Although the nervous system is very complex, there are only
two main types of cells in nerve tissue. The actual nerve
cell is the neuron. It is the "conducting" cell that transmits
impulses and the structural unit of the nervous system. The
other type of cell is neuroglia, or glial, cell. The word
"neuroglia" means "nerve glue." These cells are nonconductive
and provide a support system for the neurons. They are a special
type of "connective tissue" for the nervous system.
Neurons
Neurons, or nerve cells, carry out the functions of the nervous
system by conducting nerve impulses. They are highly specialized
and amitotic. This means that if a neuron is destroyed, it
cannot be replaced because neurons do not go through mitosis.
The image below illustrates the structure of a typical neuron.
Each neuron has three basic parts: cell body (soma), one
or more dendrites, and a single axon.
Cell Body
In many ways, the cell body is similar to other types of
cells. It has a nucleus with at least one nucleolus and contains
many of the typical cytoplasmic organelles. It lacks centrioles,
however. Because centrioles function in cell division, the
fact that neurons lack these organelles is consistent with
the amitotic nature of the cell.
Dendrites
Dendrites and axons are cytoplasmic extensions, or processes,
that project from the cell body. They are sometimes referred
to as fibers. Dendrites are usually, but not always, short
and branching, which increases their surface area to receive
signals from other neurons. The number of dendrites on a neuron
varies. They are called afferent processes because they transmit
impulses to the neuron cell body. There is only one axon that
projects from each cell body. It is usually elongated and
because it carries impulses away from the cell body, it is
called an efferent process.
Axon
An axon may have infrequent branches called axon collaterals.
Axons and axon collaterals terminate in many short branches
or telodendria. The distal ends of the telodendria are slightly
enlarged to form synaptic bulbs. Many axons are surrounded
by a segmented, white, fatty substance called myelin
or the myelin sheath. Myelinated fibers make up the white
matter in the CNS, while cell bodies and unmyelinated fibers
make the gray matter. The unmyelinated regions between the
myelin segments are called the nodes of Ranvier.
In the peripheral nervous system, the myelin is produced
by Schwann cells. The cytoplasm, nucleus, and outer cell membrane
of the Schwann cell form a tight covering around the myelin
and around the axon itself at the nodes of Ranvier. This covering
is the neurilemma,
which plays an important role in the regeneration of nerve
fibers. In the CNS, oligodendrocytes produce myelin, but there
is no neurilemma, which is why fibers within the CNS do not
regenerate.
Functionally, neurons are classified as afferent, efferent,
or interneurons (association neurons) according to the direction
in which they transmit impulses relative to the central nervous
system. Afferent, or sensory, neurons carry impulses from
peripheral sense receptors to the CNS. They usually have long
dendrites and relatively short axons. Efferent, or motor,
neurons transmit impulses from the CNS to effector organs
such as muscles and glands. Efferent neurons usually have
short dendrites and long axons. Interneurons, or association
neurons, are located entirely within the CNS in which they
form the connecting link between the afferent and efferent
neurons. They have short dendrites and may have either a short
or long axon.
Neuroglia
Neuroglia cells do not conduct nerve impulses, but instead,
they support, nourish, and protect the neurons. They are far
more numerous than neurons and, unlike neurons, are capable
of mitosis.
Tumors
Schwannomas are benign tumors of the peripheral nervous system
which commonly occur in their sporadic, solitary form in otherwise
normal individuals. Rarely, individuals develop multiple schwannomas
arising from one or many elements of the peripheral nervous
system.
Commonly called a Morton's Neuroma, this problem is fairly
common benign nerve growth and begins when the outer coating
of a nerve in your foot thickens. This thickening is caused
by irritation of branches of the medial and lateral plantar
nerves that results when two bones repeatedly rub together.
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