Parcellation of Input

In this astrocytic labyrinth, inputs are often individually enveloped and isolated from one another. In electron micrographs, wherever a neuron or a neuronal process is not engaged in a synapse, it is frequently enveloped by the somata or processes of glial cells. Yet exceptions are easily found. Small unmyelinated CNS axons are virtually bare, brushing by other neuronal and neuroglial elements. The extent of coverage of neurons varies greatly too, from heavily draped motor neurons and Purkinje cells, where only specifically directed axons effecting synapses attain surface contact, to almost bare neurons in the cerebral cortex bounded by few or no astrocytic processes (Fig. 16).

The distribution of astrocytic processes appears neither random nor to provide only mechanical support and nutrition to neurons. Cajal held that they are always disposed to prevent the contact of neuronal processes at points other than those suited to specific circuits. His belief is supported by electron microscopy. Sanford Palay and Alan Peters suggest that astrocytes ensure that terminals act in a discrete and localized manner. Each neuron has a distinctive pattern of glial investment suited to its pattern of synaptic connections. Velate astrocytes may play this role in the neuropil. A multineuronal, polysynaptic glomerulus is always heavily guarded by astrocytic sheets in instances forming capsules several layers thick. Thus, by isolating the diverse input converging on a neuron, astrocytes seem to play a key role in the communication functions of the nervous system.

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