre of the H zone that holds the thick filaments in position.

Key filaments:
- Thick filaments: Myosin — present in the A band.
- Thin filaments: Actin — present in the I band and extend into the A band.

During contraction: The I band and H zone shorten; the A band length remains constant.

Sliding Filament Theory of Muscle Contraction:

The sliding filament theory was proposed by H.E. Huxley and A.F. Huxley (1954).

Definition:
According to this theory, muscle contraction occurs due to the sliding of thin filaments (actin) over the thick filaments (myosin). The myosin heads (cross bridges) attach to the active sites on actin filaments and pull them towards the centre of the sarcomere (towards the M line), causing the sarcomere to shorten.

Key points of the theory:
- The length of both thick (myosin) and thin (actin) filaments does not change during contraction.
- The I band (only thin filaments) and H zone (only thick filaments) decrease in width.
- The A band length remains constant.
- The Z lines are pulled closer together, shortening the sarcomere.
- The overall shortening of all sarcomeres leads to contraction of the entire muscle fibre.

In summary: Muscle contraction results from the sliding of actin filaments over myosin filaments, powered by ATP hydrolysis at the myosin head (ATPase activity).

Important Steps in Muscle Contraction:

Step 1 – Neural Signal:
A motor neuron carries a nerve impulse (action potential) to the neuromuscular junction (motor end plate) of the muscle fibre.

Step 2 – Release of Neurotransmitter:
The nerve impulse causes release of acetylcholine at the neuromuscular junction, which generates an action potential in the sarcolemma (muscle cell membrane).

Step 3 – Release of Ca²⁺:
The action potential travels along the T-tubules and triggers the sarcoplasmic reticulum to release Ca²⁺ ions into the sarcoplasm.

Step 4 – Activation of Actin:
- In a resting muscle, the active sites on actin filaments are masked by troponin (a regulatory protein).
- The released $\text{Ca}^{2+}$ binds to troponin, causing a conformational change.
- This moves tropomyosin away from the active sites on actin, exposing the active sites.

Step 5 – Cross Bridge Formation:
The energised myosin head (which has already hydrolysed ATP → ADP + Pᵢ) binds to the exposed active sites on actin to form cross bridges.

Step 6 – Power Stroke (Sliding):
The myosin head bends (power stroke), pulling the actin filament towards the centre of the sarcomere (M line). This causes the thin filaments to slide inward, shortening the sarcomere.

Step 7 – Detachment and Re-energisation:
A new ATP molecule binds to the myosin head, causing it to detach from actin. ATP is hydrolysed by ATPase, re-energising the myosin head, which can then attach to the next active site on actin. This cycle repeats as long as $\text{Ca}^{2+}$ and ATP are available.

Step 8 – Relaxation:
When the nerve impulse stops, $\text{Ca}^{2+}$ is pumped back into the sarcoplasmic reticulum (active transport using ATP). Troponin re-covers the active sites on actin, cross bridges break, and the muscle relaxes.

(a) Actin is present in thin filament.

True.
Actin is indeed the major protein component of the thin filaments in a myofibril. Each thin filament is made of two 'F' actin strands along with troponin and tropomyosin.

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(b) H-zone of striated muscle fibre represents both thick and thin filaments.

False.

Corrected statement: The H-zone of striated muscle fibre represents only thick filaments (myosin). It is the central lighter region within the A band where thin (actin) filaments do not extend.

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(c) Human skeleton has 206 bones.

True.
The adult human skeletal system consists of 206 bones in total.

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(d) There are 11 pairs of ribs in man.

False.

Corrected statement: There are 12 pairs of ribs in man. Out of these, 7 pairs are true ribs, 3 pairs are false ribs, and 2 pairs are floating ribs.

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(e) Sternum is present on the ventral side of the body.

True.
The sternum (breast bone) is a flat bone present on the ventral (anterior) midline of the thorax.

