C h a p t e r 3 5
Somatosensory Function, Pain, and Headache
857
pathway is functional but the association area has been
damaged, the person can correctly describe the object
but does not recognize it as a screwdriver. This deficit is
called
astereognosis.
Anterolateral Pathway.
The anterolateral pathway
(anterior and lateral spinothalamic tracts) consists of
bilateral, multisynaptic, slow-conducting tracts (see
Chapter 34, Fig. 34-11). These tracts provide for trans-
mission of sensory information such as pain, thermal
sensations, crude touch, and pressure that does not
require discrete localization of signal source or fine dis-
crimination of intensity. The fibers of the anterolateral
pathway originate in the dorsal horns at the level of the
segmental nerve, where the dorsal root ganglion neu-
rons enter the spinal cord. These spinal neurons cross
in the anterior commissure, within a few segments of
origin, to the anterolateral column on the opposite
side of the cord, and then ascend upward toward the
brain. The anterolateral tract fibers synapse with sev-
eral nuclei in the thalamus, but en route they give off
numerous branches that travel to the reticular activat-
ing system of the brain stem. These projections provide
the basis for increased wakefulness or awareness after
strong somatosensory stimulation and for the general-
ized startle reaction that occurs with sudden and intense
stimuli. They also stimulate autonomic nervous system
responses, such as a rise in heart rate and blood pres-
sure, dilation of the pupils, and the pale, moist skin that
results from constriction of the cutaneous blood vessels
and activation of the sweat glands.
There are two subdivisions in the anterolateral path-
way: the neospinothalamic and the paleospinothalamic
tracts are important in pain perception
1
(see Fig. 35-4B).
The
neospinothalamic tract
consists of a sequence of
at least three neurons with long axons. It provides for
relatively rapid transmission of sensory information to
the thalamus. The
paleospinothalamic tract
, which is
phylogenically older, consists of bilateral, multisynaptic,
slow-conducting tracts that transmit sensory signals that
do not require discrete localization or discrimination
of fine gradations in intensity. This slower-conducting
pathway also projects into the intralaminar nuclei of the
thalamus, which have close connections with the limbic
cortical systems. This circuitry gives touch its affective
or emotional aspects, such as the particular unpleasant-
ness of heavy pressure and the peculiar pleasantness of
the tickling and gentle rubbing of the skin.
Primary
somatosensory
cortex
Medial
lemniscus
Thalamus
Medulla
Dorsal column
neuron
Spinal cord
Dorsal root
ganglion
1
2
Thalamic
neuron
3
Spinal cord
Neospinothalamic
tract
Paleospinothalamic
tract
Reticular
formation
Somatosensory
and other areas of
the cerebral cortex
B
A
Dorsal root
ganglion
FIGURE 35-4.
(A)
Rapid-transmitting discriminative (dorsal column–medial lemniscal) pathway
carrying axons mediating tactile sensation and proprioception.
(B)
Neospinothalamic and
paleospinothalamic subdivisions of the anterolateral sensory pathway.The neurons of anterolateral
pathways cross within the same segment as the cell body and ascend in the contralateral side of the
spinal cord.The neospinothalamic tract travels mainly to thalamic nuclei that have third-order fibers
projecting to the somatosensory cortex.The paleospinothalamic tract sends collaterals to the reticular
formation and other structures, from which further fibers project to the thalamus.
