Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
395
11
NEURON TYPE
MULTIPOLAR
BIPOLAR
UNIPOLAR (PSEUDOUNIPOLAR)
Functional Class: Neuron Type According to Direction of Impulse Conduction
1.
Most multipolar neurons are
interneurons
that
conduct impulses within the CNS, integrating
sensory input or motor output. May be one of a
chain of CNS neurons, or a single neuron connecting
sensory and motor neurons.
2. Some multipolar neurons are
motor neurons
that
conduct impulses along the efferent pathways from
the CNS to an effector (muscle/gland).
Essentially all bipolar neurons are
sensory neurons
that are located in
some special sense organs. For example,
bipolar cells of the retina are involved
with transmitting visual inputs from the
eye to the brain (via an intermediate
chain of neurons).
Most unipolar neurons are
sensory
neurons
that conduct impulses along
afferent pathways to the CNS for
interpretation. (These sensory neurons
are called primary or first-order sensory
neurons.)
Check Your Understanding
8.
Which structural and functional type of neuron is activated
first when you burn your finger? Which type is activated last
to move your finger away from the source of heat?
For answers, see Appendix H.
Membrane Potentials
When a neuron is adequately stimulated, an electrical impulse
is generated and conducted along the length of its axon. Tis
response, called the
action potential
or
nerve impulse
, is always
the same regardless of the source or type of stimulus, and it un-
derlies virtually all functional activities of the nervous system.
In this section, we will consider how neurons are excited or
inhibited and how they communicate with other cells. First,
however, we need to explore some basic principles of electricity
and revisit the resting membrane potential.
Basic Principles of Electricity
Te human body is electrically neutral—it has the same number
of positive and negative charges. However, there are regions
where one type of charge predominates, making those regions
positively or negatively charged. Because opposite charges at-
tract each other, energy must be used (work must be done) to
separate them. On the other hand, the coming together of op-
posite charges liberates energy that can be used to do work. For
this reason, situations in which there are separated electrical
charges of opposite sign have potential energy.
Sensory
, or
afferent
,
neurons
transmit impulses from sensory
receptors in the skin or internal organs
toward
or
into
the central
nervous system. Except for certain neurons found in some special
sense organs, virtually all sensory neurons are unipolar, and their
cell bodies are located in sensory ganglia
outside
the CNS. Only
the most distal parts of these unipolar neurons act as impulse re-
ceptor sites, and the peripheral processes are o±en very long. For
example, fibers carrying sensory impulses from the skin of your
great toe travel for more than a meter before they reach their cell
bodies in a ganglion close to the spinal cord.
Te receptive endings of some sensory neurons are naked,
in which case those terminals themselves function as sensory
receptors, but many sensory neuron endings bear receptors that
include other cell types. We describe the various types of gen-
eral sensory receptor end organs, such as those of the skin, in
Chapter 13. Te special sensory receptors (of the ear, eye, etc.)
are the topic of Chapter 15.
Motor
, or
efferent
,
neurons
carry impulses
away from
the
CNS to the effector organs (muscles and glands) of the body
periphery. Motor neurons are multipolar. Except for some neu-
rons of the autonomic nervous system, their cell bodies are lo-
cated in the CNS.
Interneurons
, or
association neurons
, lie between motor and
sensory neurons in neural pathways and shuttle signals through
CNS pathways where integration occurs. Most interneurons are
confined within the CNS. Tey make up over 99% of the neu-
rons of the body, including most of those in the CNS.
Almost all interneurons are multipolar, but there is consid-
erable diversity in both size and fiber-branching patterns. Te
Purkinje and pyramidal cells illustrated as structural variations
in ²able 11.1 are just two examples of their variety.
Impulse
Motor
neuron
Muscle
Interneuron
Impulse
Eye
Brain
Bipolar neuron
of retina of eye
Impulse
Skin
Sensory
neuron
Table 11.1
(continued)
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