The cutaneous vascular axon reflex in humans characterized by laser Doppler perfusion imaging

1. Laser Doppler perfusion imaging was used to map the cutaneous vascular axon response induced by trains of electrical skin stimuli (1 ms, 2 Hz) on the dorsum of the hand, finger and foot in twenty-four healthy subjects. Conduction anaesthesia was applied to nerves supplying the stimulated skin areas. Subtraction of images recorded before and after stimulation was used for data analysis of the intensity and area of the response. 2. The stimulation evoked a localized perfusion increase around the stimulating electrode which lasted approximately 30 min and increased in intensity and area with increasing stimulation strength to a maximum at 20 pulses and 20 mA. The intensity and area of the response was greater on the hand than on the foot. 3. Approximating the response area as a circle, the maximal perfusion increase in the hand extended 9 +/- 3 mm (mean +/- S.D.) outside the perimeter of the stimulating electrode. When stimulating within skin which had been subjected to surface anaesthesia, no response occurred, but when stimulating at the border of surface-anaesthetized skin, the perfusion increase extended 2 +/- 1 mm (mean +/- S.D.) into anaesthetized skin. 4. The results show that the perfusion increase must have been due in part to impulse conduction to, and release of transmitters from, axon endings terminating in skin outside the contact area of the probe. It is concluded that the area of perfusion increase corresponds to the size of the receptive fields of afferent polymodal C fibres.