194
Chapter 7
However, if the amount of depolarization at the fi rst node in the
afferent neuron is large enough to bring the membrane there
to threshold, action potentials are initiated, which then propa-
gate along the nerve fi ber (see Figure 7–2). The only function
of the graded potential is to trigger action potentials. (See
Figure 6–16 to review the properties of graded potentials.)
As long as the receptor potential keeps the afferent neu-
ron depolarized to a level at or above threshold, action poten-
tials continue to fi re and propagate along the afferent neuron.
Moreover, an increase in the graded potential magnitude
causes an increase in the action potential frequency in the affer-
ent neuron (up to the limit imposed by the neuron’s refractory
period) and an increase in neurotransmitter release at the affer-
ent neuron’s central axon terminal (see Figure 7–2). Although
the graded potential magnitude determines action potential
frequency,
it does not determine action potential
magnitude.
The action potential is “all-or-none,” meaning that its magni-
tude is independent of the strength of the initiating stimulus.
Factors that control the magnitude of the receptor potential
include stimulus strength, rate of change of stimulus strength,
temporal summation of successive receptor potentials (see
Figure 6–31), and a process called
adaptation.
This last process
is a decrease in receptor sensitivity, which results in a decrease in
action potential frequency in an afferent neuron despite a stim-
ulus of constant strength (
Figure 7–3
). Degrees of adaptation
vary widely among different types of sensory receptors.
Primary Sensory Coding
Converting stimulus energy into a signal that conveys the
relevant sensory information to the central nervous system is
termed
coding.
Important characteristics of a stimulus include
the type of energy it represents, its intensity, and the location
of the body it affects. Coding begins at the receptive neurons
in the peripheral nervous system.
A single afferent neuron with all its receptor endings
makes up a
sensory unit.
In a few cases, the afferent neuron
has a single receptor, but generally the peripheral end of an
afferent neuron divides into many fi ne branches, each termi-
nating with a receptor.
The area of the body that, when stimulated, leads to
activity in a particular afferent neuron is called the
receptive
eld
for that neuron (
Figure 7–4
). Receptive fi elds of neigh-
boring afferent neurons usually overlap so that stimulation of
a single point activates several sensory units. Thus, activation
at a single sensory unit almost never occurs. As we will see,
the degree of overlap varies in different parts of the body.
Stimulus Type
Another term for stimulus type (heat, cold, sound, or pressure,
for example) is stimulus
modality.
Modalities can be divided
into submodalities: Cold and warm are submodalities of tem-
perature, whereas salty, sweet, bitter, and sour are submodali-
ties of taste. The type of sensory receptor a stimulus activates
plays the primary role in coding the stimulus modality.
As mentioned earlier, a given receptor type is particularly
sensitive to one stimulus modality—the adequate stimulus—
because of the signal transduction mechanisms and ion channels
incorporated in the receptor’s plasma membrane. For example,
receptors for vision contain pigment molecules whose shape
is transformed by light. These receptors also have intracellular
mechanisms that cause changes in the pigment molecules to alter
the activity of membrane ion channels and generate a receptor
potential. In contrast, receptors in the skin do not have light-
sensitive pigment molecules, so they cannot respond to light.
All the receptors of a single afferent neuron are preferen-
tially sensitive to the same type of stimulus; for example, they
are all sensitive to cold or all to pressure. Adjacent sensory
units, however, may be sensitive to different types of stimuli.
Because the receptive fi elds for different modalities overlap, a
single stimulus, such as an ice cube on the skin, can simulta-
neously give rise to the sensations of touch and temperature.
Stimulus on
Stimulus off
Action
potential
response
Figure 7–3
Action potentials in a single afferent nerve fi ber showing adaptation
to a stimulus of constant strength. Note that the frequency of action
potentials decreases even before the stimulus is turned off.
Central nervous system
Neuron cell body
Central
terminals
Central
process
Peripheral
process
Afferent
neuron
axon
Receptive field
Peripheral terminals
with receptors
Skin
Figure 7–4
A sensory unit including the location of sensory receptors, the
processes reaching peripherally and centrally from the cell body, and
the terminals in the CNS. Also shown is the receptive fi eld of this
neuron.
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