Question

What is axon reflex?

Answer 1

Hint: Axon reflex is the reaction of the nervous system to a stimulus, which helps an organism to react physically as a reflex action to an event. Also known as Flare response, it involves three components: Redline, wheel, and flare.

Complete answer:
A reflex is an immediate reaction to a stimulus that is an involuntary, stereotyped, and automatic response from the sensory system of the body. It is a sequential event that forms an arc, starting with the receptors as the starting point and effectors as the endpoint.
An Axon reflex is a simple reaction to the stimulus that starts with a stimulus at the sensory/receptor start point, where the stimulus sets up an impulse that travels through the nerve branch (effector pathway). The impulse moves to the central point of nerve division, where it reflects down immediately through the second nerve branch (effector pathway) to the effectors (endpoint).
Axon reflex activates the local arterioles at the effector end this causes vasodilation, secretion of bicarbonates, mast cell degranulation, and/or muscle contractions. These reactions at the effector ends are also called as the Triple response:
- Red reaction: involving local vasodilation due to histamine causing the appearance of a red line on the skin within a few seconds of the stimulus.
- Wheel reaction: histamine also causes the exudation of fluid from the capillaries, and increases the capillary permeability, this causes local edema in the same region as the red line appeared. This edema takes place in 1 min.
- Flare reaction: due to axon reflex the redness slowly starts to spread and extends beyond the red line.
The most common sensory start point of the axon reflex is the skin and some of its important effectors include the mast cells, blood vessels, and sweat glands.
Thus, an axon reflex can be defined as the local response by a bifurcated axon, where activation of one branch of the axon causes the excitation of the second branch. This results in the release of neurotransmitters, affecting the innervated cells.
Figure 1: Axon reflex

Additional Information:
Transmission of a signal from one end of the nerve to another end is a type of Antidromic transmission.
The effector ends of an axon reflex are not only receptors but can also be a secretory action like the release of capsaicin activates the capsaicin sensitive pain receptors.
Axon reflex plays an important role in following functions: skin blood flow, inflammation, pain, sweating, itching and allergic reactions.

Note: A reflex involves two pathways: an afferent pathway that connects the sensory start point and the integration center, and an efferent pathway through which the message from the integration system is carried to the effectors (the end point). These two pathways involve more than one synapses, in addition to the connection between the motor neuron and the effector. But in case of Axon reflex it neither involves any synapses, nor involves the integration center.