Estimates
for the human brain range between 10 billion and 1 trillion. The
imprecision in these estimates is due almost entirely to uncertainty about
the number of granule cells in the cerebellum, a problem that can be
traced back to a study by Braitenberg & Atwood (1958). More recent work by
Lange (1975) makes a reasonably accurate estimate possible: The average
human brain (1350 gm) contains about 85 billion neurons. Of these, 12 to
15 billion are telencephalic neurons (Shariff 1953), 70 billion are
cerebellar granule cells (Lange 1975), and fewer than 1 billion are
brainstem and spinal neurons.
[A revision: In a beautiful quantitative
analysis of human cortex using the optical disector, Pakkenberg and
Gundersen (1997) have shown that the number of neocortical neurons ranges
from 15 to 31 billion and averages about 21 billion. Other forebrain
structures%u2014primarily the hippocampal region, basal ganglia, and
thalamus%u2014are likely to contain an additional 5%u20138 billion neurons. Total
neuron number in humans therefore probably averages 95%u2013100 billion. What
is perhaps more remarkable is the normal two-fold difference in
neocortical neuron number among healthy adults of normal intelligence.]
