The two types of intercellular signalling - wiring transmission and volume transmission - are involved in information transfer in neuronal-glial networks. Neurones and glia are integrated in many ways, which involve homocellular and heterocellular contacts (Figure 7.15). Homocellular contacts are represented by neuronal-neuronal chemical synapses and electrical synapses formed by gap junctions between neurones and between glial cells (wiring transmission). Hetero-cellular contacts between neurones and glia are formed by either direct synapses formed by neuronal terminals on glial cells (wiring transmission; and, at least in the case of NG2-glia, direct synapses formed by NG2-glial processes on neurones), or by close apposition of neuronal terminals and perisynaptic astroglial processes. In the latter case, neurotransmitter released by neurones activates astrocyte membranes by spillover from the synaptic cleft (volume transmission). Astroglial cells signal back to neurones through the release of neurotransmit-ters (either vesicular or transplasmalemmal), which may activate several neurones simultaneously by the way of volume transmission. At the same time, neuro-transmitters released by astrocytes may signal homocellularly and, together with astroglial-astroglial gap junctional contacts, trigger propagating calcium waves within the glial syncytium. Astrocyte-neuronal signalling may operate either locally, by feeding back to the active synaptic domain, or distantly; in the latter case the astroglial Ca2+ wave stimulated by synaptic activation conveys the signal through the astroglial network and initiates transmitter release distantly, to affect neurones not connected synaptically to the primary loci of stimulation.
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