|
The
spinothalamic tract
A diagram
of the spinothalamic tract may be viewed in Figure
1.
Nociceptors
are located in skin and muscle. They consist of small myelinated
(A -fibers)
and unmyelinated (C-fibers) axons. These axons course toward the
central nervous system in peripheral nerves. Cell bodies of these
axons are located in the dorsal root ganglia in the periphery
and in ganglia associated with cranial nerves.
In the periphery,
the central processes of these neurons course in the dorsal roots
to the posterior horn of the spinal cord, where they may course
for up to two segments before making synaptic connections onto
neurons in the gray matter of the posterior (dorsal) horn (Willis
and Westlund, 1997).
The central
terminals of the A-
and C-fibers contact cell bodies of spinothalamic tract neurons
either directly or through interneurons, and these terminals release
excitatory neurotransmitters which activate the spinothalamic
tract neurons.
The axons
of spinothalamic tract neurons cross to the opposite (contralateral)
side of the spinal cord either in the same spinal segment or within
one or two segments as the location of the cell bodies. The axons
ascend to the brain stem in the anterolateral quadrant of the
spinal white matter. The axons maintain a lateral location in
the brain stem as they ascend to the thalamus, where they terminate
primarily in the ventroposterior lateral nucleus.
The thalamocortical
axons ascend through the posterior limb of the internal capsule
on their way to terminate in the somatosensory cortex in the postcentral
gyrus in the parietal lobe. In the primary somatosensory cortex
(SI), the axons terminate in a somatotopic manner. Thalamocortical
axons also terminate in the secondary somatosensory cortex (SII),
which is located in the inferior portion of the postcentral gyrus
as well as the nearby insular cortex (Willis
and Westlund, 1997).
Perhaps surprisingly,
the roles of the different regions of cortex in pain perception
are not entirely understood. Large lesions of the postcentral
gyrus do not eliminate the perception of pain, although the ability
to localize a painful stimulus is diminished. SII has an important
role in processing nociceptive information because functional
imaging studies show increased blood flow in the region of SII
during nociceptive stimulation. The precise roles of SI and SII,
as well as nearby areas of cortex, in nociceptive processing remain
to be elucidated (Willis
and Westlund, 1997).
|