Ganong's Review of Medical Physiology, 23rd Edition

174 SECTION III Central & Peripheral Neurophysiology
To dorsal columns
I
II
III
IV
V
VI
VII
Mechanoreceptors
Mechanoreceptors
Nociceptors
Cold receptors
Nociceptors
Thermoreceptors
Mechanoreceptors
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.
Aβ
Aδ
C
from the sacral cord are positioned most medially and those
from the cervical cord are most lateral. This arrangement continues
in the medulla with lower body (eg, foot) representation
in the gracilus nucleus and upper body (eg, finger) representation
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 information
project in a highly specific way to the two somatic sensory
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 11–2 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)
Medulla oblongata
Fasciculus cuneatus
(axons of first-order
sensory neurons)
Joint stretch receptor
(proprioceptor)
Spinal cord
Postcentral
gyrus
Fasciculus gracilis
(axons of first-order
sensory neurons)
(a) Touch receptor
(b)
Thalamus
Lateral spinothalamic tract
(axons of second-order neurons)
Pain receptor
Axons of first-order
neurons (not part of
spinothalamic tract)
Temperature
receptor