Neuro‐effector characteristics of sweat glands in the human hand activated by irregular stimuli

Abstract

Intraneural electrical stimulation of cutaneous fascicles in the median nerve was performed in 24 normal subjects and the effects on sweating within the innervation zone were monitored as changes of skin resistance and water vapour partial pressure (wvpp). The aims were: (1) to investigate the response variability between repeated stimulation sequences in the same skin site and between different sites and (2) to compare quantitative effects of regular and irregular stimulation on skin resistance and wvpp. Regional axillary anaesthesia of the brachial plexus eliminated spontaneous and reflex sympathetic activity. With repeated irregular stimulation sequences skin resistance responses from the same skin site varied only slightly between trials. Differences between response curves from two skin sites in the same subject or from different subjects were also small but significantly greater (P < 0.01) than differences between responses to repeated stimulation in the same site. Irregular stimulation with average frequencies of 0.49 Hz and 3.51 Hz gave greater resistance responses than if the same number of stimuli were delivered regularly (P < 0.01). The difference was most pronounced at 0.49 Hz. At an average frequency of 0.49 Hz the stimulation usually evoked no changes of wvpp whereas an average frequency of 3.51 Hz caused an increase of wvpp which was greater with irregular than with regular stimulation in all subjects. We conclude that: (1) sweat responses to sudomotor nerve traffic vary slightly due to local factors in the skin or the terminal nerve endings and (2) irregular sudomotor nerve traffic evokes more sweat than if the same impulses occur regularly.

Firing properties of sudomotor neurones in hyperhidrosis and thermal sweating

“…the neuroeffector efficacy was increased with highly variable interspike intervals. The effect was greatest at low stimulation frequencies, and was maximal at 0.49 Hz [12], which-as we have seen-is close to the firing rates of sudomotor neurones in hyperhydrosis (0.77 Hz) and thermal sweating (0.72 Hz). We suggest that the shift towards multiple firing-and the consequent increase in the variability of interspike intervals-seen in the present study will increase the production of sweat above that expected from the relatively low mean frequencies.…”

“…Lower rows, same data after excluding cardiac intervals in which a unit was silent. Data in the right panels are from [12] intervals between the two populations (P < 0.0001). When only those cardiac intervals in which the units were active were considered (lower panels) the percentage of intervals in which the neurones generated 1, 2, 3 or 4 spikes was 60.4 ± 6.3, 22.9 ± 3.9, 9.7 ± 2.1 and 3.4 ± 1.3%, respectively.…”

“…If there are more cardiac intervals with multiple spikes than single spikes, the overall firing variability will increase. Kunimoto and colleagues [11,12] have shown that the sweat release produced by intraneural stimulation of sudomotor axons within the median nerve was greater when the stimulation trains were composed of irregular frequencies than regular frequencies, i.e. the neuroeffector efficacy was increased with highly variable interspike intervals.…”

Real-time imaging of cortical areas involved in the generation of increases in skin sympathetic nerve activity when viewing emotionally charged images

“…Those studies have used indirect markers of sympathetic outflow, typically using sweat release and heart rate. As noted above, changes in electrical skin resistance correlate very poorly with skin sympathetic nerve activity (Kunimoto et al, 1992). Ours is the first investigation to correlate changes in brain activity with changes in directly recorded SSNA during emotional state changes.…”

“…As stated by Bini et al, (1980), "no simple quantitative relationship could be seen between the size of individual sudomotor bursts and [the] accompanying electrodermal responses." Moreover, there is a very poor relationship between changes in electrical resistance (or conductance) of the skin and sudomotor activity, as produced by controlled intraneural electrical stimulation Kunimoto et al, 1991Kunimoto et al, , 1992. Indeed, because of the non-linear relationship between sudomotor activity and skin resistance, it has been argued that changes in skin resistance cannot be used to quantify changes in skin sympathetic nerve activity .…”