A Study of the Structure and Distribution of the Nexus

Nexuses, that is, fusions of plasma membranes of adjacent cells, are described in mammalian smooth and cardiac muscle, median giant axon of earthworm, frog skin, and rat submandibular gland. In smooth muscle they usually occur where a process from one cell either meets a process of, or projects into a neighboring cell. On the other hand, in mammalian heart muscle and in earthworm giant axon the nexuses occur along the intercalated disc and intercellular segmental septa, respectively. Their occurrence between these excitable cells is correlated with propagation of action potentials by an electrical rather than chemical mechanism. Since the nexuses may offer pathways for electric current between cell interiors, it seems possible that they constitute a link in the structural basis for electrical transmission in these systems. In epithelia, nexuses usually appear as part of a terminal bar complex. This is true in the rat salivary gland studied here. In the epidermis of frog skin, nexuses are less numerous between the basilar columnar cells than between the subjacent squamous cells. The nexuses which occur in epithelia in frog skin and rat salivary gland are distributed as though to provide seals against electrochemical backleaks and sites of chemical exchange between cell interiors.