Cellular structures and electrophysiological behaviour. Fine structure of smooth muscle.

Abstract

The size of smooth muscle cells is different in different organs but varies little within the same tissue. Great changes in cross-sectional area are observed in distended and contracted smooth muscle cells and during contraction and relaxation in neighbouring layers. Two basic types of intercellular junction are described: ( a ) attachment plaque, where myofilaments end and where the intercellular gap is wide; ( b ) nexus, where the adjoining cells come close together; the gap or the degree of fusion of membranes is affected by preparation procedures. The occurrence of nexuses is generally correlated with electrical coupling, although sometimes, e.g. in intestinal longitudinal muscle, there is electrical coupling but nexuses are virtually absent. Caveolae are spread over the surface of smooth muscle cells. They appear early in embryonic development. It is suggested that they represent a compartment situated between extracellular space proper and intracellular space; its ionic composition may be more readily controlled by the muscle cell itself. Caveolae disappear from muscle cells after treatment with hyaluronidase in vitro . Sacs, cisternae and tubules of sarcoplasmic reticulum are observed in the superficial region of cells in visceral muscles, closely related to caveolae and plasma membrane. Mitochondria are frequently associated with the reticular system. The suggestion that sarcoplasmic reticulum may play a similar role in smooth muscle as in skeletal muscle, in storage and release of calcium, is discussed. Accumulation of divalent cations by sarcoplasmic reticulum and the effect of incubation in Ca-rich solutions support this interpretation, although the crucial evidence for it is still lacking.