206
Chapter 7
action potentials in the afferent neuron, a series of changes
occur in components of the pain pathway—including the
ion channels in the nociceptors themselves—that alter the
way these components respond to subsequent stimuli. Both
increased and decreased sensitivity to painful stimuli can
occur. When these changes result in an increased sensitivity to
painful stimuli, known as
hyperalgesia,
the pain can last for
hours after the original stimulus is over. Thus, the pain experi-
enced in response to stimuli occurring even a short time after
the original stimulus (and the reactions to that pain) can be
more intense than the initial pain. Moreover, probably more
than any other type of sensation, pain can be altered by past
experiences, suggestion, emotions (particularly anxiety), and
the simultaneous activation of other sensory modalities. Thus,
the level of pain experienced is not solely a physical property of
the stimulus.
Analgesia
is the selective suppression of pain with-
out effects on consciousness or other sensations. Electrical
stimulation of specifi c areas of the central nervous system
can produce a profound reduction in pain, a phenomenon
called
stimulation-produced analgesia,
by inhibiting pain
pathways. This occurs because descending pathways that
originate in these brain areas selectively inhibit the trans-
mission of information originating in nociceptors (
Figure
7–17b
). The descending axons end at lower brainstem and
spinal levels on interneurons in the pain pathways as well as
on the synaptic terminals of the afferent nociceptor neurons
themselves. Some of the neurons in these inhibitory path-
ways release morphine-like endogenous opioids (Chapter 6).
These opioids inhibit the propagation of input through the
higher levels of the pain system. Thus, infusion of morphine
can provide relief in many cases of intractable pain by bind-
ing to and activating opioid receptors at the level of entry of
the active nociceptor fi
bers. This is separate from morphine’s
effect on the brain.
The body’s endogenous-opioid systems also mediate
other phenomena known to relieve pain. In recent clinical
studies, 55 to 85 percent of patients experienced pain relief
when treated with
acupuncture,
an ancient Chinese ther-
apy involving the insertion of needles into specifi c locations
on the skin. This success rate was similar to that seen when
patients were treated with morphine (70 percent). In studies
comparing morphine versus a
placebo
(injections of sugar that
patients
thought
was the drug), 35 percent of those receiving
the placebo experienced pain relief. Acupuncture is thought to
activate afferent neurons leading to spinal cord and midbrain
centers that release endogenous opioids and other neurotrans-
mitters implicated in pain relief.
It seems likely that pathways
descending from the cortex activate those same regions to exert
the placebo effect. Thus, exploiting the body’s built-in analge-
sia mechanisms can be an effective means of controlling pain.
Also of use for lessening pain is
transcutaneous electric
nerve stimulation (TENS),
in which the painful site itself or
the nerves leading from it are stimulated by electrodes placed
on the surface of the skin. TENS works because the stimula-
tion of nonpain, low-threshold afferent fi bers (e.g., the fi bers
from touch receptors) leads to the inhibition of neurons in the
pain pathways. You perform a low-tech version of this phe-
nomenon when you vigorously rub your scalp at the site of a
painful bump on the head.
Neural Pathways of the Somatosensory System
After entering the central nervous system, the afferent nerve
fi bers from the somatic receptors synapse on neurons that
form the specifi c ascending pathways going primarily to the
somatosensory cortex via the brainstem and thalamus. They
also synapse on interneurons that give rise to the nonspe-
cifi c ascending pathways. There are two major somatosen-
sory pathways (there are different pathways for sensory input
from the face). These pathways are organized differently
from each other in the spinal cord and brain (
Figure 7–19
).
The ascending
anterolateral pathway,
also called the spi-
nothalamic pathway, makes its fi rst synapse between the
sensory receptor neuron and a second neuron located in the
gray matter of the spinal cord (
Figure 7–19a
). This second
neuron crosses the opposite side and projects through the
anterolateral column of the cord to the thalamus, where it
synapses on cortically projecting neurons. The anterolateral
pathway processes pain and temperature information. The
second major pathway for somatic sensation is the
dorsal
column pathway
(
Figure 7–19b
). This, too, is named for
the section of white matter (the dorsal columns) through
which the sensory receptor neurons project to the brainstem,
where the fi rst synapse occurs. As in the anterolateral path-
way, the second synapse is in the thalamus, from which pro-
jections are sent to the somatosensory cortex.
Note that the pathways cross from the side where the
afferent neurons enter the central nervous system to the
opposite side either in the spinal cord (anterolateral system)
or in the brainstem (dorsal column system). Thus, sensory
pathways from somatic receptors on the left side of the body
terminate in the somatosensory cortex of the right cerebral
hemisphere.
In the somatosensory cortex, the endings of the axons
of the specifi c somatic pathways are grouped according to
the peripheral location of the receptors that give input to the
pathways (
Figure 7–20
). The parts of the body that are most
densely innervated—fi ngers, thumb, and lips—are repre-
sented by the largest areas of the somatosensory cortex. There
are qualifi cations, however, to this seemingly precise picture.
There is considerable overlap of the body part representations,
and the sizes of the areas can change with sensory experience.
The phantom limb phenomenon described in the fi rst sec-
tion of this chapter provides a good example of the dynamic
nature of the somatosensory cortex.
Studies of amputees have
shown that cortical areas formerly responsible for a missing
arm and hand are commonly “re-wired” to respond to sensory
inputs originating in the face (note the proximity of the corti-
cal regions representing these areas in Figure 7–20). As the
somatosensory cortex undergoes this reorganization, a touch
on a person’s cheek is often perceived as a touch on his or her
missing arm.
previous page 234 Vander's Human Physiology The Mechanisms of Body Function read online next page 236 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off