neuron is triggered. Sodium ions rush through the
plasma membrane into the cell, potassium ions
leave, and an electrical impulse is formed, which
is conducted toward the cyton. The cyton receives
the impulse and transmits it to the terminal
filaments of the axon. At this point a chemical
transmitter such as acetylcholine is released into
the SYNAPSE, a space between the axon of the
activated nerve and the dendrite receptors of
another neuron. This transmitter activates the next
nerve. In this manner the impulse is passed from
neuron to neuron down the nerve line to a cen-
tral area at approximately the speed of a bullet.
Almost immediately after being activated, the
transmitter chemical in the synapse is neutralized
by the enzyme acetylcholinesterase, and the first
neuron returns to its normal state by pumping out
the sodium ions and drawing potassium ions back
in through the plasma membrane. When these ac-
tions are completed, the nerve is ready to be trig-
gered again. A particularly strong stimulus will
cause the nerve to fire in rapid succession, or will
trigger many other neurons, thus giving a feeling
of intensity to the perceived sensation.
CENTRAL NERVOUS SYSTEM
The central nervous system consists of the
brain and spinal cord (fig. 3-39). The brain is
almost entirely enclosed in the skull, but it is con-
nected with the spinal cord, which lies in the canal
formed by the vertebral column.
The brain has two main divisions, the
cerebrum and the cerebellum. The cerebrum is the
largest and most superiorly situated portion of the
brain. It occupies most of the cranial cavity. The
outer surface is called the cortex. This portion of
the brain is also called gray matter because the
nerve fibers are unmyelinated (not covered by a
Figure 3-39.The Central nervous system.
Figure 3-40.Sheath of a neuron.
Figure 3-41.Functional areas of the brain.