198
Chapter 7
around the pencil tip is also indented, activating mechanore-
ceptors within this region (
Figure 7–10
). Exact localization
is possible because lateral inhibition removes the information
from the peripheral regions.
Lateral inhibition is utilized to the greatest degree in
the pathways providing the most accurate localization. For
example, skin hair movements, which we can locate quite well,
activate pathways that have signifi cant lateral inhibition, but
temperature and pain, which we can locate relatively poorly,
activate pathways that use lateral inhibition to a lesser degree.
Lateral inhibition is essential for retinal processing, where it
enhances visual acuity.
Stimulus Duration
Receptors differ in the way they respond to a constantly
maintained stimulus. The action potential frequency at the
beginning of the stimulus generally indicates the stimulus
strength, but after this initial response, the frequency differs
widely in different types of receptors. As
Figure 7–11
shows,
some receptors respond very rapidly at the stimulus onset,
but, after their initial burst of activity, fi re only very slowly or
stop fi
ring altogether during the remainder of the stimulus.
These are the
rapidly adapting receptors.
The rapid adap-
tation of these receptors codes for a restricted response in
time to a stimulus, and they are important in signaling rapid
change (e.g., vibrating or moving stimuli). Some receptors
adapt so rapidly that they fi re only a single action potential
at the onset of a stimulus—a so-called “on response”—while
others respond at the beginning of the stimulus and again at
its removal—so-called “on-off responses.” The rapid fading of
the sensation of clothes pressing on your skin is due to rapidly
adapting receptors.
Slowly adapting receptors
maintain their response
at or near the initial level of fi
ring regardless of the stimulus
duration (see Figure 7–11). These receptors signal slow changes
or prolonged events, such as those that occur in the joint and
muscle receptors that participate in the maintenance of upright
posture when you stand or sit for long periods of time.
Central Control of Afferent Information
All sensory signals are subject to extensive modifi cation at the
various synapses along the sensory pathways before they reach
higher levels of the central nervous system. Inhibition from
collaterals from other ascending neurons (e.g., lateral inhibi-
tion) reduces or even abolishes much of the incoming infor-
mation, as do pathways descending from higher centers in the
brain. The reticular formation and cerebral cortex, in particu-
lar, control the input of afferent information via descending
pathways. The inhibitory controls may be exerted directly by
synapses on the axon terminals of the primary afferent neu-
rons (an example of presynaptic inhibition) or indirectly via
interneurons that affect other neurons in the sensory pathways
(
Figure 7–12
).
In some cases (e.g., in the pain pathways), the afferent
input is continuously inhibited to some degree. This provides
the fl exibility of either removing the inhibition, so as to allow
a greater degree of signal transmission, or increasing the inhi-
bition, so as to block the signal more completely.
Neural Pathways in Sensory Systems
The afferent sensory neurons form the fi rst link in a chain
consisting of three or more neurons connected end to end by
synapses. A bundle of parallel, three-neuron chains together
form a
sensory pathway.
The chains in a given pathway run
parallel to each other in the central nervous system and, with
one exception, carry information to the part of the cerebral
Area of receptor activation
Skin
Inhibition
Area of inhibition
of afferent information
Excitation
Effect on action
potential frequency
Area of
excitation
Without
lateral
inhibition
Area of
sensation
With
lateral
inhibition
Figure 7–10
A pencil tip pressed against the skin activates receptors under the
pencil tip and in the adjacent tissue. The sensory unit under the tip
inhibits additional stimulated units at the edge of the stimulated
area. Lateral inhibition produces a central area of excitation
surrounded by an area where the afferent information is inhibited.
The sensation is localized to a more restricted region than that in
which all three units are actually stimulated.
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