Smooth Muscle

Smooth muscle is the major contractile component in hollow organs (such as airways and blood vessels), with its main function being regulation of content flow through these hollow pathways. It is able to carry out this function by alternating between varied degrees of contraction and relaxation to control the dimensions of the organ, which will result in an increase of decrease of resistance to flow of contents. Smooth muscle is composed of many contractile muscle cells, with a single centrally-located nucleus. These cells contain actin and myosin filaments arranged in a structure similar to sarcomeres in Skeletal muscle cells. The contraction of smooth muscles tends to be slower than that of the skeletal muscle.

The muscle itself can be controlled by both electronic signals (action potentials) and chemical signals (hormones) and, as such, may be divided into two sub-types. These are Single unit Smooth muscle - composed of electronically-coupled cells, where triggering contraction in one cell will induce contraction in all linked cells; and Multiunit Smooth muscle - composed of non-linked cells, so contraction can be triggered in one cell without causing stimulation in its neighbouring cells.

Smooth muscle activity patterns can vary, with some muscles able to carry out regular, rhythmic, 'wave-like' contraction and others constantly active (such as some sphincters).