Sensory Physiology
197
applied to the center at A or that a strong stimulus was applied
to the periphery at B. Thus, neither the intensity nor the loca-
tion of the stimulus can be detected precisely with a single
afferent neuron.
Since the receptor endings of different afferent neurons
overlap, however, a stimulus will trigger activity in more than
one sensory unit. In
Figure 7–8
, neurons A and C, stimu-
lated near the edges of their receptive fi elds where the receptor
density is low, fi re action potentials less frequently than does
neuron B, stimulated at the center of its receptive fi eld. A high
action potential frequency in neuron B occurring simultane-
ously with lower frequencies in A and C provides the brain
Skin
Stimulus
Central nervous system
ABC
Afferent neuron
Action potential frequency
ABC
Figure 7–8
A stimulus point falls within the overlapping receptive
fi elds of three afferent neurons. Note the difference in
receptor response (i.e., the action potential frequency
in the three neurons) due to the difference in receptor
distribution under the stimulus (fewer receptor endings
for A and C than for B).
+
+
+
+
+
+
+
Action potentials
in postsynaptic cell
Postsynaptic
cell
Axons of
afferent neurons
Action potentials
in afferent neuron
Excitatory synapses
Inhibitory synapses
Key
AB
C
Figure 7–9
Afferent pathways showing lateral inhibition. Three sensory units have
overlapping receptive fi elds. Because the central fi ber B at the beginning of
the pathway (bottom of fi
gure) is fi ring at the highest frequency, it inhibits
the lateral neurons (via inhibitory interneurons) to a greater extent than the
lateral neurons inhibit the central pathway.
with a more accurate localization of the stimulus near the cen-
ter of neuron B’s receptive fi
eld. Once this location is known,
the brain can use the fi ring frequency of neuron B to deter-
mine stimulus intensity.
Lateral Inhibition
The phenomenon of
lateral inhibition
is the most important
mechanism enabling the localization of a stimulus site. In lateral
inhibition, information from afferent neurons whose receptors
are at the edge of a stimulus is strongly inhibited compared to
information from the stimulus’s center.
Figure 7–9
shows one
neuronal arrangement that accomplishes lateral inhibition.
The afferent neuron in the center (B) has a higher initial fi r-
ing frequency than the neurons on either side (A and C).
The
number of action potentials transmitted in the lateral pathways
is further decreased by inhibitory inputs from inhibitory inter-
neurons stimulated by the central neuron. While the lateral
afferent neurons (A and C) also exert inhibition on the central
pathway, their lower initial fi ring frequency has less of an effect.
Thus, lateral inhibition enhances the
contrast
between the
center and periphery of a stimulated region, thereby increasing
the brain’s ability to localize a sensory input. Lateral inhibi-
tion can occur at different levels in the sensory pathways but
typically happens at an early stage.
Lateral inhibition can be demonstrated by pressing the
tip of a pencil against your fi nger. With your eyes closed, you
can localize the pencil point precisely, even though the region
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