The Neuron
Modified: 2020-06-09
The basic unit of the nervous system is the neuron or nerve cell. The
first thing we will see is that there are a lot of them. Nobody has
counted all of the neurons in the human body, but estimates run from
10-12 billion to100 billion or more. For the sake of comparison, a cockroach
has about 250,000 neurons, while a chimpanzee has 3.5 billion (with
the same warnings about numbers as above). The point to remember is
that there are a great many, and that the large number contributes to
the complexity of the nervous system. When one starts to consider the
connections between nerve cells, then the numbers truly become
astronomical because a neuron may connect to as many as 75 other
neurons. Most of the neurons in the human body, about 70%, are in the
central nervous system.
Neurons can be divided into three types, depending on their
function. Afferent or sensory neurons run from the sense organs to
the central nervous system. Efferent or motor neurons run from the
central nervous system to the muscles, and interneurons or multipolar
neurons are found chiefly within the brain and they are multiply
connected to other neurons.
Axons are a special part of a neuron. They are the part that
carries the nerve impulse down the cell. Nothing in the axon
predisposes it to carry the nerve impulse from the cell body to the
terminal buttons. If you electrically stimulate an isolated axon in
the middle, the nerve impulse will run in both directions. It is the
organization of the nerves that ensures that the impulses travel in
the proper direction.
GRAPHIC of a neuron
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