Influence of autonomic nerves on the internal and sphincter in man

The internal and sphincter receives its parasympathetic nerve supply from the sacral outflow and its sympathetic supply from the thoracicolumbar outflow of the spinal cord. In order to investigate the influence of the tonic discharge of these nerves, eight healthy subjects receiving high spinal anaesthesia (T 6-T 12) and five receiving low spinal anaesthesia (L 5-S 1) were examined. Continuous recordings of anal pressure and electromyographic activity from the external sphincter were obtained during rest and during expansion of the ampulla recti by means of an air-filled balloon. The results were compared with those obtained in an earlier study from 10 subjects with a bilateral pudendal block which paralysed the striated sphincter muscles without affecting the autonomic nerve supply to the internal sphincter. Anal pressure at rest decreased significantly more with high spinal anaesthesia (32 +/- 3-2 mm Hg) than with low (11 +/- 7-1 mm Hg) or with pudendal block (10 +/- 3-9 mm Hg) and the relaxations of the internal sphincter induced by rectal distension were somewhat smaller with high spinal anaesthesia. However, the remaining anal pressure at maximal relaxation, induced by a substantial rectal distension, was essentially the same with the three forms of anaesthesia. It is concluded that, at rest, there is a tonic excitatory sympathetic discharge to the internal anal sphincter in man. However, this seems to be without excitatory effect when the sphincter is relaxed after a substantial rectal distension. Furthermore, the results indicate that at rest there is no tonic parasympathetic discharge affecting the sphincter tone.