Glial Network

Although many astrocytic processes ensheath most of the synapses within the CNS, other astrocytic processes are connected to each other by gap junctions. The main astrocytic gap junction protein is connexin 43, but others may be present. In situ, morphological studies have shown that astrocyte gap junctions are localized between cell bodies, between processes and cell bodies, and between astrocytic end feet that surround blood vessels (Fig. 1). Gap junctions are a group of diverse channels that vary in their permeability, voltage sensitivities, and potential for modulation by intracellular factors. Thus, junctional coupling may provide a pathway for the selective exchange of small molecules (less than 1-1.4 kDa) such as ions, cyclic nucleotides, and small peptides. Gap junctions could potentially give rise to hierarchical signaling systems, in which cells of one class can transfer second messengers to cells of a second class but not vice versa. Although less frequently observed, gap junctions also occur between astrocytes and oligodendrocytes as well as between oligodendrocytes. Thus, the astrocytic syncytium extends to oligodendrocytes, allowing glial cells to form a "generalized glial syncytium.'' This large glial network extends radially across gray and white matter regions, from the spinal canal and brain ventricles to the glia limitans and the capillary epithelium.

Homologous coupling could synchronize the electrical activity of neighboring cells that serve the same functions, thereby generating a functional network. In this regard, intercellular coupling is believed to facilitate one of the major functions of astrocy-tes—maintaining the ionic balance of the extracellular fluid—by providing a vast cytoplasmic reservoir for the spatial buffering of ions taken up from the extracellular space. In particular, K+ released from active neurons would enter neighboring glial cells and be redistributed by current flow through the glial syncytium. Heterocoupling between astrocytes and oligodendrocytes has been proposed to serve K + buffering around myelinated axons.

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