192
Chapter 7
SECTION A
General Principles
A
sensory system
is a part of the nervous system that consists
of sensory receptor cells that receive stimuli from the exter-
nal or internal environment, the neural pathways that conduct
information from the receptors to the brain or spinal cord, and
those parts of the brain that deal primarily with processing the
information.
Information that a sensory system processes may or may
not lead to conscious awareness of the stimulus. For example,
while you would immediately notice a change when leaving
an air-conditioned house on a hot summer day, your blood
pressure can fl uctuate signifi cantly without your awareness.
Regardless of whether the information reaches consciousness,
it is called
sensory information.
If the information does reach
consciousness, it can also be called a
sensation.
A person’s
understanding of the sensation’s meaning is called
perception.
For example, feeling pain is a sensation, but awareness that a
tooth hurts is a perception. Sensations and perceptions occur
after the CNS modifi es or processes sensory information. This
processing can accentuate, dampen, or otherwise fi lter sensory
afferent information. At present we have little understanding of
the fi nal processing stages that cause patterns of action poten-
tials to become sensations or perceptions.
The initial step of sensory processing is the transfor-
mation of stimulus energy fi rst into graded potentials—the
receptor potentials—and then into action potentials in affer-
ent neurons. The pattern of action potentials in particular
neurons is a code that provides information about the world
even though, as is frequently the case with symbols, the action
potentials differ vastly from what they represent. Intuitively, it
might seem that sensory systems operate like familiar electrical
equipment, but this is true only up to a point. As an example,
compare telephone transmission with our auditory (hearing)
sensory system. The telephone changes sound waves into elec-
trical impulses, which are then transmitted along wires to the
receiver. Thus far the analogy holds. (Of course, the mecha-
nisms by which electrical currents and action potentials are
transmitted are quite different, but this does not affect our
argument.) The telephone receiver then changes the coded
electrical impulses back into sound waves. Here is the crucial
difference, for our brain does not physically translate the code
into sound. Instead, the coded information itself or some cor-
relate of it is what we
perceive
as sound.
Sensory Receptors
Information about the external world and about the body’s
internal environment exists in different forms—pressure, tem-
perature, light, odorants, sound waves, chemical concentra-
tion, and so on.
Sensory receptors
at the peripheral ends of
afferent neurons change this information into graded poten-
tials that can initiate action potentials, which travel into the
central nervous system. The receptors are either specialized
endings of afferent neurons (
Figure 7–1a
) or separate cells
that signal the afferent neurons by releasing chemical messen-
gers (
Figure 7–1b
).
To avoid confusion, recall from Chapter 5 that the term
receptor
has two completely different meanings. One meaning
is that of “sensory receptor,” as just defi ned. The second usage
is for the individual proteins in the plasma membrane or inside
the cell that bind specifi c chemical messengers, triggering an
intracellular signal transduction pathway that culminates in
the cell’s response. The potential confusion between these two
meanings is magnifi ed by the fact that the stimuli for some
sensory receptors (e.g., those involved in taste and smell) are
chemicals that bind to receptor proteins in the plasma mem-
brane of the sensory receptor. If you are in doubt as to which
meaning is intended, add the adjective “sensory” or “protein”
to see which makes sense in the context.
To repeat, regardless of the original form of the signal
that activates sensory receptors, the information must be trans-
lated into the language of graded potentials or action poten-
tials. The energy or chemical that impinges upon and activates
a sensory receptor is known as a
stimulus.
The process by
which a stimulus—a photon of light, say, or the mechanical
stretch of a tissue—is transformed into an electrical response
is known as
sensory transduction.
There are many types of sensory receptors, each of which
responds much more readily to one form of stimulus than to
others. The type of stimulus to which a particular receptor
responds in normal functioning is known as its
adequate
stimulus.
In addition, within the general stimulus type that
serves as a receptor’s adequate stimulus, a particular receptor
may respond best (i.e., at lowest threshold) to only a very nar-
row range of stimulus energies. For example, different indi-
vidual receptors in the eye respond best to light (the adequate
stimulus) of different wavelengths.
T
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TC
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S
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NS
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Stimulus
energy
Receptor
cell
Vesicle containing
chemical messenger
Afferent
neuron
Receptor
membrane
(b)
(a)
Figure 7–1
Schematic diagram of two types of sensory receptors. The sensitive
membrane region that responds to a stimulus is either (a) an ending
of an afferent neuron or (b) on a separate cell adjacent to an afferent
neuron. Ion channels (shown in purple) on the receptor membrane
alter ion fl
ux and initiate stimulus transduction.
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