Neural control of the internal anal sphincter motility.

Control mechanism of smooth muscle movement of the internal anal sphincter (IAS) by enteric and extrinsic neuvous systems in the dog was investigated. Responses of IAS muscle strips to electrical field stimulation (EFS) and neurotransmitter agents were recorded in vitro. The contraction response to norepinephrine or to EFS was inhibited by phentolamine. The relaxation induced by EFS was not affected by phentolamine, propranolol or atropine. The mechanical activity of smooth muscle in colon and anorectum during spontaneous defecation was recorded using strain gauge force transducers. The colon and anorectum showed the characteristic motility pattern during defecation: 1) The giant migrating contraction of the colon propagated to the rectum, 2) The relaxation of the rectum prior to the contraction, and 3) The IAS muscles continued to relax while the giant contractions of the colon were migrating to the rectum. Sacral nerves were stimulated electrically and the responses of smooth muscles in the rectum and IAS were recorded. The sacral nerve stimulation induced a relaxation followed by contraction of smooth muscle in the rectum and the relaxation in IAS. The mechanical responses of smooth muscle in the IAS were modulated by alpha-adrenergic excitatory and non-adrenergic, non-cholinergic inhibitory nerves. During defecation, the relaxation of IAS smooth muscle was associated with a characteristic motility pattern of the colon and anorectum. The enteric nervous systems may be organizing the motility of these muscles by way of the motor neurones under the control the extrinsic nervous systems.