Why does skin turn red after it is scratched?
The normal skin axon-reflex vasodilation (flare) follows skin stimulation from a simple scratch.
The scratch causes activation of unmyelinated sensory nerve terminals (C fibers).
The impulse generated by this stimulus travels antidromically, reaches a branch point, and then orthodromically arrives at a skin blood vessel, releasing one or more vasodilating peptides or adenosine triphosphate.
The released substance leads to further histamine release, activating other sensory terminals, creating a cascade of spreading flare response.
Released histamine also causes itching. Antihistamines may reduce both the flare response and the itching.
The absence of a flare response provides evidence of dysfunction of unmyelinated sensory fibers in peripheral neuropathies.
