Single-unit smooth muscle, or visceral smooth muscle is a type of smooth muscle found in the uterus, gastro-intestinal tract, and the bladder. In SUVSM, a single smooth muscle cell in a bundle is innervated by an autonomic nerve fiber. An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. Due to this property, SUVSM bundles form a syncytium that contracts in a coordinated fashion (such as uterine muscles do during childbirth).
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). A few of the cells in a given SUVSM unit may behave as pacemaker cells, generating rhythmic action potentials due to their intrinsic electrical activity. Because of its myogenic nature, single-unit smooth muscle is usually active, even when it's not receiving any neural stimulation.