39. Write an account of the chemical events taking place during muscle contraction.

In a resting muscle fibre, sarcolemma is electropositive outside and electronegative inside. This potential difference across a membrane is called resting potential. A membrane with such a resting potential is said to be polarised.

Na+ ions predominate the outside of the sarcolemma and K ions predominate the inside. Due to the difference in concentration on the two sides of the sarcolemma, potassium ions leave and sodium ions enter the muscle fibre. Sarcolemma is more permeable to K+ions than to sodium ions. Hence potassium ions leave the muscle fibre faster than sodium ions enter, and this builds a positive charge outside.

When the motor nerve impulse reaches the neuro-muscular junction, then the vesicles present in the motor end plate secrete a neurotransmitter chemical called acetylcholine. The neurotransmitter binds to the receptors on the sarcolemma makes it more permeable to Na+ than K+ so that, sodium rapidly diffuse along the concentration gradient and the electrical gradient. Now sarcolemma becomes electropositive inside and electronegative outside. This new potential difference is called action potential. Such sarcolemma is called depolarised.

Action potential stimulates the sarcoplasmic reticulum to release calcium ions which initiate the biochemical changes in muscle contraction. Calcium and megnesium ions act as co-factors for myosine ATPase enzyme which hydrolyses ATP into ADP and inorganic phosphate releasing energy.