Smooth Muscle Contraction

In a smooth muscle fiber, external stimuli like neural impulses, hormones, or mechanical stretching cause an influx of calcium ions from the interstitial fluid and sarcoplasmic reticulum into the sarcoplasm.

Within the sarcoplasm, these calcium ions bind calmodulin, a regulatory protein that activates myosin light chain kinases or MLCK

These enzymes phosphorylate and alter the myosin head conformation in the myosin filaments.

As a result, the ATPase activity of the myosin heads increases, preparing them for cross-bridge formation with actin filaments.

Upon the formation of the cross-bridges, the myosin ATPases use ATP hydrolysis to slide the actin filaments over the myosin filaments.

The sliding filaments create a corkscrew and cause the midsection of the contracting fiber to bulge.

However, in smooth muscle cells, the calcium removal from sarcoplasm is slow, which keeps the cross-bridges engaged longer, leading to sustained contraction.

Due to this moderate contraction, smooth muscles exhibit long-term tone, which assists organs like arterioles to maintain a steady blood pressure without fatigue.