The human brain contains approximately 10¹⁰ neurons to which the
neuron theory assigns the primary role in central nervous activity.
These cells occupy, however, a relatively small fraction of the total
volume. There are, for example, approximately 10 times that number of
neuroglia, cells of relatively indeterminate function. Each neuron
(consisting of cell body, dendrites and, sometimes, an axon) comes into
close contact with the dendrites of other neurones at some thousands
of places, these synapses and “ephapses” being spaced approximately 5μ
apart (1). The total number of such apparent junctions is therefore
of the order of 10¹³. In spite of infinite fine-structure variations
when viewed with slightly blurred vision, the cellular structure of
the brain is remarkably homogeneous. In the cortex, at least, the
extensions of most cells are relatively short, and when the cortex is
at rest, it appears from the large EEG alpha-rhythms that large numbers
of cells beat together in unison. Quoting again from Sperry, “In short,
current brain theory encourages us to try to correlate our subjective
psychic experience with the activity of relatively homogeneous nerve
cell units conducting essentially homogeneous impulses, through roughly
homogeneous cerebral tissue.”