Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 11

Somatosensory Pathways 175

(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 pro-
portions 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.

Studies of the sensory receiving area emphasize the very dis-

crete 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 orga-

nized 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 pari-

etal lobes. The head is represented at the inferior end of the

postcentral gyrus, and the feet at the bottom of the sylvian fis-

sure. 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 syn-

ovia and ligaments. Impulses from these organs, touch recep-

tors in the skin and other tissues, and muscle spindles are

synthesized in the cortex into a conscious picture of the posi-

tion of the body in space. Microelectrode studies indicate that

many of the neurons in the sensory cortex respond to particu-

lar movements, not just to touch or static position.

VENTROLATERAL

SPINOTHALAMIC TRACT

Fibers from nociceptors and thermoreceptors synapse on neu-

rons in the dorsal horn (Figure 11–1). A

δ

fibers terminate pri-

marily on neurons in laminae I and V, whereas the dorsal root

C fibers terminate on neurons in laminae I and II. The synap-

tic 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 mag-

netic resonance imaging (fMRI) studies in normal humans

indicate that pain activates cortical areas SI, SII, and the cingu-

late 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

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 Brod-
mann’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 11–4
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.)

Hand

Face

Tongue

Sll

Trunk

Auditory

Sl

Posterior

parietal

cortex

Visual

9

(^864312)
Intra-
abdominal
Pharynx
Tongue
Gen.
Toes
Foot
HeadNeckTrunk
Shoulder
HipLeg
Teeth, gums, and jaw
Lower lip
Lips
Upper lip
Face
Nose
EyeThumb
Index
RingLittle
HandWrist
ForearmElbowArm
Middle