The muscles you never consciously move keep you alive without asking permission
Your gut squeezes food along, your pupils shrink in bright light and your arteries tighten or relax, all without a single conscious decision. The tissue doing this is smooth muscle, controlled by the autonomic nervous system. Some of it even fires on its own, with pacemaker cells setting a rhythm and no nerve signal required.
Smooth muscle is one of the three main kinds of vertebrate muscle, alongside skeletal and cardiac. It gets its name from what it lacks: the repeating contractile units called sarcomeres that give the other two their striped look under a microscope. It lines the walls of hollow organs such as the stomach, intestines, bladder and uterus, wraps blood and lymph vessels, and runs through the airways. In the eye, smooth muscles widen or narrow the pupil and reshape the lens to focus.
Most of it is the single-unit type. Its cells are linked by numerous gap junctions, so an electrical signal spreads from cell to cell and a whole sheet contracts together, as the uterus does during childbirth. Because this kind can generate its own rhythmic signals, it is usually active even without nerve input. The rarer multi-unit type, found in the windpipe, the iris and the largest arteries, waits for a nerve to trigger it, though biologists note the neat split between the two is an oversimplification.
Each cell is a single-nucleus spindle, fat in the middle and tapered at the ends, just 30 to 200 micrometres long when relaxed, thousands of times shorter than a skeletal muscle cell. Its contractile proteins are actin and myosin, as in other muscle, but the balance is reversed: actin outnumbers myosin by anywhere from 2 to 10 times, whereas skeletal muscle is dominated by myosin. It also lacks troponin, the switch protein striated muscle uses, relying instead on calmodulin and other proteins to regulate contraction.
Instead of sarcomeres, actin threads anchor to dense bodies scattered through the cell, which pass force out to the membrane. Isolated cells have been seen contracting in a corkscrew twist. Crucially, smooth muscle can be stretched far and still contract, which is exactly what an expanding bladder or intestine needs.
Source: Smooth muscle
