Respiratory and cardiac modulation of single sympathetic vasoconstrictor and sudomotor neurones to human skin

1. The firing of single sympathetic neurones was recorded via tungsten microelectrodes in cutaneous fascicles of the peroneal nerve in awake humans. Studies were made of 17 vasoconstrictor neurones during cold-induced cutaneous vasoconstriction and eight sudomotor neurones during heat-induced sweating. Oligounitary recordings were obtained from 8 cutaneous vasconstrictor and 10 sudomotor sites. Skin blood flow was measured by laser Doppler flowmetry, and sweating by changes in skin electrical resistance within the innervation territory on the dorsum of the foot. 2. Perispike time histograms revealed respiratory modulation in 11 (65%) vasoconstrictor and 4 (50%) sudomotor neurones. After correcting for estimated conduction delays, the firing probability was higher in inspiration for both classes of neurone. Measured from the oligounitary recordings, the respiratory modulation indices were 67.7 +/- 3.9% for vasoconstrictor and 73.5 +/- 5.7% for sudomotor neurones (means +/- S.E.M.). As previously found for sudomotor neurones, cardiac rhythmicity was expressed by 7 (41%) vasoconstrictor neurones, 5 of which showed no significant coupling to respiration. Measured from the oligounitary records, the cardiac modulation of cutaneous vasoconstrictor activity was 58.6 +/- 4.9%, compared with 74.4 +/- 6.4% for sudomotor activity. 3. Both vasoconstrictor and sudomotor neurones displayed low average firing rates (0.53 and 0.62 Hz, respectively). The percentage of cardiac intervals in which units fired was 38% and 35%, respectively. Moreover, when considering only those cardiac intervals when a unit fired, vasoconstrictor and sudomotor neurones generated a single spike 66% and 67% of the time. Rarely were more than four spikes generated by a single neurone. 4. We conclude that human cutaneous vasoconstrictor and sudomotor neurones share several properties: both classes contain subpopulations that are modulated by respiration and/or the cardiac cycle. The data suggest that the intensity of a multi-unit burst of vasoconstrictor or sudomotor impulses is probably governed primarily by firing incidence and the recruitment of additional neurones, rather than by an increase in the number of spikes each unit contributes to a burst.