Calcium and mechanism of muscle contraction

Calcium has a central role in muscle contractions and it is obtained from extracellular and intracellular sources.

An elevation of intracellular calcium concentrations to about 1 uM is an absolute requirement for smooth muscle contraction, and a reduction in smooth muscle calcium concentration is considered mechanism of smooth muscle relaxation.

Calcium sits at a critical location on the muscle fiber, facilitating the interaction of the muscle proteins myosin and actin.

Stimulation of muscle fibers by nerve impulses, hormones, or stretch in the fiber increases the amount of calcium in the muscle cells and causes the muscle to contract. Calcium binds to calmodulin, causing structural changes that allow calmodulin to bind and activate myosin light chain kinase.

In the presence of calcium and calmodulin, this kinase phosphorylates a light chain of myosin. This result in structural changes in the thick filament, which then allow its interaction with actin, actomyosin ATPase activity and smooth muscle contraction.

As the cells pump calcium ions back outside, the muscle relaxes.
Calcium and mechanism of muscle contraction

Calcium and muscle contraction

Calcium has a central role in muscle contraction as the flow of calcium ions inside muscle cells muscles to contract or relax.

Muscle cells contract in response to electrical stimulation delivered by an effector nerve cell.

When stimulated, the muscle cell depolarizes and generates a n action potential. The action potential causes release of calcium from sarcoplasmic reticulum, a system of membranes that transport materials in muscle cells. Calcium ion bind to troponin, a component of the actins myofilaments.

This binding causes a change in the structure of the actin molecule that allows actin and myosin to react with each other, forming a troponin-tropomyosin complex, resulting in muscle contraction. Calcium release makes the gap between thick and thin muscle fibers lubricious so that they can contract over each other. Contraction can only form when calcium is released into muscle fibers.

One the stimulus for muscle contraction ceases, free calcium levels decreases and calcium dissociates from the regularity proteins. The muscle then relaxes.

During exercise, one cause of muscle fatigue is the impaired of calcium in muscle cells.
Calcium and muscle contraction