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  Mechanism of muscle contraction

Mechanism of muscle contraction:

Sliding-filament hypothesis:

Hanson and Huxley stated this theory in 1955. According to this theory, the contractile unit of muscle is made up of two kinds of filaments that are, myosin and actin. Such filaments during contraction slide over one other. It has been noted that both at the time of contraction and stretching, the length of the ‘A band’ remains steady. The length of the ‘I band’, on the other hand, changes in accord with the length of the muscle.

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                                                          Figure: Mechanism of muscle contraction

Due to this, the Z discs are pulled altogether. Therefore successive sarcomeres are pulled.  This leads to the contraction of muscle.

ATP as the source of Energy for contraction:

For muscle in action, energy is needed. This energy is discharged by cleaving ATP molecules to ADP+ Pi. Greater quantity of work is performed by the muscle, with bigger amount of ATP which is cleaved. This energy binds with active site of the actin filament to contract.

Neuro muscular Transmission:

The muscles are innervated by myelinated nerve fibres which originate from the spinal cord. The nerve ending makes a junction, termed as neuromuscular junction. The nerve is associated at the midpoint of muscle fiber and hence action potential in the fiber travels in both directions. Whenever a nerve impulse reaches the neuro-muscular junction, a substance known as acetylcholine is discharged. The acetylcholine persuades muscle fiber for contraction.

Initiation of muscle contraction:

Initiation of the contraction of skeletal muscle starts with action potentials in the muscle fibres. Such elicit electrical currents which spread to the interior of the fiber, where Ca ions are discharged from sarcoplasmic reticulum. The Ca ions in turn start the chemical events of the contractile process. This overall procedure of controlling muscle contraction is termed as excitation.

Maximum strength of contraction:

The highest strength of contraction of a muscle operating at a usual muscle length is around 3.5 Kg/Sq.cm. of muscle.

Changes in muscle strength:

Whenever a muscle starts to contract after a long period of rest, its primary strength of contraction might be as little as one half of its maximum strength. It gains strength after 30-50 muscle twitches. The weight lifters apply this to attain maximum capacity.

 

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