Rectal and Anal Sensation and Control

Efficient control of the rectum occurs only if the sensory afferent messages from the bowel and pelvis are correctly interpreted by the control mechanism of the brain. There is still much to be learned concerning the location and nature of the afferent receptors, of the muscles that guard continence by day and by night, and of the differential function of the sphincter muscles.

7.6.1 Intrinsic Sensory Receptors of the Anal Canal

Duthie and Gairns [27] carefully plotted the sensory nerve ends in the anal canal. They found an abundance of conventional nerve endings, such as those presumed to denote pain (free intraepithelial), touch (Meissner's corpuscles), cold (Krause end-bulbs), pressure or tension (corpuscles of Pacini and Golgi-Mazzoni), and friction (genital corpuscles), together with unnamed, unconventional receptors in the anal canal of adults, lying distal to the valves and to a point 0.5-1.5 cm cranial to these valves. These receptors were responsible for acute and fine sensory discrimination, which in the skin beyond the pecten was mediated through receptors around the hair follicles. There was a crescendo of free nerve endings and genital corpuscles on the valve line, waning in the stratified columnar zone cranial to the valves. No receptors were found in the rectal mucosa, although myelinated and nonmyelinated nerve trunks were present under the epithelium, and Meissner's plexus of ganglion cells was readily identified. The rectal mucosa of the anal canal did not appreciate any of the above stimuli when tested by the techniques used and appeared to lack the appropriate receptors. They considered that receptors may be present in the rectum to receive distension stimuli, but that they were unable to demonstrate them by present staining methods.

In two other papers, Duthie and Bennett [28] and Duthie and Watts [29] suggested that the effect of rectal distention (as assessed using balloons in these experiments) was to relax the internal sphincter and contract the external sphincter. They claimed that the relaxed internal sphincter allowed feces to contact the very sensitive and effective anal canal receptors that induced external sphincter contraction, which is thus important in the fine control of continence. We suggest in the following section that the initiating signal of distention of the rectum may not be only from the rectal mucosa. In the rectal deformities discussed, both the internal and external sphincters may be rudimentary, yet a high degree of continence can be achieved.

7.6.2 Extrinsic Sensory Receptors

Work and observations on malformation of the anus and rectum led us to evaluate the absence of receptors in the rectal mucosa in a different way. We consider that coarse perception of distention of the rectum is in part a function of the parasympathetic nerves conveying impulses from the muscle spindles in the walls of the rectum and colon, but that fine appreciation of distention, even of minor changes, is the function of the muscles surrounding the anal canal. Furthermore, the pubococcygeus and puborectalis, with their intimate sleeve-and-sling relationship to the anal canal on the cranial aspect of the valves, provide the warning of impending peristaltic progress towards the anus. With the bowel empty and at rest, no sensation is registered, but gas, solid, or liquid content moving into the sleeve-and-sling zone provides a stretch that is immediately and keenly appreciated.

Goligher and Hughes [30], in studies in adults using balloon distention of the bowel brought down in pullthrough operations, also concluded that the response to distention probably arose in structures surrounding the bowel. Similarly, Parks et al. [31] and Porter [32], in studies on the pelvic floor muscles in rectal prolapse, suggested that the receptors lie in the rectal wall and the surrounding pelvic floor muscles. Kiesewetter and Nixon [33], in their anatomic and physiologic studies of rectal sensation in patients following surgical correction of ARM, considered that the sensory receptors responsible for a measure of rectal sensation were probably present in the pu-borectalis muscle.

The investigations of Freeman et al. [34] showed that anal sensation as detected by evoked cortical responses was not present at birth, but showed maturation in the first 3-4 months of life. If the eye of a newborn kitten is kept closed for 4-5 weeks after birth and then opened, the eye is permanently blind; appropriate repetitive somatosensory stimuli during the critical interval of brain development have not occurred. On this basis, they argued that the definitive pullthrough operation should be completed by 34 months of age to achieve the best functional results [34]. The results in neonatal pullthrough operations lend support to the above hypothesis [35].

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook

Post a comment