(a) Differences between Actin and Myosin:

| Feature | Actin | Myosin |
|---|---|---|
| Filament type | Thin filament | Thick filament |
| Location in sarcomere | I band (and partly in A band) | A band |
| Molecular weight | Lower (~42 kDa per monomer) | Higher (~500 kDa) |
| Structure | Two 'F' actin strands (polymer of 'G' actin) twisted together; associated with troponin and tropomyosin | Polymer of meromyosin; has a globular head (HMM) and a rod-like tail (LMM) |
| Active sites | Has active sites for myosin (masked by troponin at rest) | Head has ATPase activity and active sites for actin |
| Role | Passive component; slides over myosin | Active component; generates force via cross bridges |

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(b) Differences between Red and White Muscles:

| Feature | Red Muscle (Slow-twitch) | White Muscle (Fast-twitch) |
|---|---|---|
| Myoglobin content | High (gives red colour) | Low (pale/white colour) |
| Mitochondria | Numerous | Few |
| Type of respiration | Aerobic (oxidative) | Anaerobic (glycolytic) |
| Fatigue | Fatigue resistant (slow to fatigue) | Fatigue quickly |
| Contraction speed | Slow but sustained | Fast but not sustained |
| Glycogen content | Less | More |
| Example | Postural muscles | Eye muscles, limb muscles for quick movement |

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(c) Differences between Pectoral and Pelvic Girdle:

| Feature | Pectoral Girdle | Pelvic Girdle |
|---|---|---|
| Location | Shoulder region (anterior) | Hip region (posterior) |
| Bones | Two clavicles + two scapulae | Two coxal (hip) bones (each formed by ilium, ischium and pubis) |
| Articulation with vertebral column | Not directly articulated (only scapula connects via muscles) | Directly articulated with the sacrum |
| Joint with limb bone | Glenoid cavity of scapula articulates with humerus | Acetabulum of coxal bone articulates with femur |
| Function | Supports forelimbs (upper limbs) | Supports hindlimbs (lower limbs) and bears body weight |
| Strength | Less strong | More strong and rigid |

Correct Matching:

| Column I | Column II |
|---|---|
| (a) Smooth muscle | (iv) Involuntary |
| (b) Tropomyosin | (ii) Thin filament |
| (c) Red muscle | (i) Myoglobin |
| (d) Skull | (iii) Sutures |

Justification:
- (a) Smooth muscle → (iv) Involuntary: Smooth (visceral) muscles are non-striated and involuntary, not under conscious control.
- (b) Tropomyosin → (ii) Thin filament: Tropomyosin is a regulatory protein associated with the thin (actin) filament, along with troponin.
- (c) Red muscle → (i) Myoglobin: Red muscles appear red due to the high content of the oxygen-binding pigment myoglobin.
- (d) Skull → (iii) Sutures: The flat bones of the skull are joined by immovable fibrous joints called sutures.

Types of Movements Exhibited by Cells of the Human Body:

The cells of the human body exhibit three main types of movements:

1. Amoeboid Movement:
- Exhibited by specialised cells such as macrophages and leucocytes (WBCs) in the blood.
- Movement occurs by the formation of pseudopodia (false feet) due to streaming of cytoplasm.
- Cytoskeletal elements like microfilaments are involved.
- Example: WBCs migrate out of blood vessels (diapedesis) and engulf pathogens.

2. Ciliary Movement:
- Exhibited by cells lining the respiratory tract (trachea, bronchi) and the fallopian tubes (oviducts).
- Cilia are hair-like projections that beat rhythmically.
- In the respiratory tract, cilia move mucus and trapped dust particles outward.
- In the fallopian tube, cilia help in the movement of the ovum towards the uterus.

3. Muscular Movement:
- This is the most common type of movement in the human body.
- Exhibited by muscle cells (muscle fibres) through contraction and relaxation.
- Responsible for movements of limbs, locomotion, movement of food through the alimentary canal (peristalsis), pumping of blood by the heart, etc.
- Involves the sliding of actin and myosin filaments (sliding filament theory).

