| Popis: |
The gap junction is one form of membrane specialization that appears in the electron microscope as a closely apposed region of plasma membrane separating adjacent cells in contact by a 2- to 5-nm-wide “gap,” from which the name gap junction is derived (Revel and Karnovsky, 1967). In freeze-fracture replicas viewed by electron microscopy, gap junctions are composed of tightly packed clusters of particles (Kreutziger, 1968). Each particle in a gap junction presumably represents the site of a single intramembrane channel that connects with the adjacent cell allowing a direct exchange or “communication” of ions and other small molecules. The functions of these “communicating channels” and the spectrum of activities they regulate are now beginning to be realized. In excitable tissues, gap junctions are synonymous with electrical synapses and are sometimes referred to as a nexus (Dewey and Barr, 1962). Gap junctions allow rapid signaling and the coordination of smooth and cardiac muscle contraction. It is apparent, however, that the role of gap junctions in the nervous system can be more complex and that they may also modify or regulate the behavior of discrete populations of neurons (Getting, 1974; Getting and Willows, 1974; Rayport and Kandel, 1980; Dudek et al., 1983; Marder, 1984; Bennett et al., 1985). The functions of gap junctions in nonexcitable tissues include, but are not limited to, tissue homeostasis (metabolic cooperation, i.e., the sharing of metabolites), coordination of a response to hormones or neurotransmitters, and, in lower invertebrates, adhesion. |
