272 Chapter 10
of the neuron. Changes in the firing rate of primary sensory
neurons affect the firing of second- and third-order neurons,
which in turn affects the firing of those neurons in the post-
central gyrus that receive input from the third-order neurons.
Indirectly, therefore, neurons in the postcentral gyrus can be
said to have receptive fields in the skin.
The area of each receptive field in the skin depends on the
density of receptors in the region. In the back and legs, where
a large area of skin is served by relatively few sensory endings,
the receptive field of each neuron is correspondingly large. In
the fingertips, where a large number of cutaneous receptors
serve a small area of skin, the receptive field of each sensory
neuron is correspondingly small.
The greater the number of sensory receptors serving an
area of the body, and the correspondingly smaller the receptive
field of each, the greater will be the sensory acuity (sharpness
of sensation) from that area. Two separate points of touch can
be either resolved or blurred together, depending on the density
of receptors and the sizes of their receptive fields. Also, resolu-
tion is improved by less convergence of sensory information
on higher-order neurons as the sensory information is transmit-
ted to the brain for perception.transmitted by C fibers. These are the slowest-conducting sen-
sory axons because they are the thinnest and are unmyelinated.
The sensory neurons in the dorsal roots of spinal nerves pro-
ject to the dorsal horn of the spinal cord, which is organized
into laminae (layers). Neurons in lamina 1 receive information
regarding noxious stimuli arriving in A-delta and C fibers. Neu-
rons in deeper layers receive information from both noxious
and non-noxious stimuli, whereas neurons in the deepest layer,
lamina V, receive information from all the cutaneous senses. It
is likely that somatic and visceral sensory information converge
in this layer and contribute to referred pain (discussed shortly).
The primary sensory afferents synapse with second-order
association neurons that project to the thalamus. Their axons
cross over to the contralateral side and ascend to the brain in
the lateral spinothalamic tracts. Fibers that mediate touch and
pressure ascend in the anterior spinothalamic tract. Fibers of
both spinothalamic tracts synapse with third-order neurons in the
thalamus, which, in turn, project to the post-central gyrus. Notice
that somatesthetic information is always carried to the postcen-
tral gyrus in third-order neurons. Also, because of crossing-over,
somatesthetic information from each side of the body is projected
to the postcentral gyrus of the contralateral cerebral hemisphere.
Because all somatesthetic information from the same area
of the body projects to the same area of the somatosensory cor-
tex, a “map” of the body can be drawn on the postcentral gyrus
to represent sensory projection points (see fig. 8.7). This map
is distorted, however, because it shows larger areas of cortex
devoted to sensation in the face and hands than in other areas
in the body. This disproportionately large area of the cortex
devoted to the face and hands reflects the higher density of sen-
sory receptors in these regions.
The impulses from nociceptors that are delivered to the
somatosensory cortex provide information about the body loca-
tion and intensity of pain. However, the emotional component
of pain—the sense of “hurt”—is probably a result of impulses
projecting from the thalamus to the cingulate gyrus, particu-
larly the anterior cingulate gyrus (see chapter 8, fig. 8.18). The
cingulate gyrus is a part of the limbic system, a group of brain
structures involved in emotion.
Pain that is felt in a somatic location (such as the left arm)
may not be the result of nociceptor stimulation in that body
region, but may instead be the result of damage to an internal
organ (such as the heart). This is a referred pain (the specific
example given is known as angina pectoris ). Another example
of a referred pain is when pain in the back, under the right
scapula, is caused by a gallstone when the gallbladder con-
tracts. Referred pains are believed to result because visceral
sensory and somatic sensory neurons can synapse on the same
interneurons in the spinal cord. These, in turn, project to the
thalamus and from there to the particular somatic location (the
left arm, for example) on the somatosensory cortex.
Receptive Fields and Sensory Acuity
The receptive field of a neuron serving cutaneous sensation is
the area of skin that, when stimulated, changes the firing rate
CLINICAL APPLICATION
The phantom limb phenomenon is common in amputees,
who may experience complete sensations from the missing
limbs. These sensations are sometimes useful—for example,
in fitting prostheses into which the phantom has seemingly
entered. However, pain in the phantom is experienced by
about 70% of amputees, and the pain can be severe and per-
sistent. One older explanation for phantom limbs is that the
nerves remaining in the stump can grow into inflamed nodules
called neuromas, and these may generate nerve impulses that
are transmitted to the brain and interpreted as arising from the
missing limb. However, a phantom limb may occur in cases
where the limb has not been amputated, but the nerves that
normally enter from the limb have been severed. Or it may
occur in individuals with spinal cord injuries above the level
of the limb, so that sensations from the limb do not enter the
brain. Current theories propose that the phantom may be pro-
duced by brain reorganization caused by the absence of the
sensations that would normally arise from the missing limb.
Such brain reorganization has been demonstrated, but has
not satisfactorily explained the origin of phantom pain.Two-Point Touch Threshold
The approximate size of the receptive fields serving light touch
can be measured by the two-point touch threshold test. In
this procedure, the two points of a pair of calipers are lightly
touched to the skin at the same time. If the distance between
the points is sufficiently great, each point will stimulate a