The neuron

The functional unit of the nervous system is the neuron,
which consists of a cell body and its thread-like
processes – one or more dendrites and a single axon.
The axon is commonly referred to as the nerve fi bre
and varies in length from a fraction of a millimetre
to a metre or more. Most of the nerve cell bodies are
located in or near the CNS. Their processes, which
are capable of transmitting impulses, may lie totally
within the CNS, may extend from the CNS system for
great distances, or may lie entirely outside the CNS.
The dendrites conduct impulses toward the cell body,
and the axon conducts impulses away from the cell
body. Nerve impulses are relayed from one neuron to
another across synapses. At these meeting points the
surface membranes of the two neurons are very close
together, but there is never any cytoplasmic continuity.
The width of the extracellular space separating the
pre- and post-synaptic membranes, the synaptic cleft,
is generally about 25 nm. Transmission of an impulse
across a synapse is chemical rather than electrical. It
depends on the release of a chemical transmitter from
the presynaptic terminal, which then diffuses across
the synaptic cleft and interacts with receptor sites on
the postsynaptic membrane so as to cause a specifi c
change in its ionic permeability.