CBSE Class 11 Biology Chapter 20: Locomotion and Movement NCERT Solutions

A sarcomere is the fundamental contractile unit of a muscle fiber. It is the region between two successive Z-lines. The diagram below illustrates the structure of a sarcomere, highlighting its key components and regions:

The main components include:

• Actin Filaments (Thin Filaments): Primarily composed of actin, along with regulatory proteins tropomyosin and troponin. They are anchored to the Z-lines.
• Myosin Filaments (Thick Filaments): Composed mainly of myosin.
• Z-line: A dense protein structure that bisects the I-band and serves as an anchor for actin filaments.
• I-band (Isotropic band): Contains only actin filaments. It appears lighter under a microscope and shortens during contraction.
• A-band (Anisotropic band): Contains the entire length of the myosin filament, overlapping with actin filaments at its ends. It appears darker and its length remains constant during contraction.
• H-zone: The central region of the A-band where only myosin filaments are present, without overlap from actin filaments. It shortens during contraction.

A visual representation of a sarcomere is crucial for understanding muscle contraction. [Placeholder for an actual diagram image if available in source, otherwise describe structure]

The sliding filament theory is the widely accepted explanation for how muscle contraction occurs. It proposes that during contraction, the thin filaments (actin) slide past the thick filaments (myosin) within each sarcomere. This sliding action causes the sarcomere to shorten, leading to the overall contraction of the muscle fiber. Key points of the theory include:

• Muscle fibers contain alternating light (I-bands) and dark (A-bands) regions due to the arrangement of actin and myosin filaments.
• Actin filaments are found in the I-band, and myosin filaments are found in the A-band.
• The Z-lines, which anchor the actin filaments, are located at the center of the I-band.
• The H-zone is the central part of the A-band where there is no overlap between actin and myosin filaments.
• During contraction, the myosin heads interact with actin filaments, pulling them towards the center of the sarcomere.
• This sliding causes the I-bands and H-zones to shorten, while the A-band's length remains unchanged. The sarcomere as a whole shortens, resulting in muscle contraction.

Muscle contraction is a complex process initiated by a nerve impulse and executed through a series of coordinated steps involving the interaction of actin and myosin filaments. The key steps are:

1. Initiation of Signal: A signal, in the form of an action potential, travels down a motor neuron and reaches the neuromuscular junction (the synapse between the neuron and the muscle fiber's sarcolemma).
2. Neurotransmitter Release: At the neuromuscular junction, the neuron releases acetylcholine (a neurotransmitter) into the synaptic cleft. This neurotransmitter binds to receptors on the sarcolemma, generating an action potential in the muscle fiber.
3. Calcium Ion Release: The action potential generated on the sarcolemma propagates into the muscle fiber and triggers the release of calcium ions (Ca^{2+}) from the sarcoplasmic reticulum (an internal storage organelle for calcium).
4. Activation of Actin Sites: The increased concentration of Ca^{2+} in the sarcoplasm (muscle cell cytoplasm) plays a crucial role. Calcium ions bind to troponin, a protein complex located on the actin filaments. This binding causes a conformational change in troponin, which in turn moves tropomyosin, another protein that normally covers the active sites on actin.
5. Cross-bridge Formation: With the active sites on actin now exposed, the myosin heads (which are part of the thick filaments and possess ATPase activity) can bind to these sites, forming cross-bridges between the actin and myosin filaments.
6. Power Stroke: Upon binding, the myosin head undergoes a conformational change, often referred to as the 'power stroke'. This involves the pulling of the actin filament towards the center of the sarcomere (M-line). This action requires energy derived from the hydrolysis of ATP.
7. Cross-bridge Detachment: After the power stroke, a new ATP molecule binds to the myosin head. This binding causes the myosin head to detach from the actin filament.
8. ATP Hydrolysis and Re-cocking: The bound ATP is then hydrolyzed to ADP and inorganic phosphate (P_i), releasing energy. This energy is used to 're-cock' the myosin head into its high-energy, ready-to-bind position.
9. Repetitive Cycle: This cycle of binding, power stroke, detachment, and re-cocking repeats as long as calcium ions are present and ATP is available. Each cycle results in further sliding of the actin filaments over the myosin filaments, leading to the shortening of the sarcomere and thus, muscle contraction.
10. Relaxation: Muscle relaxation occurs when the nerve impulse stops. Calcium ions are actively pumped back into the sarcoplasmic reticulum, reducing their concentration in the sarcoplasm. This causes tropomyosin to cover the actin binding sites again, preventing cross-bridge formation and allowing the muscle to return to its resting length.