Mechanically evoked cortical potentials: A physiological approach to assessment of anorectal sensory pathways

Background: Normal defaecation involves activation of anorectal mechanoreceptors responsive to pressure and stretch. The aim of this study was to develop selective anal and rectal mucosal light-touch stimulation suitable for measurement of cortical evoked potentials (EPs) in order to explore the sensory arm of these pathways. New method: A novel device was manufactured to deliver selective rectal and/or anal light-touch stimulation using a shielded inter-dental brush mounted on a rotating stepper motor (1 Hz, 1 ms, 15° rotation). Resultant somatosensory EPs recorded with a 32-channel cortical multi-electrode array were compared to those elicited by electrical anorectal stimulation (2 mm anal plug electrode [1 Hz, 1 ms, 10 V]). Results: Eighteen anaesthetized female Wistar rats (body mass 180-250 g) were studied. Electrical and mechanical stimulation provoked similar maximal response amplitudes (electrical anorectal 39.0 μV[SEM 5.5], mechanical anal 42.2 μV[8.1], mechanical rectal 45.8 μV[9.0]). Response latency was longer following mechanical stimulation (electrical anorectal 8.8 ms[0.5], mechanical anal 16.4 ms[1.1], mechanical rectal 18.3 ms[2.5]). The extent of activated sensory cortex was smaller for mechanical stimulation. Sensory inferior rectal nerve activity was greater during anal compared to rectal mechanical in a subgroup of 4 rats. Evoked potentials were reproducible over 40 min in a subgroup of 9 rats. Comparison with existing methods: Cortical EPs are typically recorded in response to non-physiological electrical stimuli. The use of a mechanical stimulus may provide a more localized physiological method of assessment. Conclusions: To the authors' knowledge these are the first selective brush-elicited anal and rectal EPs recorded in animals and provide a physiological approach to testing of anorectal afferent pathways.