with all the multitudinous axonal and dendritic fibers and astrocytic
processes, is called neuropil (Greek pilos = hair). When looked at with
the high magnification of an electron microscope, neuropil appears
as a dense mass of solidly packed structures. Whatever gaps exist be-
twixt and between structures are very narrow, at most 20 nanometers
or so. There are numerous chemical and electrical synapses—perhaps
upwards of one hundred million synapses in a mere cubic millimeter
of neuropil.
Figure 18.5. Electron micrograph of neuropil from rat hippocampus. Den-
drites, axons, and cell bodies are shown in cross section. Many mitochondria
and neurotransmitter storage vesicles are visible. Scale bar = 1 micrometer.