Describe the structure of skeletal muscle and explain the mechanism of muscle contraction.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in two lines. Skeletal muscle is striated, voluntary muscle attached to bone. It contracts when actin and myosin filaments slide past each other.
2 · Draw the levels of organisation. [Diagram: muscle → fascicle → fibre → myofibril → sarcomere, with epimysium, perimysium, endomysium labelled]
3 · Structure of a muscle fibre.
Sarcolemma and sarcoplasm.
Sarcoplasmic reticulum stores calcium; T-tubules carry the impulse.
Mitochondria, glycogen and myoglobin.
Myofibrils of actin and myosin.
4 · Draw a sarcomere. [Diagram: Z line, I band, A band, H zone, M line, thick and thin filaments]
5 · Sliding filament steps.
Impulse and acetylcholine at the neuromuscular junction.
Impulse down the T-tubules; Ca2+ released.
Ca2+ binds troponin; tropomyosin moves.
Cross-bridges form; power stroke.
ATP detaches the head; cycle repeats.
Relaxation: Ca2+ pumped back.
6 · Types of contraction. Concentric, eccentric, isometric and isokinetic, one example each.
7 · Nutrition link. ATP from carbohydrate and fat, calcium and magnesium for contraction, protein for repair.
8 · Close in one line. Contraction is a calcium-triggered, ATP-driven sliding of actin over myosin.
Scoring tip: Two neat labelled diagrams (levels and sarcomere) carry many marks; draw them before writing the steps.
