Structural organization of the gap junction network in the cerebral cortex

Neurons in mammalian brains are heavily interconnected by specialized junctions called chemical synapses. At chemical synapses, signals are transmitted rather indirectly between connected neurons with some delay. Recent studies have revealed additional channels for neuronal communication, that is, gap junctions. As electrical signals are directly transmitted through gap junctions without delay, gap junctions can mediate synchronous activities of coupled neurons. In the cerebral cortex, both morphological and physiological studies have established frequent occurrence of gap junctions between dendrites of GABAergic interneurons of particular types. However, it remains unclear how the dendritic network of these interneurons is extended three-dimensionally within the tissue architecture. The present article reviews recent findings on structural aspects of the gap junction network in the cerebral cortex. The three-dimensional structure of the network is discussed on a basis of contrasting tissue organization in the two cortical areas, the hippocampus and neocortex. The controversial issue on the presence and function of distal gap junctions is addressed. Quantitative analysis of the distribution of gap junctions along dendritic profiles suggests lateral continuity of the network that extends in a boundless manner. Functional implications of this lateral continuity are discussed with particular reference to variability in local cell assemblies.