Mechanism of Muscle Contraction
தசைச் சுருக்க இயக்கமுறை
Calcium does not pull anything. It binds troponin, which moves tropomyosin off actin's active sites — only then can myosin heads grab actin. That single step is NEET's favourite question in this chapter.
The idea
Muscle contraction is best explained by the sliding filament theory: contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. It is initiated by a signal sent by the central nervous system via a motor neuron. A motor neuron with the muscle fibres connected to it constitutes a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is the neuromuscular junction or motor end plate. A neural signal reaching this junction releases the neurotransmitter acetylcholine, which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in Ca2+ level leads to the binding of calcium with a subunit of troponin on actin filaments, thereby removing the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards the centre of the A band; the Z lines attached to these actins are also pulled inwards, causing shortening of the sarcomere, i.e. contraction. During shortening the I bands get reduced, whereas the A bands retain their length. The myosin, releasing ADP and Pi, goes back to its relaxed state; a new ATP binds and the cross bridge is broken; ATP is again hydrolysed by the myosin head and the cycle of cross-bridge formation and breakage repeats, causing further sliding. The process continues till Ca2+ ions are pumped back to the sarcoplasmic cisternae, resulting in the masking of actin filaments; this causes the Z lines to return to their original position, i.e. relaxation. Repeated activation of muscles can lead to accumulation of lactic acid due to anaerobic breakdown of glycogen, causing fatigue.
Why it matters
Muscles must contract only when the nerve says so; hiding myosin's binding sites until calcium arrives makes the nerve signal the on-off switch.
NEET lens
Sequence: motor neuron → ACh at neuromuscular junction → action potential in sarcolemma → Ca2+ released from sarcoplasmic reticulum → Ca2+ binds troponin → active sites unmasked → myosin head (using ATP energy) forms cross bridge → actin pulled towards centre of A band → Z lines pulled in. During contraction: I band shortens, H zone disappears, A band unchanged, sarcomere shortens. Ca2+ pumped back → relaxation. Lactic acid → fatigue.
Clinical significance
Muscle cramps and fatigue after heavy exercise are linked to lactic acid build-up from anaerobic glycolysis. Low blood calcium makes nerves and muscles over-excitable and causes tetany.
Introduction
According to the sliding filament theory, a muscle shortens because thin actin filaments slide over thick myosin filaments — without either filament changing its own length.
Core concept
Motor unit = motor neuron + its muscle fibres. Neuromuscular junction (motor end plate) → acetylcholine → action potential in sarcolemma → Ca2+ released into sarcoplasm → Ca2+ binds troponin → masking of active sites removed → myosin head (energy from ATP hydrolysis) binds actin = cross bridge → actin pulled towards centre of A band → Z lines pulled inward → sarcomere shortens. Bands: I band reduces, A band retains length, H zone reduces/disappears. Myosin releases ADP + Pi → new ATP binds → cross bridge breaks → ATP hydrolysed → cycle repeats. Relaxation: Ca2+ pumped back to sarcoplasmic cisternae → actin masked → Z lines return. Fatigue: lactic acid from anaerobic breakdown of glycogen.
Go deeper
Sustained contraction due to lack of relaxation between successive stimuli is called tetanus (a physiological state, different from the disease tetanus and from tetany caused by low Ca2+).
From Class 10
Class 10 said muscles contract when nerves stimulate them. This concept shows what happens inside the sarcomere when they do.