The Evolving View of Astrocytes

Abstract

Did you know that glial cells are more numerous than neurons in the brain? Scientists have found that one type of glial cell that is prevalent in the cortex-the astrocyte-communicates with its brethren, sends information to neurons, and controls blood flow to regions of brain activity. Because of all these properties, and since the cortex is believed responsible for cognition, the role of astrocytes in sleep, learning, and memory is being determined.

Figure 1:

In addition to the electrically active neurons, the brain contains numerous glial cell types, including astrocytes, oligodendrocytes, and microglia. Astrocytes are the most plentiful of these glial cells and have unique physical attributes: They contract blood vessels as well as neuronal synapses at a structure called the tripartite synapse (rectangular box and see figure 2). Consequently, they play important roles in synaptic development and modulation/homeostasis as well as in the delivery of nutrients from the circulation to neurons and act as an intermediate to relay neuronal activity to the vasculature to control blood flow.

Figure 2:

The astrocytic process is the third active element forming the tripartite synapse. (a) Electron micrograph showing a presynaptic (Pre) and postsynaptic (Post) terminal enwrapped by the astrocytic process (green) forming the tripartite synapse. (b) The close association of the astrocytic process with the presynaptic and postsynaptic terminals exerts crucial roles in clearing K⁺ ions that accumulate following neuronal activity, and in the uptake of the synaptic transmitter glutamate by the activity of plasma-membrane glutamate transporters. Additionally, neurotransmitter release from presynaptic terminals can activate astrocytic receptors that induce Ca2+ elevations which in turn triggers the release of gliotransmitters from these cells. Courtesy of Cell Press