Not all smooth muscle cells are exposed directly to the neurotransmitter, however. As discussed above, smooth muscle cells make contact with neighboring cells by gap junctions. As in cardiac muscle, contraction is propagated from cell to cell via gap junctions, thus producing coordinated activity within a smooth muscle bundle or layer. The gap junction between two smooth muscle cells was originally designated a nexus, a term still in use.
Smooth muscle cells also secrete connective tissue matrix.
Smooth muscle cells have organelles typical of secretory cells. A well-developed rER and Golgi apparatus are found in the perinuclear zone. Smooth muscle cells synthesize both type IV (basal lamina) collagen and type III (reticular) collagen as well as elastin, proteoglycans, and multiadhesive glycoproteins. Except at the gap junctions, smooth muscle cells are surrounded by an external lamina. In some locations, such as the walls of blood vessels and the uterus, smooth muscle cells secrete large amounts of both type I collagen and elastin.
Renewal, Repair, and Differentiation
Smooth muscle cells are capable of dividing to maintain or increase their number.