Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder and uterus.

In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature and fear.

In single-unit smooth muscle a single cell in a bundle is innervated by an autonomic nerve fiber (myogenic).

Multiunit smooth muscle is found in the trachea, in the iris of the eye, and lining the large elastic arteries.

This is due to the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements.

[8] In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle.

There is also a lot of actin (mainly beta-actin) that does not take part in contraction, but that polymerizes just below the plasma membrane in the presence of a contractile stimulant and may thereby assist in mechanical tension.

Also, all three of these proteins may have a role in inhibiting the ATPase activity of the myosin complex that otherwise provides energy to fuel muscle contraction.

The adherens junctions consist of large number of proteins including alpha-actinin (α-actinin), vinculin and cytoskeletal actin.

[8] The adherens junctions are scattered around dense bands that are circumfering the smooth muscle cell in a rib-like pattern.

When complexes of actin and myosin contract, force is transduced to the sarcolemma through intermediate filaments attaching to such dense bands.

Smooth muscle cells may secrete a complex extracellular matrix containing collagen (predominantly types I and III), elastin, glycoproteins, and proteoglycans.

The sarcolemma also contains caveolae, which are microdomains of lipid rafts specialized to cell signaling events and ion channels.

Also, contraction, as well as relaxation, can be induced by a number of physiochemical agents (e.g., hormones, drugs, neurotransmitters – particularly from the autonomic nervous system).

Smooth muscle in various regions of the vascular tree, the airway and lungs, kidneys and vagina is different in their expression of ionic channels, hormone receptors, cell-signaling pathways, and other proteins that determine function.

For instance, blood vessels in skin, gastrointestinal system, kidney and brain respond to norepinephrine and epinephrine (from sympathetic stimulation or the adrenal medulla) by producing vasoconstriction (this response is mediated through alpha-1 adrenergic receptors).

Pulmonary blood vessels within the lung are unique as they vasodilate to high oxygen tension and vasoconstrict when it falls.

These responses to carbon dioxide and oxygen by pulmonary blood vessels and bronchiole airway smooth muscle aid in matching perfusion and ventilation within the lungs.

Further different smooth muscle tissues display extremes of abundant to little sarcoplasmic reticulum so excitation-contraction coupling varies with its dependence on intracellular or extracellular calcium.

[citation needed] Recent research indicates that sphingosine-1-phosphate (S1P) signaling is an important regulator of vascular smooth muscle contraction.

The Rhoa and Rac portion of the signaling pathway provides a calcium-independent way to regulate resistance artery tone.

Single unit smooth muscle displays numerous gap junctions and these tissues often organize into sheets or bundles which contract in bulk.

The reproductive, digestive, respiratory, and urinary tracts, skin, eye, and vasculature all contain this tonic muscle type.

[1] Activation consists of phosphorylation of a serine on position 19 (Ser19) on the MLC20 light chain, which causes a conformational change that increases the angle in the neck domain of the myosin heavy chain,[8] which corresponds to the part of the cross-bridge cycle where the myosin head is unattached to the actin filament and relocates to another site on it.

[8] Isolated preparations of vascular and visceral smooth muscle contract with depolarizing high potassium balanced saline generating a certain amount of contractile force.

For example, cyclic nucleotides can relax arterial smooth muscle without reductions in crossbridge phosphorylation, a process termed force suppression.

In invertebrate smooth muscle, contraction is initiated with the binding of calcium directly to myosin and then rapidly cycling cross-bridges, generating force.

Clams and other bivalve mollusks use this catch phase of smooth muscle to keep their shell closed for prolonged periods with little energy usage.

Activating arteriole smooth muscle can decrease the lumenal diameter 1/3 of resting so it drastically alters blood flow and resistance.

Activation of aortic smooth muscle doesn't significantly alter the lumenal diameter but serves to increase the viscoelasticity of the vascular wall.