The human vasodilator axon reflex - an exclusively peripheral phenomenon?

The effect of regional anesthesia of the brachial plexus on the size and intensity of the histamine-induced axon reflex flare (neurogenic inflammation) of the forearm and the upper arm was compared to that of the contralateral arm as control in humans. No changes in the axon reflex could be assessed. Thus the lateral spread of the axon reflex flare must be transmitted by peripheral nerve branches not affected by the anesthesia in the axilla. This excludes the existence of physiologically relevant amounts of proximal branchpoints, DRG neurons with multiple peripheral axons or spinal interneurons transmitting action potentials between peripheral C-afferents involved in the axon reflex flare. Mechanoinsensitive C-fibres are known to be activated by histamine and to be responsible for the neuropeptide release in the skin inducing the axon reflex flare. Reports on those proximal connections can therefore obviously not extend to mechanoinsensitive C-fibres and do not explain the origin of neurogenic inflammation in humans without prior sensitization.