Interactions between sympathetic vasoconstrictor outflow and C nociceptor-induced antidromic vasodilatation.

The interaction between orthodromic sympathetic vasoconstrictor and antidromic vasodilator effects of C-fiber stimulation was studied in normal human volunteers. Excitation of C fibers was achieved through administration of intermittent painful intraneural microstimulation to upper limb nerves, while recording cutaneous thermal emission profiles in the projected fields of cutaneous sensation. During initial stimulation, when both systems are co-activated, skin temperature decreased. Temperature increased after termination of the stimulus. Eventually, temperature decreased again upon renewed stimulation. Thus, sympathetic vasoconstriction was found to override the antidromic vasodilator effect induced by stimulation of C nociceptors. When interpreting abnormal deviations of skin temperature in patients with peripheral nerve disorders, the dynamic interplay between opposing vasomotor effects driven by sympathetic and somatic systems must be taken into consideration.