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

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.