Sensory discrimination and dynamic activity in the anorectum: evidence using a new ambulatory technique

The anal canal in health is extremely sensitive to thermal stimuli, If temperature sensation plays a part in sensory discrimination two conditions must be fulfilled: there must be a temperature gradient along the anorectum and rectal contents must be able to come into contact with the sensitive anal mucosa. In a study of 53 normal subjects we demonstrated this temperature gradient with a median temperature difference between the rectum and lower, mid and upper anal canal of 0.4, 0.2 and 0.1 degrees C respectively. The second condition was examined in 15 normal ambulatory subjects by measuring mid-anal sphincter (SP) and rectal pressures (RP) with a microtransducer catheter. The signals were digitalized and recorded in a portable electronic memory for later computer display, and analysis. Marked spontaneous sphincter relaxation resulting in equalization of RP and SP occurred 7 times per hour (1-4). The conscious sensation of the presence of flatus was associated with an SP reduction of 30 mmHg (20-50 mmHg) and an RP increase of 7 mmHg (0-15 mmHg), such that RP greater than or equal to SP in 80 per cent of 144 recorded events. Using this new technique we have demonstrated the highly dynamic nature of the anal sphincter. Several times an hour the sphincter relaxes with subsequent equalization of rectal and anal pressures, allowing entry of rectal contents into the anal canal so that its presence and nature can be determined.