Smooth muscle contraction is a complex process vital for various bodily functions, from maintaining blood vessel tension to facilitating the movement of food through the digestive tract. Unlike striated muscles, smooth muscle contraction begins more slowly and lasts longer.
The onset of contraction is triggered by an increase in calcium ions within the sarcoplasm, similar to the process in striated muscle. However, smooth muscles have a relatively smaller reservoir of the sarcoplasmic reticulum, the primary calcium storage site in striated muscles. Instead, calcium ions infiltrate the smooth muscle sarcoplasm from both the interstitial fluid and the sarcoplasmic reticulum. The absence of transverse tubules, replaced by structures called caveolae in smooth muscle, means that calcium takes longer to reach the central filaments, contributing to the slower onset of contraction typical of smooth muscles.
In the sarcoplasm, the regulatory protein calmodulin binds these calcium ions, activating myosin light chain kinases or MLCK enzymes. These enzymes alter the conformation of myosin heads in the thick filaments through phosphorylation. The ATPase activity of the myosin heads increases, preparing them to form cross-bridges with actin filaments. The myosin ATPases hydrolyze ATP to slide actin filaments over myosin filaments, resulting in a muscle contraction. Unlike skeletal muscles, the calcium removal from smooth muscle sarcoplasm is slow, which keeps the cross-bridges engaged longer, leading to sustained contraction and delaying relaxation.
Smooth muscle also differs from skeletal muscle in that its thick and thin filaments are not arranged into sarcomeres, conferring the ability to stretch considerably while maintaining contractile functionality. When stretched, smooth muscle initially contracts and increases tension but soon undergoes a stress-relaxation response, allowing the muscle to lengthen while reducing tension. This phenomenon, called plasticity, allows the smooth muscle to contract over a range of lengths four times greater than skeletal muscle. Plasticity is crucial in organs like the stomach and bladder, which must accommodate varying volumes without significantly altering internal pressure.
