Single-unit visceral smooth muscle is myogenic it can contract regularly without input from a motor neuron (as opposed to multiunit smooth muscle, which is neurogenic - that is, its contraction must be initiated by an autonomic nervous system neuron). (such as the uterine muscles during childbirth). Due to this property, single-unit bundles form a syncytium that contracts in a coordinated fashion making the whole muscle contract or relax. An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. In single-unit smooth muscle a single cell in a bundle is innervated by an autonomic nerve fiber (myogenic). It is not found in the heart which has cardiac muscle. Most smooth muscle is of the single-unit type, and is found in the walls of most internal organs (viscera) and lines blood vessels (except large elastic arteries), the urinary tract, and the digestive tract.
Smooth muscle is grouped into two types: single-unit smooth muscle, also known as visceral smooth muscle, and multiunit smooth muscle. Structure Gross anatomy Ī series of axon-like swellings, called varicosities from autonomic neurons, loosely form motor units through the smooth muscle. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature or fear. In the eyes, the ciliary muscles, a type of smooth muscle, dilate and contract the iris and alter the shape of the lens. There is smooth muscle in the tracts of the respiratory, urinary, and reproductive systems. In the walls of blood vessels, and lymph vessels, (excluding blood and lymph capillaries) it is known as vascular smooth muscle. Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder and uterus. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.
It is divided into two subgroups, single-unit and multiunit smooth muscle.
Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations ( bands or stripes). Smooth muscle tissue, highlighting the inner circular layer (nuclei then rest of cells in pink), outer longitudinal layer (nuclei then rest of cells), then the serous membrane facing the lumen of the peritoneal cavity
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