Write a short note on role of calcium ions in contraction and relaxation of muscles.

Calcium ions play a major role in contraction and relaxation of muscles. 

1. Calcium ions are released from the sarcoplasm during muscle contraction and stored in sarcoplasmic reticulum during muscle relaxation. 

2. When a skeletal muscle is excited and an action potential travels along the T tubule, the concentration of calcium ions increases. 

3. These calcium ions bind to troponin which in turn undergoes a conformational change that causes tropomyosin to move away from the myosin-binding sites on actin. Once these binding sites are free, myosin heads bind to them to form crossbridges and the muscle fiber contracts. 

4. The decrease in calcium ion concentration in the sarcoplasmic reticulum causes tropomyosin to slide back and block the myosin binding sites on actin. This causes the muscle to relax. 

[Note: Students can scan the adjacent Q.R code to get conceptual clarity with the aid of a relevant video.]