130 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”of a nerve impulse ranges from 0.1 to about 100 meters per second,
depending on the diameter of the axon (the bigger the faster) and the
degree of insulation (the more myelination, the faster). This is a very
slow process compared to an electric wire.^ However, the slowness “buys
time” for modulation and subtle adjustments, a feature not possible with
a metallic conductor.^2
The size of the nerve cell body ranges from a few thousandths of a
millimeter to a tenth of a millimeter, whereas the length of an axon can
be as long as a meter in a person. It has been estimated that the total
length of all the axons in a human brain exceeds 100,000 miles. A neuron
can receive incoming messages from as many as a thousand neurons, and
can transmit to as many others. The complexity of neuronal connections
in a single human brain dwarfs the relationship among all the stars in the
universe. The synapse, which connects a nerve terminal to a dendrite,
consists of a pre-synaptic portion (axonal side, releasing neurotransmit-
ters), a post-synaptic portion (dendritic side, receiving neurotransmit-
ters) and an intervening gap called a synaptic cleft (Fig. 7.4). Usually, the
portion of the dendrite that forms a synapse is a specialized area (a small
protrusion) called the dendritic spine (see Fig. 7.1). Chemical communi-
cation between neurons is a one-way street, made possible by the release
Fig. 7.2. Interconnection of numerous neurons in the human cerebral cortex as
revealed by silver stain. [Source: S. Ramon y Cajal, public domain.]