Sensory and motor neurons make up the peripheral
nervous system. Visceral motor neurons form the
autonomic nervous system, a specialized subdivision
of the PNS that will be discussed later in this chapter.
Interneuronsare found entirely within the central
nervous system. They are arranged so as to carry only
sensory or motor impulses, or to integrate these func-
tions. Some interneurons in the brain are concerned
with thinking, learning, and memory.
A neuron carries impulses in only one direction.
This is the result of the neuron’s structure and loca-
tion, as well as its physical arrangement with other
neurons and the resulting pattern of synapses. The
functioning nervous system, therefore, is an enormous
network of “one-way streets,” and there is no danger
of impulses running into and canceling one another
out.
NERVES AND NERVE TRACTS
A nerveis a group of axons and/or dendrites of many
neurons, with blood vessels and connective tissue.
Sensory nervesare made only of sensory neurons.
The optic nerves for vision and olfactory nerves for
smell are examples of nerves with a purely sensory
function. Motor nervesare made only of motor neu-
rons; autonomic nerves are motor nerves. A mixed
nervecontains both sensory and motor neurons. Most
of our peripheral nerves, such as the sciatic nerves in
the legs, are mixed nerves.
The term nerve tractrefers to groups of neurons
within the central nervous system. All the neurons in
a nerve tract are concerned with either sensory or
motor activity. These tracts are often referred to as
white matter; the myelin sheaths of the neurons give
them a white color.
THE NERVE IMPULSE
The events of an electrical nerve impulse are the same
as those of the electrical impulse generated in muscle
fibers, which is discussed in Chapter 7. Stated simply,
a neuron not carrying an impulse is in a state of polar-
ization, with Na ions more abundant outside the
cell, and Kions and negative ions more abundant
inside the cell. The neuron has a positive charge on
the outside of the cell membrane and a relative nega-
tive charge inside. A stimulus (such as a neurotrans-
mitter) makes the membrane very permeable to Na
ions, which rush into the cell. This brings about
depolarization, a reversal of charges on the mem-
brane. The outside now has a negative charge, and the
inside has a positive charge.
As soon as depolarization takes place, the neuron
membrane becomes very permeable to Kions, which
rush out of the cell. This restores the positive charge
outside and the negative charge inside, and is called
repolarization. (The term action potentialrefers to
depolarization followed by repolarization.) Then the
sodium and potassium pumps return Naions outside
and Kions inside, and the neuron is ready to respond
to another stimulus and transmit another impulse. An
action potential in response to a stimulus takes place
very rapidly and is measured in milliseconds. An indi-
vidual neuron is capable of transmitting hundreds of
action potentials (impulses) each second. A summary
of the events of nerve impulse transmission is given in
Table 8–2.
Transmission of electrical impulses is very rapid.
The presence of an insulating myelin sheath increases
the velocity of impulses, since only the nodes of
Ranvier depolarize. This is called saltatory conduc-
tion. Many of our neurons are capable of transmitting
impulses at a speed of many meters per second.
Imagine a person 6 feet (about 2 meters) tall who stubs
his toe; sensory impulses travel from the toe to the
brain in less than a second (crossing a few synapses
along the way). You can see how the nervous system
can communicate so rapidly with all parts of the body,
and why it is such an important regulatory system.
At synapses, nerve impulse transmission changes
from electrical to chemical and depends on the release
of neurotransmitters. Although diffusion across
synapses is slow, the synapses are so small that this
does not significantly affect the velocity of impulses in
a living person.THE SPINAL CORD
The spinal cordtransmits impulses to and from the
brain and is the integrating center for the spinal cord
reflexes. Although this statement of functions is very
brief and sounds very simple, the spinal cord is of
great importance to the nervous system and to the
body as a whole.
Enclosed within the vertebral canal and the menin-
ges, the spinal cord is well protected from mechanicalThe Nervous System 171