Sympathetic activity influences the vascular axon reflex in the skin

The interaction between changes of skin blood flow evoked by centrally mediated reflexes and local axon reflexes was studied in healthy subjects. Axon reflexes were evoked on the dorsum of the hand by transcutaneous electrical stimulation and reflex changes of blood flow by changes of ambient temperature, deep breath and emotional changes of blood flow by changes of ambient temperature, deep breath and emotional stress. Skin blood flow was measured by two laser-Doppler flowmeters, the probes of which were situated 6-8 mm from the stimulating electrode (monitoring net flow responses) and several centimetres away (monitoring generalized reflex responses only). The axon reflex responses were markedly diminished by body cooling but did not change during body warming. In warm subjects, a deep breath and emotional stress caused transient reductions of the flow response evoked by the electrical stimulation. Regional anaesthesia of the nerve(s) innervating the stimulated skin area led to marked increases of axon reflex responses in cold subjects, but no changes occurred in warm subjects. The anaesthesia also eliminated the transient flow reductions evoked by deep breaths and emotional stress. Since the applied stimuli are known to change skin sympathetic activity, it is concluded that sympathetic (presumably vasoconstrictor) impulses destined for the skin may reduce axon reflex responses.