The astrocyte (blue-gray, above) contacts many of the synapses between axonal boutons and the spines from a segment of dendrite.
In the cerebellum, Bergmann glia completely ensheath most parallel fiber-Purkinje cell synapses. In the hippocampus, however, relationships between astrocytes and synapses are much more variable. Only 60% of the synapses in stratum radiatum of hippocampal area CA1 have any astrocytic coverage at all around their spine-axon interface. The amount of coverage ranges from 1% to 100%. Click on one of the synapses above to see the degree to which it is surrounded by the astrocyte.
For more quantitative details see:
Ventura R, Harris KM (1999) Three-dimensional relationships between hippocampal synapses and astrocytes. J Neuroscience 19(16):6897-6906. (888K PDF)