Ganong's Review of Medical Physiology, 23rd Edition

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FIGURE 11–3 Brain areas concerned with somatic sensation,
and some of the cortical receiving areas for other sensory
modalities in the human brain. The numbers are those of Brodmann’s
cortical areas. The primary auditory area is actually located in
the sylvian fissure on the top of the superior temporal gyrus and is not
normally visible in a lateral view of the cortex.
(Figure 11–3). Not only is there detailed localization of the
fibers from the various parts of the body in the postcentral
gyrus, but also the size of the cortical receiving area for
impulses from a particular part of the body is proportionate to
the use of the part. The relative sizes of the cortical receiving
areas are shown dramatically in Figure 11–4, in which the proportions
of the homunculus have been distorted to correspond
to the size of the cortical receiving areas for each. Note that the
cortical areas for sensation from the trunk and back are small,
whereas very large areas are concerned with impulses from the
hand and the parts of the mouth concerned with speech.
FIGURE 11–4 Sensory homunculus, drawn overlying a
coronal section through the postcentral gyrus. Gen., genitalia.
(Reproduced, with permission, from Penfield W, Rasmussen G: The Cerebral Cortex of
Man. Macmillan, 1950.)
8 6
Sl
4
Lips
Sll
Nose
Face
Tongue
Thumb
Eye
Upper lip
Ring
Middle
Index
Pharynx
Intraabdominal
Hand
Face
Tongue
Auditory
Little
Lower lip
Teeth, gums, and jaw
Hand
Wrist
Trunk
Forearm
Elbow
3 1 2
Arm
Shoulder
Head
Neck
Trunk
Hip
Leg
Foot
Toes
Gen.
Posterior
parietal
cortex
Visual
CHAPTER 11 Somatosensory Pathways 175
Studies of the sensory receiving area emphasize the very discrete
nature of the point-for-point localization of peripheral
areas in the cortex and provide further evidence for the general
validity of the law of specific nerve energies (see Chapter 8).
Stimulation of the various parts of the postcentral gyrus gives
rise to sensations projected to appropriate parts of the body.
The sensations produced are usually numbness, tingling, or a
sense of movement, but with fine enough electrodes it has
been possible to produce relatively pure sensations of touch,
warmth, and cold. The cells in the postcentral gyrus are organized
in vertical columns, like cells in the visual cortex. The
cells in a given column are all activated by afferents from a
given part of the body, and all respond to the same sensory
modality.
SII is located in the superior wall of the sylvian fissure, the
fissure that separates the temporal from the frontal and parietal
lobes. The head is represented at the inferior end of the
postcentral gyrus, and the feet at the bottom of the sylvian fissure.
The representation of the body parts is not as complete
or detailed as it is in the postcentral gyrus.
Conscious awareness of the positions of the various parts of
the body in space depends in part on impulses from sense
organs in and around the joints. The organs involved are
slowly adapting spray endings, structures that resemble Golgi
tendon organs, and probably Pacinian corpuscles in the synovia
and ligaments. Impulses from these organs, touch receptors
in the skin and other tissues, and muscle spindles are
synthesized in the cortex into a conscious picture of the position
of the body in space. Microelectrode studies indicate that
many of the neurons in the sensory cortex respond to particular
movements, not just to touch or static position.
VENTROLATERAL
SPINOTHALAMIC TRACT
Fibers from nociceptors and thermoreceptors synapse on neurons
in the dorsal horn (Figure 11–1). Aδ fibers terminate primarily
on neurons in laminae I and V, whereas the dorsal root
C fibers terminate on neurons in laminae I and II. The synaptic
transmitter secreted by afferent fibers subserving fast mild
pain is glutamate, and the transmitter subserving slow severe
pain is substance P.
The axons from these neurons cross the midline and ascend
in the ventrolateral quadrant of the spinal cord, where they
form the ventrolateral spinothalamic tract (Figure 11–2).
Fibers within this tract synapse in the VPL. Other dorsal horn
neurons that receive nociceptive input synapse in the reticular
formation of the brain stem (spinoreticular pathway) and
then project to the centrolateral nucleus of the thalamus.
Positron emission tomographic (PET) and functional magnetic
resonance imaging (fMRI) studies in normal humans
indicate that pain activates cortical areas SI, SII, and the cingulate
gyrus on the side opposite the stimulus. In addition, the
mediofrontal cortex, the insular cortex, and the cerebellum are
activated. These technologies were important in distinguishing