排便の生理学の最近の知見

Recent Advances in Physiology of Defecation

はじめに

　排便は，糞便がその貯留所ともいうべきS字状結腸をこえて直腸に入ってきた時に起こる．直腸が糞塊によって伸展されると，直腸の強力な正蠕動が発来し，便を肛門から排出しようとする．他方，直腸の伸展は便意を感じさせ，その結果，外肛門括約筋，肛門挙上筋，腹筋，呼息筋が肛門の開大，腹圧の上昇を起こし，排便を促進する．この排便を起こす機序に結腸および直腸内壁内神経叢が主役を演じ，仙髄排便中枢および高位の中枢（脳幹および大脳辺縁系）がこれを調節していると考えられる．そこでこれらのことについて，これまで得られた当教室の知見を混えながら，排便反射について考察してみたい．

　1. Even in the rectum isolated from the central nervous system, the defecation occurs. Mucosal stimulation of the colon or rectum excites the motility above and inhibits it below the spot stimulated.

Because the responses disappear completely after intravenous administration of CE or tetrodotoxin, it is supposed that the receptors are located in the mucosa and the reflex center in the Auerbach's plexus. Such a view is substantiated by the experimental results of perfusion of the colon and the insertion of the bolus into the colonic lumen: The perfusion of the colonic lumen with the pressure of 12 cm H₂O of over causes the mass peristalsis, resulting in the outflow of fluid (Nanba, 1960). The insertion of the bolus into the colonic lumen produces anally inhibition of the colonic motility and orally the mass peristalsis (Nakayanla, 1963). It is considered that such a mechanism plays a role on the colonic and rectal movements to cause defecation.

　2. Distension of the denervated rectum causes relaxation of the internal anal sphincter. It is therefore concluded that the stretch receptors are located in the mucosa or the muscular layer, and that the responses are elicited by a mechanism similar to that mentioned above.

　3. Adrenergic agents sometimes cause contraction and sometimes relaxation of the internal anal sphincter. It is suggested that the internal sphincter muscle contains α-adrenergic receptors subserving contraction, and fi-receptors subserving relaxation, and that it is generally insensitive or less sensitive to acetylcholine.

　4. Many investigators have reported that electric stimulation of the amygdaloid nuclei and periarnygclaloid cortex causes defecation and micturition. Watanabe in the author's laboratory has demonstrated that electric stimulation of the amygdaloicl nuclei or peria mygdaloid cortex produces the contraction or relaxation of the colon, rectum and internal anal sphincter in dogs.

　5. Judging from the questionaires about defecation among the medical students, the author presumes a possibility that the limbic system participates in the morning defecation, and also gives his opinion on the question whether or not the higher defecation center is located in the medulla oblongata.