Neuronal Signaling and the Structure of the Nervous System
159
g. In myelinated nerve fi bers, APs manifest saltatory conduction.
h. APs can be triggered by depolarizing graded potentials in
sensory neurons, at synapses, or in some cells by pacemaker
potentials.
Additional Clinical Examples
I. Multiple sclerosis is an autoimmune disease in which
antibodies attack the myelin sheaths surrounding neurons.
The resulting failure of action potential propagation causes
neurological disability that can vary in severity and rate of
progression. The causes of MS are currently unknown and
there is no cure, but drugs that suppress immune system
function slow the progression of the disease.
SECTION B KEY TERMS
absolute refractory period
154
action potential
151
action potential
propagation
155
after-hyperpolarization
152
all-or-none
154
current
144
decremental
150
depolarized
149
electrical potential
144
electrogenic pump
148
equilibrium potential
146
excitability
151
excitable membrane
151
Goldman-Hodgkin-Katz
(GHK) equation
147
graded potential
149
hyperpolarized
149
inactivation gate
152
leak potassium channels
147
ligand-gated channels
151
mechanically gated
channels
151
negative feedback
152
Nernst equation
146
Ohm’s law
144
overshoot
149
pacemaker potential
157
positive feedback
152
potential
144
potential difference
144
receptor potential
156
relative refractory period
155
repolarizing
149
resistance
144
resting membrane potential
144
saltatory conduction
155
subthreshold potential
153
subthreshold stimulus
153
summation
151
synaptic potential
156
threshold potential
152
threshold stimulus
153
voltage-gated channels
152
SECTION B CLINICAL TERMS
local anesthetics
154
tetrodotoxin
154
multiple sclerosis
155
Xylocaine
®
154
Novocaine
®
154
SECTION B REVIEW QUESTIONS
1. Describe how negative and positive charges interact.
2. Contrast the abilities of intracellular and extracellular fl
uids
and membrane lipids to conduct electrical current.
3. Draw a simple cell; indicate where the concentrations of Na
+
,
K
+
, and Cl
are high and low and the electrical potential
difference across the membrane when the cell is at rest.
4. Explain the conditions that give rise to the resting membrane
potential. What effect does membrane permeability have on this
potential? What role do Na
+
/K
+
-ATPase membrane pumps play
in the membrane potential? Is this role direct or indirect?
5. Which two factors involving ion diffusion determine the
magnitude of the resting membrane potential?
6. Explain why the resting membrane potential is not equal to the
potassium equilibrium potential.
7. Draw a graded potential and an action potential on a graph
of membrane potential versus time. Indicate zero membrane
potential, resting membrane potential, and threshold potential;
indicate when the membrane is depolarized, repolarizing, and
hyperpolarized.
8. List the differences between graded potentials and action
potentials.
9. Describe how ion movement generates the action potential.
10. What determines the activity of the voltage-gated sodium
channel?
11. Explain threshold and the relative and absolute refractory
periods in terms of the ionic basis of the action potential.
12. Describe the propagation of an action potential. Contrast this
event in myelinated and unmyelinated axons.
13. List three ways in which action potentials can be initiated in
neurons.
SECTION C
Synapses
As defi ned earlier, a synapse is an anatomically specialized junc-
tion between two neurons, at which the electrical activity in a
presynaptic neuron infl uences the electrical activity of a post-
synaptic neuron. Anatomically, synapses include parts of the
presynaptic and postsynaptic neurons and the extracellular space
between these two cells. According to the latest estimate, there
are approximately 10
14
(100 trillion!) synapses in the CNS.
Activity at synapses can increase or decrease the likelihood
that the postsynaptic neuron will fi re action potentials by produc-
ing a brief, graded potential in the postsynaptic membrane. The
membrane potential of a postsynaptic neuron is brought closer
to threshold (i.e., depolarized) at an
excitatory synapse,
and it
is either driven farther from threshold (i.e., hyperpolarized) or
stabilized at its resting potential at an
inhibitory synapse.
Hundreds or thousands of synapses from many different
presynaptic cells can affect a single postsynaptic cell
(conver-
gence),
and a single presynaptic cell can send branches to affect
many other postsynaptic cells (
divergence,
Figure 6–24
).
Convergence allows information from many sources to infl u-
ence a cell’s activity; divergence allows one information source
to affect multiple pathways.
The level of excitability of a postsynaptic cell at any
moment (i.e., how close its membrane potential is to thresh-
old) depends on the number of synapses active at any one time
and the number that are excitatory or inhibitory. If the mem-
brane of the postsynaptic neuron reaches threshold, it will
generate action potentials that are propagated along its axon
to the terminal branches, which in turn infl uence the excit-
ability of other cells.
Functional Anatomy of Synapses
There are two types of synapses:
electrical
and
chemical.
At
electrical synapses, the plasma membranes of the pre- and post-
synaptic cells are joined by gap junctions (Chapter 3). These
allow the local currents resulting from arriving action potentials
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