The brain of an average human being contains as many as 100 billion nerve cells, or
neurons. Billions more neurons are found in other parts of the nervous system. Yet
a single neuron holds many of the secrets of behavior and mental activity.
Neurons vary widely in size and shape, but they are all specialized to receive and
transmit information. Like other cells, the
neuron’s cell body is made up of a nucleus, which contains a complete set of chromosomes
and genes; cytoplasm, which keeps the cell alive; and a cell membrane,
which encloses the whole cell. What makes a neuron different from other cells are
the tiny fibers that extend out from the cell body, enabling a neuron to perform its
special job: receiving and transmitting messages. The short fibers branching out
around the cell body are dendrites (from the Greek word for “tree”). Their role is
to pick up incoming messages from other neurons and transmit them to the cell
body. The single long fiber extending from the cell body is an axon (from the Greek
for “axle”). The axon’s job is to carry outgoing messages to neighboring neurons or
to a muscle or gland. Axons vary in length from 1 or 2 millimeters to 3 feet (for
example, in adults, the axons that run from the brain to the base of the spinal cord
or from the spinal cord to the tip of the thumb). Although a neuron has only one
axon, near its end the axon splits into many terminal branches. When we talk about
a nerve (or tract), we are referring to a group of axons bundled together like wires
in an electrical cable.