Key pointsr The internal anal sphincter (IAS) develops tone important for maintaining high anal pressure and continence whereas tone in the rectum is less.r To investigate tone generation, the electrical properties [membrane potential (E m ) and slow waves (SWs)] and morphology of the mouse IAS and distal rectum were compared.r SWs were greatest in amplitude and frequency at the distal end of the IAS and declined toward the rectum. SWs were also coordinated to a greater degree in the circumferential than the oral direction.r The circular muscle was divided into 'minibundles' in the IAS but not rectum. Intramuscular interstitial cells of Cajal and platelet-derived growth factor receptor alpha-positive cells were present in each minibundle making each a possible candidate for SW generation.r The features that distinguish the IAS from rectum (i.e. depolarized E m , larger and higher frequency SWs and multiunit configuration) are all properties that are predicted to result in greater tone generation. AbstractThe internal anal sphincter (IAS) develops tone and is important for maintaining a high anal pressure while tone in the rectum is less. The mechanisms responsible for tone generation in the IAS are still uncertain. The present study addressed this question by comparing the electrical properties and morphology of the mouse IAS and distal rectum. The amplitude of tone and the frequency of phasic contractions was greater in the IAS than in rectum while membrane potential (E m ) was less negative in the IAS than in rectum. Slow waves (SWs) were of greatest amplitude and frequency at the distal end of the IAS, declining in the oral direction. Dual microelectrode recordings revealed that SWs were coordinated over a much greater distance in the circumferential direction than in the oral direction. The circular muscle layer of the IAS was divided into five to eight 'minibundles' separated by connective tissue septa whereas few septa were present in the rectum. The limited coordination of SWs in the oral direction suggests that the activity in adjacent minibundles is not coordinated. Intramuscular interstitial cells of Cajal and platelet-derived growth factor receptor alpha-positive cells were present in each minibundle suggesting a role for one or both of these cells in SW generation. In summary, three important properties distinguish the IAS from the distal rectum: (1) a more depolarized E m ; (2) larger and higher frequency SWs; and (3) the multiunit configuration of the muscle. All of these characteristics may contribute to greater tone generation in the IAS than in the distal rectum.

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