The smooth muscle is a type of muscular tissue which consists of elongated contractile cells lacking transverse striation. It forms the middle-layer of arteries and veins, the wall of gastrointestinal tract (oesophagus, stomach, intestines), the respiratory passages (bronchi and bronchioles), and the urogenital tubes. The contraction of the smooth muscle narrows the lumen (diameter) of arteries and respiratory passages. The movement of this muscular tissue is involuntary and is controlled by the autonomous nervous system, with its network of nerve ganglia.
The smooth muscle is composed of elongated, tapering cells which are between 10 and 18 μm (micrometer) in diameter and a range in length of 20 μm. Each cell (fiber) has a central rod-shaped nucleus, which is so long that it becomes pleated when the cell shortens, becoming snake-like in appearance. Mitochondria and stored glycogen lie mostly near the poles of the nucleus in the cytosol. The smooth muscle fiber are shorter in small arteries than those in the uterine wall during pregnancy.
The smooth muscle cells are individually capable of energy-efficient partial contraction as they are able to maintain tonus (muscular tension tone) more or less indefinitely. Thus, smooth muscle has the important function of regulating the luminal diameter of hollow organs and most tubes in the body. In the digestive tract, uterine tubes, and ureter, it undergoes slow rhythmic contractions, resulting in successive way of contractions, which are called peristalsis or peristaltic waves.
Electron microscopy, reveals thick filaments (containing myosin) and thin filaments (having actin) in smooth muscle cells cut perpendicular to its long axis. These filaments are less formally arranged in smooth muscle cells than they are in skeletal muscle cells (myocytes/fibers).
The smooth muscle cell contraction mechanism has been a topic of considerable research interest. Most researchers believe that the thin filaments are relatively stable and are inserted into the plasma membrane by α-actinin molecules. On the other hand, thick, myosin-rich filaments are relatively labile, except immediately preceding contraction. Aside from the sympathetic and parasympathetic nerve innervation, the smooth muscle fibers also have autonomous motility, which is modulated by the muscle innervation.
Below, micrograph of a smooth muscle cell. A) Living muscle fiber relaxed; B) same muscle fiber fully-contracted; C) scanning electro-micrograph of a fully-contracted smooth muscle fiber.
The micrograph shows the snake-like pleated nuclei, which are the characteristic feature of contracted of smooth muscle fibers.
