Events In Skeletal Muscle Contraction

SUMMARY OF THE EVENTS (STEPS) IN SKELETAL MUSCLE CONTRACTION I. Stimulation 1. Nerve impulse arrives at the neuromuscular junction. 2. Vesicle in the end plate of motor axon release acetylcholine which diffuses across nerve-muscle gap. 3. Acetylcholine depolarizes sarcolemma, and is then inactivated by acetylcholinesterase. 4. T tubules and Z lines spread the action potential throughout the muscle fibre. 5. Electrical depolarization changes the permeability of S.R., causing somehow the release of calcium ions from the S.R. lumen into the cytoplasm of the muscle fibre. 6. Ca++ bind to troponin, making tropomyosin move uncover the active sites on actin. One complete swiveling cycle of myosine head shortens the sarcomere by about 1% of its original length. II. Contraction 7. ATP joints meromyosin head that hydrolyses. ATP into ADP and inorganic phosphate, releasing energy. The energy raises the head to high-energy state. An enzyme myosin ATPase catalyzes the reaction in the presence of Ca2+ and Mg2+ ions. Myosin ATPase ATP ⎯⎯⎯⎯⎯⎯⎯⎯ → ADP + Pi + Energy Ca ++ , Mg ++ 8. The energized head straightens and joins active site on actin, forming a cross-bridge. 9. The head now releases ADP and inorganic phosphate and relaxes to its low-energy state and position. This pulls the actin myofilament towards the centre of the sarcomere, that shortens. 10. A new ATP molecule joins the head, detaching it form the actin myofilament. 11. Above four steps (7 – 10) are repeated many times during one contraction to draw the thin myofilaments further inward. III. Relaxation 1. Now the calcium ions are quickly returned to the sarcoplasmic reticulum by active transport with energy from ATP. 2. Troponin-tropomyosin complex shifts back to block the active sites on actin. 3. The thin myofilaments return to their original resting position. The muscle is now relaxed. 4. In the relaxation period, sarcolemma becomes normal or repolarized as the sodium-potassium exchange pump starts working. 5. In case of repeated contractions, ATP is replenished by the action : ADP + Phosphocreatine = ATP + Creatine. 6. In case of excessive muscular activity, the muscle fibres run into oxygen debt and stop contractions due to accumulation of lactic acid. 7. In a resting muscle, the excess lactic acid is oxidized. This enables the muscle to resume contractions. 8. EFFICIENCY OF A MUSCLE A steam engine can convert only about 10% of the heat energy of fuel into useful work, the rest is wasted. A muscle can use 50 – 70% of the chemical energy of glucose in mechanical work of contraction. The remaining energy changes into heat, which is not entirely wasted because it is used to maintain the body temperature. If we avoid muscle contraction, the heat produced elsewhere in the body is insufficient to keep it warm in a cold place. Under such a condition, muscles start contracting involuntarily (shivering), producing heat to restore normal body temperature.

RED AND WHITE SKELETAL MUSCLE FIBRES Humans have in their skeletal muscles two kinds of striated muscle fibres : tonic or red or slow and twitch or white or fast. A muscle may consist of only tonic fibres, or only twitch fibres, or a mixture of both. 1. Tonic (Slow, Red) Muscle Fibres. These are thin, dark red and slow contracting muscle fibres. They contain a high content of a hemeprotein pigment called myoglobin, abundant mitochondria, low glycogen content and poorly formed sarcoplasmic reticulum. Myoglobin imparts them dark red colour and stores oxygen as oxymyoglobin. Latter’s oxygen by aerobic oxidation in mitochondria provides energy for muscle contraction. Little lactic acid accumulates in this respiration. This enables the red muscle fibres to carry on slow and sustained contractions for long periods without fatigue. The tonic muscle fibres are innervated by thin, slow conducting nerve fibres. The body muscles meant for sustained work at a slow rate for a prolonged duration are composed mostly or entirely of red muscle fibres. The extensor muscles of the back in man remain in sustained contraction to maintain erect posture against gravity. Therefore they are rich in red muscle fibres.

2.

Twitch (Fast, White) Muscle Fibres. These are much thicker, lighter in colour and fast-contracting muscle fibres. They have a low content of myoglobin, few mitochondira, abundant glycogen granules and well formed sarcoplasmic reticulum. They derive energy for their fast contractions mainly by anaerobic oxidation, accumulate lactic acid during strenuous work and soon get fatigued. The twitch muscle fibres are innervated by thick, fast conducting nerve fibres. The body muscles, which are meant for fast and strenuous work for short durations, are composed mostly or entirely of white muscle fibres. The muscles that move eyeballs are very rich in white fibres.