Ganong's Review of Medical Physiology, 23rd Edition

174
SECTION III
Central & Peripheral Neurophysiology

from the sacral cord are positioned most medially and those

from the cervical cord are most lateral. This arrangement con-

tinues in the medulla with lower body (eg, foot) representation

in the gracilus nucleus and upper body (eg, finger) representa-

tion in cuneate nucleus. The medial lemniscus is organized

dorsal to ventral representing from neck to foot.

Somatotopic organization continues through the thalamus

and cortex. VPL thalamic neurons carrying sensory informa-

tion project in a highly specific way to the two somatic sen-

sory areas of the cortex:

somatic sensory area I (SI)

in the

postcentral gyrus and

somatic sensory area II (SII)

in the

wall of the sylvian fissure. In addition, SI projects to SII. SI

corresponds to

Brodmann’s areas 3, 2, and 1.

Brodmann was

a histologist who painstakingly divided the cerebral cortex

into numbered areas based on their histologic characteristics.

The arrangement of projections to SI is such that the parts of

the body are represented in order along the postcentral gyrus,

with the legs on top and the head at the foot of the gyrus

FIGURE 11–1
Schematic representation of the terminations
of the three types of primary afferent neurons in the various
layers of the dorsal horn of the spinal cord.

I

II

III

IV

V

VI

VII

Mechanoreceptors

Mechanoreceptors

Nociceptors

Cold receptors

Nociceptors

Thermoreceptors

Mechanoreceptors

Aδ

C

Aβ

To dorsal columns

FIGURE 11–2
Ascending tracts carrying sensory information from peripheral receptors to the cerebral cortex. (a)
Dorsal-column
pathway mediating touch, vibratory sense, and proprioception.
(b)
Ventrolateral spinothalamic tract mediating pain and temperature.
(From Fox
SI,
Human Physiology.
McGraw-Hill, 2008.)

Axons of

third-order

neurons

Cerebral

cortex

Medial lemniscal tract

(axons of second-order

neurons)

Fasciculus cuneatus

(axons of first-order

sensory neurons)

Joint stretch receptor

(proprioceptor)

Spinal cord

Fasciculus gracilis

(axons of first-order

sensory neurons)

Medulla oblongata

Touch receptor

Temperature

receptor

Pain receptor

Axons of first-order

neurons (not part of

spinothalamic tract)

Lateral spinothalamic tract

(axons of second-order neurons)

Thalamus

Postcentral

gyrus

(a) (b)