The internal anal sphincter in the cat: a study of nervous mechanisms affecting tone and reflex activity

1. Smooth muscle activities in rectum and internal anal sphincter have been recorded using intraluminal balloons.2. Reflex activation of the sphincter, caused by distension of the rectum, has been assessed before and after various combinations of blocking drugs.3. Responses to stimulation of hypogastric or sacral nerves, or to the administration of drugs with autonomic actions have been tested before and after various combinations of blocking drugs.4. Results indicate that the tone of the internal anal sphincter is influenced by a number of neural mechanisms. These include motor pathways involving both alpha-adrenergic and cholinergic mechanisms and inhibitory pathways involving both beta-adrenergic and non-adrenergic non-cholinergic mechanisms.5. Cholinergic contractions of the sphincter were converted to relaxations after alpha-adrenergic blockade. This indicates that the contractions are an indirect effect operating through an adrenergic reflex. Cholinergic relaxations may also be indirect and operate through reflex inhibition secondary to rectal contractions.6. Sphincteric motor activity is controlled largely through alpha-adrenergic mechanisms by adrenergic nerves acting directly on the muscle. beta-Adrenergic inhibitory mechanisms are thought to operate indirectly via ganglia.7. The over-all control of the sphincter is by complex reflex mechanisms involving numerous pathways and the activity of the sphincter at any one time is determined by the net balance between motor and inhibitory influences.8. Sacral nerve stimulation indicated that it contains cholinergic nerves to the rectum, non-adrenergic non-cholinergic inhibitory axons to the sphincter and variable numbers of adrenergic axons to the sphincter.9. Responses of the sphincter to drugs and nerve stimulation were often variable, as has been described many times in the literature. It is considered that this is due to complex combinations of indirect reflex effects, secondary to activation of structures outside the sphincter, operating with or against direct effects on the sphincter itself.