Sensory Physiology
195
Stimulus Intensity
How do we distinguish a strong stimulus from a weak one
when the information about both stimuli is relayed by action
potentials that are all the same size? The frequency of action
potentials in a single receptor is one way, because increased
stimulus strength means a larger receptor potential and more
frequent action potential fi ring (review Figure 7–2).
As the strength of a local stimulus increases, receptors on
adjacent branches of an afferent neuron are activated, result-
ing in a summation of their local currents.
Figure 7–5
shows
a record of an experiment in which increased stimulus inten-
sity to the receptors of a sensory unit is refl ected in increased
action potential frequency in its afferent neuron.
In addition to increasing the fi ring frequency in a single
afferent neuron, stronger stimuli usually affect a larger area
and activate similar receptors on the endings of
other
afferent
neurons. For example, when you touch a surface lightly with a
fi nger, the area of skin in contact with the surface is small, and
only the receptors in that skin area are stimulated. Pressing
down fi rmly increases the area of skin stimulated. This “call-
ing in” of receptors on additional afferent neurons is known as
recruitment.
Stimulus Location
A third type of information to be signaled is the location of
the stimulus—in other words, where the stimulus is being
applied. It should be noted that in vision, hearing, and smell,
stimulus location is interpreted as arising from the site from
which the stimulus originated rather than the place on our
body where the stimulus was actually applied. For example,
we interpret the sight and sound of a barking dog as occurring
in that furry thing on the other side of the fence rather than
in a specifi c region of our eyes and ears. We will have more to
say about this later; we deal here with the senses in which the
stimulus is located to a site on the body.
Stimulus location is coded by the site of a stimulated
receptor, as well as by the fact that action potentials from each
receptor travel along unique pathways to a specifi c region of
the CNS associated only with that particular modality and
body location. These distinct anatomical pathways are some-
times referred to as
labeled lines.
The precision, or
acuity,
with which we can locate and discern one stimulus from an
adjacent one depends upon the amount of convergence of
neuronal input in the specifi c ascending pathways: The greater
the convergence, the less the acuity. Other factors affecting
acuity are the size of the receptive fi eld covered by a single
sensory unit (
Figure 7–6a
), the density of sensory units, and
the amount of overlap in nearby receptive fi elds. For example,
it is easy to discriminate between two adjacent stimuli (two-
point discrimination) applied to the skin on your lips, where
the sensory units are small and numerous, but it is harder to
do so on the back, where the relatively few sensory units are
large and widely spaced (
Figure 7–6b
). Locating sensations
from internal organs is less precise than from the skin because
Action
potentials
Skin
Time
Pressure
(mmHg)
180
120
60
Glass probe
Afferent neuron
Figure 7–5
Action potentials from an afferent fi ber leading from the pressure receptors of a single sensory unit increase in frequency as branches of the
afferent neuron are stimulated by pressures of increasing magnitude.
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