Smooth muscle

Smooth muscle is controlled involuntarily by the autonomic nervous system and is comprised of cells which are uninucliated, spindle shaped and lack striations. Smooth muscle tissue can be subdivided into two categories depending on whether the cells contract collectively and simultaneously, referred to as single-unit smooth muscle, or work independently, multi-unit smooth muscle.

Single-unit smooth muscle

Single-unit smooth muscle, commonly called visceral muscle, is found on the walls of hollow organs in the digestive tract, urinary tract, and reproductive system. It is formed by groups of smooth muscle cells interconnected by gap junctions which allow the transmission of action potentials between cells meaning motor neurons can stimulate more than one cell simultaneously. This allows for coordinated contraction of many cells at once. Single-unit smooth muscle cells also contain desmosomes which prevent muscle tissues from separating upon contraction. Single-unit smooth muscle is myogenic, which means that there is no need for the input of a motor neuron to contract.

Multi-unit smooth muscle

Multi-unit smooth muscle is found mostly in large blood vessels, large airways to the lung, the muscles of the eye, and the erector pili muscles. Unlike single-unit smooth muscle cells, each cell is an independent unit, innervated by at least one moto-neuron. This allows for more refined control of movements due to the independent contraction of each cell. In addition, this type of smooth muscle is neurogenic, which means that an autonomic nervous system neuron must initiate its contraction.

Figure 1. Single-unit and multi-unit smooth muscle.