91影视

The stimulation of a myofibril by a nerve impulse results in an increase in the intracellular Ca²⁺ concentration. This higher concentration of calcium triggers the conformational change in the troponin-tropomyosin complex.

The incident exposes the site on actin where the myosin head binds, thereby increasing myosin鈥檚 affinity for actin by ~10,000 fold.

At a lower calcium concentration, the troponin-tropomyosin complex assumes its resting conformation, hence blocking the myosin binding to actin, causing the muscle to relax.

ATP binds to a myosin head in a manner that causes myosin鈥檚 actin-binding site to open up, releasing the bound actin. The hydrolysis of the bound ATP to ADP and Pi puts the myosin head into its 鈥渉igh energy鈥� conformation.

The myosin head then binds to an actin monomer weakly. The release of Pi from the myosin strengthens the binding of myosin and actin.

Hence, the thin filaments contract only in the presence of Ca²⁺ during muscle contraction. The hydrolysis of ATP provides the force required for the contraction.