Differences between Skeletal Muscle and Cardiac Muscle:

| Feature | Skeletal Muscle | Cardiac Muscle |
|---|---|---|
| Location | Attached to bones (limbs, trunk, face, etc.) | Present only in the wall of the heart |
| Striations | Striated (shows cross striations) | Striated (shows cross striations) |
| Nature of control | Voluntary (under conscious control) | Involuntary (not under conscious control) |
| Branching | Fibres are unbranched, long and cylindrical | Fibres are branched |
| Nuclei | Multinucleate (many nuclei per fibre, peripherally located) | Uninucleate (single, centrally placed nucleus) |
| Intercalated discs | Absent | Present (join adjacent cardiac muscle cells, allow rapid conduction of impulse) |
| Fatigue | Fatigues relatively quickly | Does not fatigue (contracts rhythmically throughout life) |
| Rhythm | Contracts only when stimulated | Shows rhythmic, spontaneous contractions (autorhythmicity) |
| Function | Locomotion and body movements | Pumping of blood |

Conclusion: Both skeletal and cardiac muscles are striated, but they differ fundamentally in their control (voluntary vs. involuntary), structure (branching, intercalated discs) and function.

Types of Joints:

(a) Atlas / Axis:
Pivot joint
The atlas rotates around the odontoid process (dens) of the axis, allowing the head to rotate from side to side (e.g., shaking the head to say 'no').

(b) Carpal / Metacarpal of thumb:
Saddle joint
This joint allows movement in two planes (flexion/extension and abduction/adduction), giving the thumb its wide range of motion and opposability.

(c) Between phalanges:
Hinge joint
The interphalangeal joints allow movement in only one plane — flexion and extension (like the hinge of a door).

(d) Femur / Acetabulum (hip joint):
Ball and socket joint
The rounded head (ball) of the femur fits into the cup-shaped acetabulum (socket) of the coxal bone, allowing movement in multiple planes.

(e) Between cranial bones:
Fibrous joint (Sutures)
The flat bones of the cranium are joined by immovable fibrous joints called sutures, held together by dense fibrous connective tissue. No movement is permitted.

(f) Between pubic bones in the pelvic girdle:
Cartilaginous joint (Symphysis pubis)
The two pubic bones are joined by fibrocartilage, forming a slightly movable cartilaginous joint called the pubic symphysis.

(a) All mammals (except a few) have 7 (seven) cervical vertebrae.

*(This is a characteristic feature of mammals — almost all mammals, from giraffes to humans to whales, have exactly 7 cervical vertebrae.)*

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(b) The number of phalanges in each limb of human is 14.

*(Each hand/foot has 14 phalanges: 2 in the thumb/big toe (proximal + distal) and 3 in each of the remaining 4 digits (proximal + middle + distal) = 2 + (3 × 4) = 14.)*

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(c) Thin filament of myofibril contains 2 'F' actins and two other proteins namely troponin and tropomyosin.

*(Troponin is a complex of three subunits that, along with tropomyosin, regulates the binding of myosin to actin.)*

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(d) In a muscle fibre, $\text{Ca}^{2+}$ is stored in the sarcoplasmic reticulum.

*(The sarcoplasmic reticulum is the smooth endoplasmic reticulum of muscle fibres; it releases $\text{Ca}^{2+}$ upon stimulation and reabsorbs it during relaxation.)*

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(e) 11th and 12th pairs of ribs are called floating ribs.

*(These ribs are not connected to the sternum at all — they are free at their anterior ends, hence called 'floating ribs'.)*

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(f) The human cranium is made of 8 bones.

*(The 8 cranial bones are: 1 frontal, 2 parietal, 1 occipital, 2 temporal, 1 sphenoid, and 1 ethmoid.)*