Rectal inhibition by inferior rectal nerve stimulation in dogs: recognition of a new reflex - the 'voluntary anorectal inhibition reflex'

Objective The effect of inferior rectal nerve (IRN) stimulation on the rect um was studied, postulating that nerve stimulation might inhibit rectal con tractility and could thus be used in the management of defecation disorders . Method The IRN was exposed through a para-anal incision in 12 dogs (18.2 +/ - 3.3 SD kg, seven male, five female) and a cuff-type electrode was applied to the nerve. A balloon introduced into the rectum was filled with saline in increments of 5 ml. The rectal and rectal neck (anal canal) pressures, a nd the electromyographic (EMG) activity of the external anal sphincter (EAS ) and internal anal sphincter (IAS) were recorded until the balloon was exp elled to the exterior. The test was repeated until the expulsion volume was reached, and the IRN was stimulated (pulse width 200 mu /s, charge density 2 to 6 mu Ci/cm(2)per phase). The test was performed again following indiv idual anaesthetization of the EAS and the IAS, Results At a mean rectal distending volume of 38.3 +/- 2.3 ml, the rectal p ressure increased (P < 0.01), rectal neck pressure declined (P < 0.01), the EAS and IAS EMGs disappeared, and the balloon was expelled. IRN stimulatio n at a distending volume of 38.3 +/- 2.3 ml increased the EMG activity of t he EAS, whereas the rectal pressure and IAS EMG did not change (P > 0.05) a nd the balloon was not expelled. With IRN stimulation at the distending vol ume of 38.3 +/- 2.3 ml while the EAS was anaesthetized, the rectal pressure increased (P < 0.01), disappeared, and the balloon was expelled. Upon repe tition of IRN stimulation during anaesthetization of the IAS, the rectal pr essure remained high and the balloon was not expelled. Conclusion It is suggested that the EAS produces continence by a twofold ac tion. The EAS prevents IAS relaxation on rectal contraction, with a resulti ng rectal relaxation. A reflex relationship is postulated to exist between failure of the IAS to relax and rectal relaxation, We call this reflex rela tionship 'voluntary anorectal inhibition reflex'. Secondly, the EAS mechani cally compresses the rectal neck. It seems that contraction of the EAS, whi ch is a striated muscle, mechanically occludes the rectal neck for a few se conds - enough for the rectum to relax in a reflex manner as an effect of t he voluntary anorectal inhibition reflex. Eur J Gastroenterol Hepatol 13:41 3-418 (C) 2001 Lippincott Williams & Wilkins.