Discharge patterns of intramural mechanoreceptive afferents during selective distension of the cat's rectum

Abstract

The afferent input from the rectum to the central nervous system (CNS) has yet to be thoroughly characterized. The characteristics of mechanoreceptive rectal afferents have been studied in unanaesthetized decerebrate cats. Following lumbo-sacral laminectomy, single-unit activity (occasionally multi-unit activity) was recorded from centrally cut filaments of the sacral dorsal roots (predominantly S2), while a balloon was inflated in the rectum. Starting from their background activities (mean 15.1 imp sec-1, SD 7.6 imp sec-1), afferent discharge rate increased with increasing balloon pressure (mean threshold 6.3 mmHg, SD 3.6 mmHg). The dependence of firing rate on intrarectal pressure began to flatten out at 25 mmHg (mean; SD 10 mmHg). For 22 out of 29 units (76%) complete saturation occurred at 35 mmHg (mean; SD 15 mmHg) with a maximum discharge rate of 31 imp sec-1 (mean; SD 12.6 imp sec-1). In a number of recording sessions, cyclical rectal contractions were observed. In these cases, changes in firing of the units were closely related to changes in intrarectal pressure. Pressure-related afferent activity could be enhanced by parasympathomimetic drugs which augmented rectal contractions. We conclude that sacral dorsal roots contain afferents from low-threshold mechanoreceptors located in the rectal wall, and that these afferents monitor the filling state and contraction level of the rectum.

Locality-dependent descending reflex motor activity in the anal canal–cholinergic and nitrergic contributions in the rat model

“…Hence, we were able to characterize the physiological topography of locality-dependent activation of the descending reflex pathways involved in the integrative neuronal circuitry of the anal canal. Rectal wall distension is a factor that influences the excitation of mechanoceptors that are involved in monitoring the filling state and the contraction level of the rectum in humans and cats [24,25] . Detection of mechanical deformation by mechanoceptors containing specialized intraganglionic laminal endings, which are found in dense afferent innervations, has been described in the guinea pig rectum [26,27] .…”

Neurogenic adaptation contributes to the afferent response to mechanical stimulation

“…Previous studies on mesenteric afferent signaling used pressure, volume, and diameter as stimulus parameters (22,25). However, most likely, the receptors expressed on these sensory nerve endings are directly activated by mechanical forces and deformations in the intestinal wall rather than by changes in the luminal volume and pressure.…”

Impaired Proximal Colonic Motor Response to Rectal Mechanical and Chemical Stimulation in Obstructed Defecation

“…The experimental observations that rectal balloon distention initiates the urge to defecate 17,37 have led to the notion that the sensations for the rectal fullness and the urge to defecate originate from tonically active rectal intraluminal mechanoreceptors. 38,39 However, there is a body of evidence to suggest that, under normal physiologic conditions, the rectum plays little or no role in the urge sensation. For example, the sense of rectal filling and defecatory urge can persist or be initiated even after surgical bypass of the rectum.…”