Neuron



Neurons (also called nerve cells) are the primary cells of the nervous
system. They are found in the brain, the spinal cord and in the peripheral
nerves and ganglia.

Neurons have a quasi-amoeboid shape, consisting of a single long projection
called an axon and shorter, typically branching projections called
dendrites. Some types of neurons, such as Purkinje cells, have over 1000
dendrites. The body of a neuron, from which the axon and dendrites project,
is called the soma and holds the nucleus of the cell. The nucleus typically
occupies most of the volume of the soma and is much larger in diameter than
the axon and dendrites, which typically are only about a micrometer thick or
less. Neurons join to one another and to other cells through synapses.

A defining feature of neurons is their ability to become "electrically
excited"--that is, to undergo an action potential--and to convey this
excitation rapidly along their axons as an impulse. The narrow cross section
of axons and dendrites lessens the metabolic expense of conducting action
potentials, although fatter axons convey the impulses more rapidly,
generally speaking.

Many neurons have insulating sheaths of myelin around their axons, which
enable their action potentials to travel faster than in unmyelinated axons
of the same diameter (see saltatory conduction under action potential).
Formed by glial cells, the myelin sheathing normally runs along the axon in
sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Neurons
and glia make up the two chief cell types of the nervous system.

An action potential that arrives at its terminus in one neuron may provoke
an action potential in another through release of neurotransmitter molecules
across the synaptic gap.

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