Chapter 13
The Peripheral Nervous System and Reflex Activity
489
13
difficult because pain is an intensely personal experience that
cannot be measured objectively.
Pain receptors are activated by extremes of pressure and tem-
perature as well as a veritable soup of chemicals released from
injured tissue. Histamine, K
1
, ATP, acids, and bradykinin are
among the most potent pain-producing chemicals. All of these
chemicals act on small-diameter fibers.
When you cut your finger, you may have noticed that you
first felt a sharp pain followed some time later by burning or
aching. Sharp pain is carried by the smallest of the myelinated
sensory fibers, the A delta fibers, while burning pain is car-
ried more slowly by small nonmyelinated C fibers. Both types
of fibers release the neurotransmitters
glutamate
and
substance
P
, which activate second-order sensory neurons. Axons from
these second-order neurons ascend to the brain via the spino-
thalamic tract and other pathways.
If you cut your finger while fighting off an attacker, you might
not notice the cut at all. How can that be? ±e brain has its own
pain-suppressing analgesic systems in which the endogenous
opioids such as
endorphins
and
enkephalins
(p. 417) play a key
role. Various nuclei in the brain stem, including the periaque-
ductal gray matter of the midbrain, relay descending cortical
and hypothalamic pain-suppressing signals. Descending fibers
activate interneurons in the spinal cord, which release enkepha-
lins. Enkephalins are inhibitory neurotransmitters that quash
the pain signals generated by nociceptors.
Pain Tolerance
We all have the same
pain threshold
—that is, we begin to per-
ceive pain at roughly the same stimulus intensity. However, our
tolerance to pain varies widely. When we say that someone is
“sensitive” to pain, we mean that the person has a low
pain toler-
ance
rather than a low pain threshold.
A number of genes help determine a person’s pain tolerance
and response to pain medications. ±e genetics of pain is cur-
rently an area of intense research, aimed at allowing an indi-
vidual’s genes to determine the best pain treatment.
Homeostatic Imbalance
13.1
Normally the body maintains a steady state that correlates in-
jury and pain. Long-lasting or very intense pain inputs, such
as limb amputation, can disrupt this system, leading to
hyper-
algesia
(pain amplification), chronic pain, and
phantom limb
pain
. Intense or long-lasting pain activates
NMDA receptors
,
the same receptors that strengthen neural connections during
certain kinds of learning. Essentially, the spinal cord
learns
hy-
peralgesia. In light of this, it is crucial that health professionals
effectively manage pain early to prevent chronic pain from be-
coming established.
Phantom limb pain
(pain perceived in tissue that is no longer
present) is a curious example of hyperalgesia. Until recently,
surgical limb amputations were conducted under general an-
esthesia only and the spinal cord still experienced the pain of
amputation. Epidural anesthetics block neurotransmission in
the spinal cord, and using them during surgery greatly reduces
the incidence of phantom limb pain.
on the nature of the message (which is, aFer all, just an action
potential). Each sensory fiber is analogous to a “labeled line”
that tells the brain “who” is calling—a taste bud or a pressure
receptor—and from “where.” ±e brain always interprets the
activity of a specific sensory receptor (“who”) as a specific sensa-
tion, no matter how it is activated.
²or example, pressing on your eyeball activates photorecep-
tors, but what you “see” is light. ±e exact point in the cortex
that is activated always refers to the same “where,” regardless of
how it is activated, a phenomenon called
projection
. Electri-
cally stimulating a particular spot in the visual cortex causes you
to “see” light in a particular place.
Let’s examine the major features of sensory perception.
Perceptual detection
is the ability to detect that a stimulus
has occurred. ±is is the simplest level of perception. As a
general rule, inputs from several receptors must be summed
for perceptual detection to occur.
Magnitude estimation
is the ability to detect how
intense
the
stimulus is. Perceived intensity increases as stimulus inten-
sity increases because of frequency coding (see ²igure 11.13).
Spatial discrimination
allows us to identify the site or pat-
tern of stimulation. A common tool for studying this quality
in the laboratory is the
two-point discrimination
test. ±e
test determines how close together two points on the skin
can be and still be perceived as two points rather than as one.
±is test provides a crude map of the density of tactile recep-
tors in the various regions of the skin. ±e distance between
perceived points varies from less than 5 mm on highly sensi-
tive body areas (tip of the tongue) to more than 50 mm on
less sensitive areas (the back).
Feature abstraction
is the mechanism by which a neuron or
circuit is tuned to one feature, or property, of a stimulus in
preference to others. Sensation usually involves an interplay
of several stimulus features.
²or example, one touch tells us that velvet is warm, compress-
ible, and smooth but not completely continuous, each a feature
that contributes to our perception of “velvet.” ²eature abstrac-
tion enables us to identify more complex aspects of a sensation.
Quality discrimination
is the ability to differentiate the sub-
modalities of a particular sensation. Each sensory modality has
several
qualities
, or submodalities. ²or example, taste is a sen-
sory modality and its submodalities include sweet and bitter.
Pattern recognition
is the ability to take in the scene around
us and recognize a familiar pattern, an unfamiliar one, or one
that has special significance for us. ²or example, we can look
at an image made of dots and recognize it as the portrait of a
familiar face. We can listen to music and hear a melody, not
just a string of notes.
Perception of Pain
Everyone has suffered pain—the cruel persistence of a head-
ache, the smart of a bee sting or a cut finger. Although we may
not appreciate it at the time, pain is invaluable because it warns
us of actual or impending tissue damage and strongly motivates
us to take protective action. Managing a patient’s pain can be
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