Reticulospinal vasomotor neurons in the RVL mediate the somatosympathetic reflex.

Morrison SF, Reis DJ.
Am J Physiol. 1989 May;256(5 Pt 2):R1084-97.

   In this paper, they wanted to see if the RVLM neurons responsible for basal sympathetic tone were also responsible for the somatosympathetic reflex. To do this, they stimulated the sciatic nerve to increase SNA while monitoring RVLM unit activity and efferent splanchnic SNA.
   When they examined the effects of individual sciatic stimuli, they found 2 separate SNA were evoked in response. By varying stimulus intensity, they determined that these were not due to two separate populations of sciatic fibers. However they did estimate afferent conduction velocity (at the lumbar dorsal root) and found 2 lightly myelated types and 1 unmyelinated type when they gave a strong enough stimulus. This suggests that the sensory afferents that drive a sympathetic response are the lightly myelinated ones and that the C-fibers play a much smaller role in this effect.
   They next used the classic technique of doing transections to find out which regions are necessary for the response, and found that the lower brainstem and below is sufficient. They then checked that the RVLM was involved in the reflex by microinjection of kainic acid, which caused a depolarization block that was able to reduce the SNA response to sciatic stimulation by ~90%.
   To examine how individual neurons played into the response, they found barosensitive neurons and determined that they were spinally projecting and verified (in post processing) that they were cardiovascular-related and probably functioned to stimulate SNA. They then found that most of these neurons would show early and late action potentials linked to sciatic stimuli, very similar to what was seen for splanchnic SNA. The neurons also increased their rate of firing for about 20 milliseconds after each of the evoked action potentials.
   Finally, in the discussion, they dedicate a lot of space to the argument (supported by addition of afferent and efferent conduction velocities) that the somatosympathetic effect occurs primarily by sensory neurons in the sciatic bundle synapsing onto two populations of neurons in the spinal cord, which have different conduction velocities but both directly excite the neurons in the RVLM that are responsible for stimulating the splanchnic nerve. - DH