Locomotion and Movement — NCERT Solutions

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.

(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.

(c) Human skeleton has 206 bones.

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

(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.

(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:

(b) Differences between Red and White Muscles:

(c) Differences between Pectoral and Pelvic Girdle: