Point
2
Intro
In gross terms a
lesion of the anterolateral system will result in a
deficit in pain, temperature and crude touch
on the contralateral side of the body
below the level of the lesion. That is, a lesion of
the ALS in the upper cervical region results in a pain and
temperature deficit of the entire contralateral side
of the body. A lesion of the ALS in the upper thoracic cord
will result in a deficit in pain and temperature from the
thorax and lower extremities on the contralateral
side because the fibers which are interrupted at this level
convey pain and temperature information from these areas.
The upper extremity is safe because the fibers
conveying information regarding pain and temperature from
this region are still reaching the thalamus (VPL) and in
turn the cortex. These fibers lie in the anterolateral
system ROSTRAL to the lesion and are not interrupted.
The contralateral
deficits in pain and temperature following a lesion of the
ALS are more detectable than the deficit in crude
touch. Why?? Because the dorsal column system is intact and
it "covers" for the loss during the neurological exam. For
example, following a lesion of the left ALS at C2
there is a deficit in crude touch on the right
(contra.) side but the dorsal columns are OK on the
right. Of course, if you have a lesion of the dorsal
columns only, you still have the crude touch being conveyed
by the ALS. For example, following a lesion of the
right fasciculus gracilis, you still have crude touch
from the right lower extremity because the left ALS
is fine and dandy!!!
We know that the central processes of delta
and C fibers ASCEND approximately 2 levels before
they synapse in the dorsal horn. In other words, dorsal
horn cells which send their axons across into the
anterolateral system receive their pain and temperature
information from TWO spinal segments below. Thus,
instead of the gross approximation of deficits of pain and
temperature below the level of the lesion on the
contralateral side, it is best to say the deficits start two
levels below the lesion of the ALS and includes everything
below this level. Of course, the deficits are contralateral.
For example, a lesion of the ALS at T1
will result in deficits in pain, temperature, and crude
touch from T3 (2 segments below) and below on the
contralateral side of the body.