372
UNIT THREE
P
ainful muscle cramps, convulsions, paraly-
sis, and anesthesia can each result from
changes in the permeability of axons to
particular ions. A number of substances alter
axon membrane permeability to ions.
Calcium ions are required to close sodium
channels in axon membranes during an action
potential. If calcium is deficient, then sodium
channels remain open, and sodium ions dif-
fuse through the membrane continually so that
impulses are transmitted repeatedly. If these
spontaneous impulses travel along axons to
skeletal muscle F
bers, the muscles continuously
spasm (tetanus or tetany). This can happen to
women during pregnancy as the developing
fetus uses maternal calcium. Tetanic contraction
may also occur when the diet lacks calcium or
vitamin D or when prolonged diarrhea depletes
the body of calcium.
A small increase in the concentration of
extracellular potassium ions causes the rest-
ing potential of nerve F
bers to be less negative
(partially depolarized). As a result, the threshold
potential is reached with a less intense stimulus
than usual. The a±
ected F
bers are excitable, and
the person may experience convulsions.
If the extracellular potassium ion concentra-
tion is greatly decreased, the resting potentials
of the nerve F
bers may become so negative that
action potentials are not generated. In this case,
impulses are not triggered, and muscles become
paralyzed.
Certain anesthetic drugs, such as procaine,
decrease membrane permeability to sodium ions.
In the tissue fluids surrounding an axon, these
drugs prevent impulses from passing through
the affected region. Consequently, the drugs
keep impulses from reaching the brain, prevent-
ing perception of touch and pain.
10.3
CLINICAL APPLICATION
Factors Affecting Impulse Conduction
Neurotransmitters
The nervous system produces at least thirty different types
of neurotransmitters. Some neurons release only one type
of neurotransmitter; others produce two or three types.
Neurotransmitters include
acetylcholine,
which stimulates
skeletal muscle contractions (see chapter 9, p. 290); a group
If a different neurotransmitter binds other receptors and
increases membrane permeability to potassium ions, these
ions diffuse outward, hyperpolarizing the membrane. An
action potential is now less likely to occur, so this change
is called an
inhibitory postsynaptic potential
(IPSP). Some
inhibitory neurotransmitters open chloride ion channels. In
this case, if sodium ions enter the cell, negative chloride ions
are free to follow, opposing the depolarization.
In the brain and spinal cord, each neuron may receive
the synaptic knobs of a thousand or more axons on its
dendrites and cell body
(fig. 10.20)
. Furthermore, at any
moment, some of the postsynaptic potentials are excitatory
on a particular neuron, while others are inhibitory.
The integrated sum of the EPSPs and IPSPs determines
whether an action potential results. If the net effect is more
excitatory than inhibitory, threshold may be reached and
an action potential triggered. Conversely, if the net effect is
inhibitory, no impulse is transmitted.
Summation of the excitatory and inhibitory effects of the
postsynaptic potentials commonly takes place at the trigger
zone, usually in a proximal region of the axon, but found
also in the distal peripheral process of some sensory neu-
rons. This region has an especially low threshold for trigger-
ing an action potential; thus, it serves as a decision-making
part of the neuron.
PRACTICE
14
Describe a synapse.
15
Explain the function of a neurotransmitter.
16
Distinguish between an EPSP and an IPSP.
17
Describe the net e±
ects of EPSPs and IPSPs.
Neuron
cell body
Nucleus
Presynaptic
knob
Presynaptic
axon
FIGURE 10.20
The synaptic knobs of many axons may
communicate with the cell body of a neuron.
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