Structural function creation of the functional microarchitecture of the brain

Protoplasmic astrocytes in the grey matter are organized in a very particular way, with each astrocyte controlling its own three-dimensional anatomical territory (Figure 7.3). The overlap between territories of neighbouring astroglial cells is minimal and it does not exceed five per cent, i.e. astrocytes contact each other only by the most distal processes. Individual astrocytes establish contacts with blood vessels, neurones and synapses residing within their anatomical domain. Astrocytic processes show a very high degree of morphological plasticity; many of these processes send very fine expansions, the lamellopodia and filopodia, which contact synaptic regions. These lamellopodia and filopodia are in fact motile, and may expand or shrink at a speed of several ^m per minute. The lamellopodia show gliding movements along neuronal surfaces and filopodia are able to rapidly protrude towards or retract from the adjacent neuronal membranes or synaptic structures.

Using clearly delineated anatomical territories, astrocytes divide the whole of grey matter (both in the brain and in the spinal cord) into separate domains, the elements of which (neurones, synaptic terminals and blood vessels) are integrated via the processes of protoplasmic astrocytes; the membranes of a single astrocyte

Protoplasmic Astrocyte

Figure 7.3 Astrocytic domains form the micro-architecture of grey matter. Each single astrocyte occupies a well-defined territory; astroglial contacts occur only through distal processes and overall overlap between astrocyte territories does not exceed three to five per cent. The astrocytic domains are organized in rows along the vessels, which are typically positioned in the narrow interface between astrocytes as shown on the scheme. (Modified from Nedergaard M, Ransom B, Goldman SA (2003) New roles for astrocytes: redefining the functional architecture of the brain. Trends Neurosci 26, 523-530)

Figure 7.3 Astrocytic domains form the micro-architecture of grey matter. Each single astrocyte occupies a well-defined territory; astroglial contacts occur only through distal processes and overall overlap between astrocyte territories does not exceed three to five per cent. The astrocytic domains are organized in rows along the vessels, which are typically positioned in the narrow interface between astrocytes as shown on the scheme. (Modified from Nedergaard M, Ransom B, Goldman SA (2003) New roles for astrocytes: redefining the functional architecture of the brain. Trends Neurosci 26, 523-530)

may cover about 100 000 to 2 millions (in humans) synapses present in its domain. The astrocytic processes provide for local signalling within the domain, as their membranes that contact neurones, synapses and blood vessels are packed with receptors, which sense the ongoing activity. Signals activated by glial receptors may propagate through the astrocyte cytoplasm, thus integrating distant parts of the domain. Importantly, the processes of the same astrocyte are often directly coupled via gap junctions, which establish diffusion shortcuts, allowing the local metabolic signals to rapidly spread through these processes, bypassing the soma.

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