Smooth muscle fibers are spindle-shaped, and, like striated muscle, can tense and relax. In the relaxed state, each cell is spindle-shaped, 20-500 micrometers in is ~6:1 in striated muscle and ~15:1 in smooth muscle.Fact: date=September 2008 Smooth muscle does not contain the protein troponin; instead calmodulin (which takes on the regulatory role in smooth muscle), caldesmon and calponin are significant proteins expressed within smooth muscle.

However, there is an organized cytoskeleton consisting of the intermediate filament proteins vimentin and desmin, along with actin filaments. Actin filaments attach to the sarcolemma by focal adhesions or in a spiral corkscrew fashion, and contractile proteins can organize into zones of actin and myosin along the axis of the cell.

The sarcolemma possess microdomains specialized to cell-signaling events and ion channels called caveolae. These invaginations in the sarcoplasma contain a host of receptors (prostacyclin, endothelin, serotonin, muscarinic receptors, adrenergic receptors), second messenger generators (adenylate cyclase, Phospholipase C), G proteins (RhoA, G alpha), kinases (rho kinase-ROCK, Protein kinase C, Protein Kinase A), ion channels (L type Calcium channels, ATP sensitive Potassium channels, Calcium sensitive Potassium channels) in close proximity. The caveolae are often in close proximity to sarcoplasmic reticulum or mitochondria, and have been proposed to organize signaling molecules in the membrane.

Function

To maintain organ dimensions against forces, cells are fastened to one another by adherens junctions. As a consequence, cells are mechanically coupled to one another such that contraction of one cell invokes some degree of contraction in an adjoining cell. Gap junctions couple adjacent cells chemically and electrically, facilitating the spread of chemicals (e.g., calcium) or action potentials between smooth muscle cells. Smooth muscle may contract spontaneously (via ionic channel dynamic or Cajal pacemaker cells) or be induced by a number of physiochemical agents (e.g., hormones, drugs, neurotransmitters - particularly from the autonomic nervous system), and also mechanical stimulation (such as stretch).

Smooth muscles have been divided into "single unit" and "multi-unit" or into "phasic" and "tonic" types based on the characteristics of the contractile patterns and characteristics of the smooth muscle. Multi-unit smooth muscle lines the large airways to the lungs and large blood vessels. The ciliary muscles within the eye and the arrector pili muscle of the skin are also multiunit. This smooth muscle contains few gap junctions and the autonomic nervous system innervates each smooth cell and regulates them like motor units so graded responses can occur. Single unit smooth muscle lines all the hollow organs and is most common. This type smooth muscle tends to contract rhythmically, is coupled by numerous gap junctions, and often exhibits spontaneous action potential. Another nomenclature separates smooth muscle by contractile pattern. It may contract phasically with rapid contraction and relaxation, or tonically with slow and sustained contraction. The reproductive, digestive, respiratory, and urinary tracts, skin, eye, and vasculature all contain this tonic muscle type. For example, contractile function of vascular smooth muscle is critical to regulating the lumenal diameter of the small arteries-arterioles called resistance vessels. The resistance arteries contribute significantly to setting the level of blood pressure. Smooth muscle contracts slowly and may maintain the contraction (tonically) for prolonged periods in blood vessels, bronchioles, and some sphincters. In the digestive tract, smooth muscle contracts in a rhythmic peristaltic fashion, rhythmically forcing foodstuffs through the digestive tract as the result of phasic contraction. There are differences in the myosin heavy and light chains that also correlate with these differences in contractile patterns and kinetics of contraction between tonic and phasic smooth muscle.