b. Neurons
are
classified according to number of
processes extending from the cell body:
Multipolar neurons have several
dendrites and one axon. They are
found in the brain and spinal cord.
Neurons Are Electrically Excitable Cells
Designed for Transmitting Information
Like other cells, neurons have organelles such as endoplasmic
reticulum (ER), mitochondria, and nuclei in their cell bodies
that maintain normal cell functions, including protein synthe-
sis and metabolism. Like muscle cells, neurons are electrically
excitable and specialized for transmitting electrical impulses
(Figure 7.3a).
Unlike most other cells, neurons have cellular extensions
that are commonly referred to as nerve fibers. There are two
different types of nerve fibers. A neuron always has one axon.
This fiber conducts electrical impulses away from the cell body
and is involved in signalling motor activity in other cells. The
other fiber is called a dendrite. This fiber conducts impulses
toward the cell body and is associated with sensory nerve re-
ceptors. A neuron can have multiple dendrites.
Neurons are classified in different ways. Structurally they
are labeled as multipolar, bipolar, or unipolar, depending on
the number of processes extending from the cell body (Figure
7.3b). Functionally, neurons fall into one of three classes:
Bipolar neurons
have one dendrite
and one axon. They are found in the
retina of the eye, the inner ear, and
olfactory areas of the brain.
Unipolar neurons
have dendrites
and axons fused into one process.
Most sensory neurons are unipolar.
Dendrites
Trigger zone
Peripheral
process
Myelin sheath
Cell body
Central
process
Axon
terminal
Sensory neurons, or afferent neurons (AF-er-ent NOO-
ronz), either have specialized receptors themselves or are
connected to separate sensory receptors. The somato-
sensory and autonomic sensory neurons are unipolar
neurons, while the special sense receptors are attached
to bipolar neurons. Sensory neurons transmit impulses
toward the CNS through spinal or cranial nerves.
Motor neurons, or efferent neurons (EF-er-ent NOO-
ronz), are mostly multipolar neurons that carry infor-
mation from the CNS to effectors through cranial or spinal
nerves.
Interneurons
(in'-ter-NOO-ronz)
extend only for short
distances and contact nearby neurons in the brain, spinal
cord, or a ganglion. A ganglion (plural is
g a n g lia )
is a
collection of nerve cell bodies found in the peripheral
nervous system. Similar collections of nerve cell bodies in
the CNS are referred to as nuclei. Interneurons are mostly
multipolar and comprise the vast majority of neurons in the
body.
Neurons can conduct action potentials over a range of dis-
tances from just a few millimeters for an interneuron to over
a meter for some of the sensory and motor neurons. To enable
the impulses to be conducted rapidly
over these distances, most neuronal
fibers are covered by layers of
myelin
,
which serves to insulate the axon and
speed the conduction of impulses.
Now let’s take a closer look at the supporting cells of the
nervous system, neuroglia or glial cells.
myelin
(Mf-e-lin)
A multilayered axon
covering that consists
of lipid and protein.
Nerve Cells "Talk" to Each Other 195
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