Axon reflex

An axon reflex is a nervous reaction that is triggered by an axon via its collaterals, the so-called axon collaterals. This process occurs without synaptic impulse transmission.

The axon reflex is characterized by a so-called "triple response", an inflammatory triple response to a mechanical stimulus (neurogenic inflammation):

• Local erythema after about 30 seconds
• Lateral erythema spreading beyond the site of mechanical irritation (spreading flare)
• Circumscribed oedema of the skin (wheal formation).

The excitation impulse is conducted through afferent nociceptive fibers orthodromically until the next encounter with an afferent fiber. When the branching site of an axon is excited, even primarily unexcited sensory endings can be depolarized by a so-called "antidromic excitation propagation". This results in the release of neuropeptides such as substance P, CGRP or somatostatin. Substance P in particular is able to activate mast cells with consecutive histamine release. The result is a "neurogenic inflammation".

Axon reflexes explain e.g. the red dermographism in urticaria. Pathological reactions to axon reflexes also seem to play a role in hyperhidrosis of the patient with atopic eczema (Kijima A et al. 2012).

Since nociceptive afferences innervate almost all tissues of the human organism, a neurogenic inflammation can affect further and very different organs.

Finally, the terms "axon reflex" and "antidromic vasodilatation" describe vasodilatation as a result of noxic stimulation of afferent nerve endings.

1. Kijima A et al (2012) Abnormal axon reflex-mediated sweating correlates with high state of anxiety in atopic dermatitis. Allergol Int 61:469-473.
2. Tavee JO et al (2014) Sural sensory nerve action potential, epidermal nerve fiber density, and quantitative sudomotor axon reflex in the healthy elderly. Muscle nerve 49:564-569.