Smooth muscle cells are a heterogeneous collection of cell types that function in many different organs, where they regulate a variety of functions such as vascular smooth muscle control of blood vessel tone, emptying of the bladder, tonic contractions of sphincters such as the urethra and the rhythmical contraction of the gastrointestinal tract, uterus and ureter. In order to carry out these multiple functions, a number of different signaling mechanisms have evolved to control smooth muscle contractility.
There is a certain degree of uniformity with regard to smooth muscle cell structure and there also are some general mechanisms that are responsible for smooth muscle cell excitation–contraction coupling . In addition to this excitatory mechanism, there also are signalling pathways responsible for smooth muscle cell relaxation . However, when it comes to the smooth muscle activation mechanisms , each muscle has to be considered separately especially with regard to the way in which excitatory stimuli bring about the increase in Ca2+ necessary to trigger contraction.
Most smooth muscle cells are excited by membrane depolarization that then activates VOCs to provide the Ca2+ signal to induce contraction. For many of these muscles, this depolarization is induced by the interstitial cells of Cajal that have a pacemaker function resembling that of the sino-atrial node in the heart. Other smooth muscles have an endogenous pacemaker mechanism while others are activated directly by neurotransmitters released by nerves.
Smooth muscle cell structure
Smooth muscle cells have a spindle shape, and are usually lined up alongside each other and are connected through gap junctions (Module 7: Figure smooth muscle cell structure ). They are about 100 μm long and 5–10 μm wide. The cell surface has numerous caveolae and many of these make close contact with the sarcoplasmic reticulum (Module 6: Figure smooth muscle caveolae).
By comparison with skeletal and cardiac muscle, the contractile system composed of actin and myosin is somewhat disorganized, with actin filaments radiating away from localized densities dotted around the plasma membrane. Smooth muscle cell contraction also differs from that found in skeletal and cardiac muscle in that there are two different mechanisms for regulating the contractile processes (Module 7: Figure smooth muscle cell E-C coupling.