[Antidromic vasodilation caused by A-delta afferents in the frog]

In anesthetized immobilized frog we recorded changes in hind leg volume evoked by electrical stimulation of peripheral end of the sciatic nerve. The ranges of the stimulus amplitudes sufficient to induce vasodilator or vasoconstrictor reactions were estimated. In a separate set of experiments thresholds of A alpha beta, A delta and C-afferent fibers excitation were evaluated by recording waves of compound action potentials in VIII-X dorsal roots. It was found that vasodilation is elicited by the stimuli of virtually the same intensity range as the excitation of A delta afferent fibers, including low threshold one. Consequently we concluded that in frog these myelinated afferent fibers are capable of dilating the blood vessels by antidromic action. This finding is in contrast with antidromic vasodilation in mammals which is known to result mainly from the impulses of the unmyelinated afferent fibers.