379
CHAPTER TEN
Nervous System I
b. Slightly later, potassium channels open and
potassium ions diffuse outward, repolarizing the
membrane.
c. This rapid change in potential is an action
potential.
d. Many action potentials can occur before active
transport reestablishes the original resting
potential.
e. The propagation of action potentials along a nerve
F
ber is an impulse.
5. All-or-none response
a. A nerve impulse is an all-or-none response. If a
stimulus of threshold intensity is not applied to an
axon, an action potential is not generated.
b. All the impulses conducted on an axon are the
same.
6. Refractory period
a. The refractory period is a brief time following
passage of a nerve impulse when the membrane is
unresponsive to an ordinary stimulus.
b. During the absolute refractory period, the
membrane cannot be stimulated; during the
relative refractory period, the membrane can be
stimulated with a high-intensity stimulus.
7. Impulse conduction
a. An unmyelinated axon conducts impulses that
travel over its entire surface.
b. A myelinated axon conducts impulses that travel
from node to node.
c. Impulse conduction is more rapid on myelinated
axons with large diameters.
10.7
SYNAPTIC TRANSMISSION (PAGE 371)
Neurotransmitter molecules diffuse across the synaptic
cleft and react with receptors in the postsynaptic neuron
membrane.
1. Synaptic potentials
a. Some neurotransmitters can depolarize the
postsynaptic membrane, possibly triggering an
action potential. This is an excitatory postsynaptic
potential (EPSP).
b. Others hyperpolarize the membrane, inhibiting an
action potential. This is an inhibitory postsynaptic
potential (IPSP).
c. EPSPs and IPSPs are summed in a trigger zone of
the neuron.
2. Neurotransmitters
a. The nervous system produces at least thirty types
of neurotransmitters.
b. Calcium ions diffuse into synaptic knobs
in response to action potentials, releasing
neurotransmitters.
c. Neurotransmitters are quickly decomposed or
removed from synaptic clefts.
3. Neuropeptides
a. Neuropeptides are chains of amino acids.
b. Some neuropeptides are neurotransmitters or
neuromodulators.
c. They include enkephalins, endorphins, and
substance P.
(2) If a peripheral axon is severed, its distal
portion will die, but under the infl
uence of
nerve growth factors, the proximal portion
may regenerate and reestablish connections, if
a tube of connective tissue guides it.
(3) SigniF
cant regeneration is not likely in the
CNS.
10.5
THE SYNAPSE (PAGE 365)
A synapse is a junction between two cells. A synaptic
cleft is the gap between parts of two cells at a synapse.
Synaptic transmission is the process by which the impulse
in the presynaptic neuron signals the postsynaptic cell.
1. A nerve impulse travels along the axon to a synapse.
2. Axons have synaptic knobs at their distal ends that
secrete neurotransmitters.
3. The neurotransmitter is released when a nerve
impulse reaches the end of an axon, and the
neurotransmitter diffuses across the synaptic cleft.
4. A neurotransmitter reaching a postsynaptic neuron or
other cell may be excitatory or inhibitory.
10.6
CELL MEMBRANE POTENTIAL
(PAGE 365)
A cell membrane is usually polarized as a result of an
unequal distribution of ions on either side. Channels
in membranes that allow passage of some ions but not
others control ion distribution.
1. Distribution of ions
a. Membrane ion channels, formed by proteins, may
be always open or sometimes open and sometimes
closed.
b. Potassium ions pass more readily through resting
neuron cell membranes than do sodium and
calcium ions.
c. A high concentration of sodium ions is on the
outside of the membrane, and a high concentration
of potassium ions is on the inside.
2. Resting potential
a. Large numbers of negatively charged ions, which
cannot diffuse through the cell membrane, are
inside the cell.
b. In a resting cell, more positive ions leave the cell
than enter it, so the inside of the cell membrane
develops a negative charge with respect to the
outside.
3. Local potential changes
a. Stimulation of a membrane affects its resting
potential in a local region.
b. The membrane is depolarized if it becomes less
negative; it is hyperpolarized if it becomes more
negative.
c. Local potential changes are graded and subject to
summation.
d. Reaching threshold potential triggers an action
potential.
4. Action potentials
a. At threshold, sodium channels open and sodium
ions diffuse inward, depolarizing the membrane.
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