Topic: Physiology of muscle contraction
Question: Discuss in detail about the organization of skeletal muscle. Give a brief note on general mechanism of muscle contraction.
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Skeletal muscle is organized into multiple levels:
1. Muscle fibers: Long, cylindrical cells that contain actin and myosin myofilaments for contraction. Each fiber is a single muscle cell.
2. Fascicles: Groups of 10-100 muscle fibers ensheathed in connective tissue called perimysium. Fascicles bind together to form the body of the muscle.
3. Muscle: Composed of multiple fascicles, ensheathed in epimysium connective tissue. Skeletal muscles typically attach to bones via tendons at either end.
4. Muscle groups: Muscles that act synergistically to perform specific movements. Acting together, muscle groups are responsible for coordinated motion and posture.
Mechanism of muscle contraction:
1. Neural stimulation: An impulse travels down a motor neuron to reach the neuromuscular junction, triggering release of acetylcholine. ACh binds to receptors on the muscle fiber, depolarizing the membrane.
2. Excitation-contraction coupling: Depolarization activates dihydropyridine receptors in the T-tubule, triggering calcium release from the sarcoplasmic reticulum (SR) via ryanodine receptors. Intracellular calcium levels rise.
3. Calcium binding to troponin: The rise in calcium ions causes calcium to bind to troponin proteins on actin filaments, changing their shape.
4. Binding sites exposed: The conformational change in troponin exposes myosin binding sites on actin filaments.
5. Cross-bridge formation: Myosin heads bind to the exposed actin binding sites, forming cross-bridges between the thick and thin filaments.
6. Power stroke: When myosin releases ADP and phosphate, the myosin heads bend, pulling the actin filament toward the center of the sarcomere. This results in filament sliding over one another, causing muscle shortening (contraction).
7. Relaxation: Calcium is pumped back into the SR by Ca2+ ATPase, reducing cytosolic levels. Calcium dissociates from troponin, covering myosin binding sites. Cross-bridges detach, actin/myosin filaments relax to their resting position.
Acetylcholine is broken down by acetylcholinesterase, ending stimulation. The highly organized structure of skeletal muscle, from fibers to muscle groups, allows for powerful, controlled and coordinated contractions essential for locomotion, posture, and movement. Excitation-contraction coupling translates neural signals into the mechanical force of muscle fibers contracting and shortening, using calcium ions to rapidly initiate and terminate the process.
Muscle organization and the molecular mechanism of contraction highlight the beautifully intricate yet efficient nature of neuromuscular physiology. Disruption at any level, whether structural, chemical or neural, can have significant consequences on strength, mobility, independence and quality of life. An understanding of these processes is fundamental to diagnosis and management of neuromuscular disorders.
