Extracellular Space

From: <a href="https://synapseweb.clm.utexas.edu/astrocytes">Astrocytes | SynapseWeb</a>

Modulating ECS volume fraction through astrocytic swelling and filopodia extension

Astrocytes are an important determinant of the volume fraction of the extracellular space. Astrocytic swelling has been shown to occur as a result of both glutamate and adenosine receptor stimulation (Hansson, 1994; Bourke et al., 1983). Filopodial extension, or outgrowth of membrane, can be induced in astrocytes by focal application of glutamate (Cornell-Bell et. al., 1992). This filopodial extension can change the astrocytic size and morphology, possibly also altering the volume fraction occupied by extracellular space.

Synaptic function is sensitive to the extracellular volume fraction. As the volume fraction decreases, the concentration of components of the extracellular space increases and this could, theoretically, increase a neuron’s responsiveness to a given amount of neurotransmitter inputted. In fact, it has been shown that when the extracellular volume fraction is decreased as a result of increasing the concentration of K+, which is known to induce astrocytic swelling, the excitability of the neurons increases to the extent that seizures result (reviewed in Porter and McCarthy, 1997; Traynelis and Dingledine, 1989). Also, furosemide, which is known to block astrocytic swelling, has been recently shown to prevent electrically induced seizures (Hochman et al., 1995; Porter and McCarthy, 1997). These results suggest that astrocytes, through modulation of their size, can influence the excitability of neurons.