371
CHAPTER TEN
Nervous System I
second. Clinical Application 10.3 discusses factors that infl
u-
ence nerve impulse conduction.
PRACTICE
9
Summarize how a resting potential is achieved.
10
Explain how a polarized axon responds to stimulation.
11
List the major events of an action potential.
12
DeF
ne
refractory
period.
13
Explain how impulse conduction di±
ers in myelinated and
unmyelinated axons.
10.7
SYNAPTIC TRANSMISSION
Released neurotransmitter molecules diffuse across the syn-
aptic cleft and react with speciF c molecules called
receptors
in the postsynaptic neuron membrane. Effects of neurotrans-
mitters vary. Some open ion channels, and others close
them. These ion channels respond to neurotransmitter mol-
ecules, so they are called
chemically-gated,
in contrast to the
voltage-gated ion channels that participate in action poten-
tials. Changes in chemically-gated ion channels create local
potentials, called
synaptic potentials,
which enable one
neuron to affect another.
Synaptic Potentials
Synaptic potentials can depolarize or hyperpolarize the
receiving cell membrane. ±or example, if a neurotransmitter
binds to a postsynaptic receptor and opens sodium ion chan-
nels, the ions diffuse inward, depolarizing the membrane,
possibly triggering an action potential. This type of mem-
brane change is called an
excitatory postsynaptic potential
(EPSP), and it lasts for about 15 milliseconds.
Impulse Conduction
An unmyelinated axon conducts an impulse over its entire
surface. A myelinated axon functions differently. Myelin
contains a high proportion of lipid that excludes water and
water-soluble substances. Thus, myelin serves as an electri-
cal insulator and prevents almost all fl
ow of ions through the
membrane that it encloses.
It might seem that the myelin sheath would prevent con-
duction of a nerve impulse, and this would be true if the
sheath were continuous. However, nodes of Ranvier between
Schwann cells or oligodendrocytes interrupt the sheath (see
F g. 10.3). At these nodes, the axon membrane has channels
for sodium and potassium ions that open during a threshold
depolarization.
When a myelinated axon is stimulated to threshold, an
action potential occurs at the trigger zone. This causes an
electric current to fl
ow away from the trigger zone through
the cytoplasm of the axon. As this local current reaches the
F
rst node, it stimulates the membrane to its threshold level,
and an action potential occurs there, sending an electric
current to the next node. Consequently, in a nerve impulse
traveling along a myelinated axon, action potentials occur
only at the nodes. The action potentials appear to jump
from node to node, so this type of impulse conduction is
called
saltatory conduction.
Conduction on myelinated
axons is many times faster than conduction on unmyeli-
nated axons
(f
g. 10.19)
.
The diameter of the axon also affects the speed of nerve
impulse conduction—the greater the diameter, the faster the
impulse. An impulse on a thick, myelinated axon, such as
that of a motor neuron associated with a skeletal muscle,
might travel 120 meters per second, whereas an impulse on
a thin, unmyelinated axon, such as that of a sensory neu-
ron associated with the skin, might move only 0.5 meter per
++
++
Action
potential
+
+
+
++
++
++
++
++
++
Electric current
Nodes
Axon
Schwann cells
(a)
Action
potential
+
+
+
++
++
++
++
++
++
++
++
(b)
+ +
++
+ +
++
++
++
++
++
Action
potential
+
+
+
(c)
+
+
+
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FIGURE 10.19
On a myelinated axon, a nerve impulse appears to jump from node to node.
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