A. Shafik et O. El-sibai, Rectal inhibition by inferior rectal nerve stimulation in dogs: recognition of a new reflex - the 'voluntary anorectal inhibition reflex', EUR J GASTR, 13(4), 2001, pp. 413-418
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.