Neuronal Signaling and the Structure of the Nervous System
141
efferent neurons varies according to the complexity of the action
they control. The knee-jerk refl ex elicited by tapping below the
kneecap requires no interneurons—the afferent neurons inter-
act directly with efferent neurons. In contrast, when you hear a
song or smell a certain perfume that evokes memories of some-
one you once knew, millions of interneurons may be involved.
Table 6–1
summarizes the characteristics of the three
functional classes of neurons.
The anatomically specialized junction between two neu-
rons where one neuron alters the electrical and chemical activ-
ity of another is called a
synapse.
At most synapses, the signal
is transmitted from one neuron to another by neurotransmit-
ters, a term that also includes the chemicals efferent neurons
use to communicate with effector cells (e.g., a muscle cell).
The neurotransmitters released from one neuron alter the
receiving neuron by binding with specifi c protein receptors on
the membrane of the receiving neuron. (Once again, do not
confuse this use of the term
receptor
with the sensory recep-
tors at the peripheral ends of afferent neurons.)
Most synapses occur between an axon terminal of one
neuron and a dendrite or the cell body of a second neuron.
Sometimes, however, synapses occur between two dendrites or
between a dendrite and a cell body or between an axon terminal
and a second axon terminal. A neuron that conducts a signal
toward a synapse is called a
presynaptic neuron,
whereas a neu-
ron conducting signals away from a synapse is a
postsynaptic
neuron.
Figure 6–5
shows how, in a multineuronal pathway, a
single neuron can be postsynaptic to one cell and presynaptic to
another. A postsynaptic neuron may have thousands of synaptic
junctions on the surface of its dendrites and cell body, so that
signals from many presynaptic neurons can affect it.
Glial Cells
Neurons account for only about 10 percent of the cells in the
central nervous system. The remainder are
glial cells,
also
called neuroglia (
glia
= glue). However, because neurons
branch more extensively than glia do, neurons occupy about
50 percent of the volume of the brain and spinal cord.
Glial cells surround the soma, axon, and dendrites of
neurons and provide them with physical and metabolic sup-
port (
Figure 6–6
). As noted earlier, one type of glial cell, the
oligodendrocyte, forms the myelin covering of CNS axons.
A second type of glial cell, the
astrocyte,
helps regu-
late the composition of the extracellular fl uid in the central
nervous system by removing potassium ions and neurotrans-
mitters around synapses. Another important function of astro-
cytes is to stimulate the formation of tight junctions between
the cells that make up the walls of capillaries found in the
central nervous system. This forms the
blood-brain barrier,
which prevents toxins and other substances from entering the
brain. Astrocytes also sustain the neurons metabolically—for
example, by providing glucose and removing ammonia. In
developing embryos, astrocytes guide neurons as they migrate
to their ultimate destination, and they stimulate neuronal
growth by secreting growth factors. In addition, astrocytes
have many neuron-like characteristics. For example, they have
ion channels, receptors for certain neurotransmitters and the
Table 6–1
Characteristics of Three
Classes of Neurons
I
.
AFFERENT
NEURONS
A. Transmit information into the central nervous system
from receptors at their peripheral endings
B. Cell body and the long peripheral process of the axon are in
the peripheral nervous system; only the short central process
of the axon enters the central nervous system
C. Most have no dendrites (do not receive inputs from other
neurons)
II
.
EFFERENT
NEURONS
A. Transmit information out of the central nervous system to
effector cells, particularly muscles, glands, or other neurons
B. Cell body, dendrites, and a small segment of the axon are
in the central nervous system; most of the axon is in the
peripheral nervous system
III
.
INTERNEURONS
A. Function as integrators and signal changers
B. Integrate groups of afferent and efferent neurons into
refl
ex circuits
C. Lie entirely within the central nervous system
D. Account for 99 percent of all neurons
Figure 6–5
A neuron postsynaptic to one cell can be presynaptic to another.
Arrows indicate direction of neural transmission.
Presynaptic
Postsynaptic
Axon
Presynaptic
Presynaptic
Postsynaptic
Presynaptic
Postsynaptic
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