118
UNIT 2
Basic Tissues
Neurons and Synapses
Figure 7-1A.
The neuron: The building block of the
nervous system.
Neurons
contain the organelles that are common to all cells:
a cell membrane, nucleus, endoplasmic reticulum, and cyto-
plasm. In addition, neurons possess two types of specialized
processes,
dendrites
and
axons
. The plasma membranes of den-
drites and of the cell body itself contain specialized receptors
that react to the release of
neurotransmitters
. These molecules
produce a change in polarization of the membrane. The mem-
brane of axons, by contrast, is specialized to transmit electro-
chemical signals called
action potentials
. An action potential is
a wave of membrane depolarization that maintains its size as it
travels along the axon. When the action potential reaches the
end of the axon, neurotransmitters are released from the
axon
terminals
that infl uence the next neuron in line. Some axons
are surrounded by a lipid-rich sheath called
myelin
, which
facilitates the rapid conduction of action potentials.
J. Lynch
Dendrites
Soma
Nucleus
Myelin
Axon
Axon terminals
A
J. Lynch
Unipolar
Bipolar
Axon
Axon
D
D
D
T
T
T
T
D
Axon
collateral
Multipolar
B
Figure 7-1B.
Types of neurons.
Neurons
can be classiF ed on the basis of the shapes of their cell
bodies and the general arrangement of their axons and den-
drites.
Unipolar neurons
have a single process attached to a
round cell body. This process typically divides and forms a long
axon extending from sensory receptors in the various tissues of
the body to synaptic terminals in the CNS.
Bipolar neurons
have
a process extending from each end of the cell body. This type of
neuron is found primarily in the eye, ear, vestibular end organs,
and olfactory system.
Multipolar neurons
have many dendrites
extending from the cell body and a single axon (although the
axon may split into two or more
collateral axons
after it leaves
the cell body). Multipolar neurons are the most numerous in
the nervous system and have many different shapes and sizes.
(D, dendrites; T, axon terminals [“terminal boutons” or “bou-
tons terminaux”] with synaptic endings.
Red arrows
indicate
the direction of information transmission.)
J. Lynch
C
Purkinje
Unipolar
Stellate
Spinal
motor
Pyramidal
Figure 7-1C.
Some representative neurons.
Drawings
from Golgi-stained tissue.
Neurons
come in a wide variety of shapes and sizes, depend-
ing on their function and location.
Purkinje cells
are found in
the cerebellar cortex.
Pyramidal cells
are the most numerous
cells in the cerebral cortex.
Stellate cells
are also located in the
cerebral cortex. Multipolar motor neurons are found in the
anterior horn of the spinal cord (
spinal motor neurons
) and in
the motor nuclei of cranial nerves. Other types of multipolar
neurons are found in many central and autonomic nervous
system sites.
Unipolar neurons
have cell bodies in the poste-
rior root ganglia of the spinal cord. Their peripheral processes
contact sensory receptors in the skin, muscles, and internal
organs; their central processes form synapses on neurons in
nuclei of the CNS. There are many additional types of neurons
in the nervous system, but these represent some of the most
common types and demonstrate the wide variety that exists.
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