The human vasodilator axon reflex - an exclusively peripheral phenomenon?

Area of Science:

• Neuroscience
• Dermatology
• Anesthesiology

Background:

• Neurogenic inflammation, characterized by axon reflex flare, is a key component of inflammatory responses.
• Histamine is known to activate mechanoinsensitive C-fibers, initiating neuropeptide release and subsequent flare.
• The precise neural pathways mediating the axon reflex flare in humans remain incompletely understood.

Purpose of the Study:

• To investigate the role of brachial plexus regional anesthesia in modulating the histamine-induced axon reflex flare.
• To determine if anesthesia affects the spread and intensity of neurogenic inflammation in the forearm and upper arm.

Main Methods:

• Human subjects underwent regional anesthesia of the brachial plexus.
• A histamine-induced axon reflex flare was elicited on the anesthetized and contralateral control arms.
• The size and intensity of the flare were assessed to evaluate the effect of anesthesia.

Main Results:

• No significant changes in the size or intensity of the axon reflex flare were observed following brachial plexus anesthesia.
• The lateral spread of the flare was not inhibited by the regional anesthesia.

Conclusions:

• The findings suggest that the axon reflex flare's spread is mediated by peripheral nerve branches not blocked by axillary brachial plexus anesthesia.
• This implies a lack of significant contribution from proximal neural connections, such as DRG neurons with multiple peripheral axons or spinal interneurons, in transmitting the flare response in humans.
• The study excludes physiologically relevant roles for these proximal pathways in human neurogenic inflammation initiated by histamine via mechanoinsensitive C-fibers.