A smooth muscle contraction is a type of muscle contraction that takes place within the walls of many of the body’s internal structures such as the blood vessels, organs of the digestive tract, and reproductive organs. Unlike skeletal muscles, which are found close to the skin and which perform voluntary movements of the body’s joints like walking or lifting a fork to eat, smooth muscles perform automatic, unconscious functions like pushing the blood through the arteries or digested food through the intestines. The smooth muscle contraction differs from the skeletal muscle contraction in that it causes the entire wall of the vessel to undulate, moving as a unit to transport the vessel’s contents.
Forming an internal layer of the wall of an organ or vessel just behind that vessel’s mucous membrane, smooth muscle is flat and continuous. Found lining the walls of such structures as the arteries and veins, the esophagus, stomach, and intestines, and the bladder and uterus, this type of muscle is typically fusiform in shape. Fusiform muscle fibers are each shaped like a spindle, and while skeletal muscle fibers can take this shape as well, skeletal muscle contraction occurs linearly in a direction that is parallel to the fiber’s length while smooth muscle contraction involves the expansion and contraction of muscle tissue in multiple directions. Smooth muscle contraction therefore produces a kind of undulating, squeezing movement known as peristalsis that pushes blood, partially digested food, and reproductive cells in a single direction in slow waves.
This type of muscle contraction is controlled not by the central nervous system in the brain but by the autonomic nervous system, the portion of the peripheral nervous system that coordinates involuntary actions like breathing and heart rate. Based on how the muscle cells are penetrated by the functional units of the autonomic nervous system, the nerves, smooth muscle is classified into two muscle types: single-unit and multiunit. In single-unit smooth muscle, one cell within a movable unit of smooth muscle is innervated by one nerve. The nerve impulse spreads to the adjacent cells in the sheet of muscle like lightning striking the ground, causing a collective contraction by the entire unit. Multiunit smooth muscle contraction takes place in individual cells where very small, fine motor control is required, as in dilating and enlarging the iris of the eye. The vast majority of the body’s smooth muscle is single-unit.
Within the muscle cell, molecules arranged linearly in filaments called actin and myosin provide the mechanism of smooth muscle contraction. By forming rows of interlinked chains, much like chain mail armor, they make contraction in multiple directions possible. Several proteins known as calmodulin, caldesmon, and calponin facilitate this muscle contraction by binding to calcium ions. It is this binding action that changes the molecular structure of the smooth muscle cell and thus brings about the contraction.