Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
407
11
to 10,000 axon terminals making synapses and are stimulated
by an equal number of other neurons. Outside the central ner-
vous system, the postsynaptic cell may be either another neuron
or an effector cell (a muscle cell or gland cell).
Now let’s look at the two varieties of synapses:
electrical
and
chemical
.
Electrical Synapses
Electrical synapses
, the less common variety, consist of gap junc-
tions like those found between certain other body cells. Tey
contain protein channels, called connexons, that intimately con-
nect the cytoplasm of adjacent neurons and allow ions and small
molecules to flow directly from one neuron to the next. Tese
neurons are
electrically coupled
, and transmission across these
synapses is very rapid. Depending on the nature of the synapse,
communication may be unidirectional or bidirectional.
A key feature of electrical synapses between neurons is that
they provide a simple means of synchronizing the activity of all
interconnected neurons. In adults, electrical synapses are found
in regions of the brain responsible for certain stereotyped move-
ments, such as the normal jerky movements of the eyes. Tey
also occur in axoaxonal synapses in the hippocampus, a brain
region intimately involved in emotions and memory.
Electrical synapses are far more abundant in embryonic
nervous tissue, where they permit exchange of guiding cues
during early neuronal development so that neurons can con-
nect properly with one another. As the nervous system devel-
ops, chemical synapses replace some electrical synapses. Gap
junctions also exist between glial cells of the CNS.
Chemical Synapses
In contrast to electrical synapses, which are specialized to allow
the flow of ions between neurons,
chemical synapses
are special-
ized to allow the release and reception of chemical neurotrans-
mitters. A typical chemical synapse is made up of two parts:
A knoblike
axon terminal
of the presynaptic neuron, which
contains many tiny, membrane-bounded sacs called
synap-
tic vesicles
, each containing thousands of neurotransmitter
molecules
A neurotransmitter
receptor region
on the postsynaptic neu-
ron’s membrane, usually located on a dendrite or the cell body
Although close to each other, presynaptic and postsynaptic
membranes are always separated by the
synaptic clef
, a fluid-
filled space approximately 30 to 50 nm (about one-millionth of
an inch) wide. (If an electrical synapse with its gap junctions
is like the threshold of a doorway between neurons, a synaptic
cleF is like a good-size lake between them.)
Because the current from the presynaptic membrane dis-
sipates in the fluid-filled cleF, chemical synapses effectively
prevent a nerve impulse from being
directly
transmitted from
one neuron to another. Instead, an impulse is transmitted via a
chemical event
that depends on the release, diffusion, and recep-
tor binding of neurotransmitter molecules and results in
unidi-
rectional communication
between neurons.
the smaller somatic sensory fibers transmitting sensory impulses
from the skin (such as pain and small touch fibers).
What happens when an action potential arrives at the end of
a neuron’s axon? Tat is the subject of the next section.
Homeostatic Imbalance 11.3
A number of chemical and physical factors impair impulse
propagation. Local anesthetics like those used by your dentist
act by blocking voltage-gated Na
1
channels. As we have seen,
no Na
1
entry—no AP.
Cold and continuous pressure interrupt blood circulation,
hindering the delivery of oxygen and nutrients to neuron pro-
cesses and impairing their ability to conduct impulses. ±or exam-
ple, your fingers get numb when you hold an ice cube for more
than a few seconds, and your foot “goes to sleep” when you sit on
it. When you remove the cold object or pressure, impulses are
transmitted again, leading to an unpleasant prickly feeling.
Check Your Understanding
11.
Which is bigger, a graded potential or an action potential?
Which travels farthest? Which initiates the other?
12.
An action potential does not get smaller as it propagates
along an axon. Why not?
13.
Why does a myelinated axon conduct action potentials faster
than a nonmyelinated axon?
14.
If an axon receives two stimuli close together in time, only
one AP occurs. Why?
For answers, see Appendix H.
The Synapse
Define synapse. Distinguish between electrical and
chemical synapses by structure and by the way they
transmit information.
Te operation of the nervous system depends on the flow of
information through chains of neurons functionally connected
by synapses. A
synapse
(sin
9
aps), from the Greek
syn
, “to clasp
or join,” is a junction that mediates information transfer from
one neuron to the next or from a neuron to an effector cell—it’s
where the action is.
Synapses between the axon endings of one neuron and the
dendrites of other neurons are
axodendritic synapses
. Tose
between axon endings of one neuron and cell bodies (soma) of
other neurons are
axosomatic synapses
(Figure 11.16)
. Less
common (and far less understood) are synapses between axons
(
axoaxonal
), between dendrites (
dendrodendritic
), or between
cell bodies and dendrites (
somatodendritic
).
Te neuron conducting impulses toward the synapse is the
presynaptic neuron
, and the neuron transmitting the electrical
signal away from the synapse is the
postsynaptic neuron.
At a
given synapse, the presynaptic neuron sends the information,
and the postsynaptic neuron receives the information. As you
might anticipate, most neurons function as both presynaptic
and postsynaptic neurons. Neurons have anywhere from 1000
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