| |
Components
of pain
The sensation
of acute pain has two components, corresponding to the two major
fiber types myelinated and unmyelinated (Figure
2).
- The first,
sometimes termed "fast pain," is a sharp, pricking
pain similar to the brief pain associated with sticking a pin
into the skin.
- The second,
"slow pain," follows fast pain and has a burning
quality. Slow pain typically has a suffering quality associated
with it.
The faster
conducting myelinated A -fibers
mediate fast pain, which reaches SI in the classic spinothalamic
tract system. This pathway is most likely utilized to detect the
location of the nociceptive stimulus.
The slower
conducting unmyelinated C-fibers mediate slow pain, which primarily
utilizes the spinoreticular pathway and the branches of the spinothalamic
tract that relay through the reticular formation. SII is probably
involved in the suffering sensation, because individuals with
damage to SI with preservation of SII still experience pain.
The connections
of the intralaminar nuclei with the limbic system as well as to
the hypothalamus possibly mediate the emotional and autonomic
reactions to slow pain, respectively (Willis
and Westlund, 1997).
There are
three additional pathways that receive somatic nociceptive input:
spinoparabrachial tract, spinomesencephalic tract, and spinohypothalamic
tract. Since the terminations of the axons of these three pathways
are involved with control of autonomic function, these pathways
probably contribute to the autonomic components that may accompany
pain, such as sweating or the fight-or-flight response.
|