![]() ![]() In addition, there are large diameter free nerve endings within the epithelium. There is profuse innervation of the anal canal with a variety of specialized sensory nerve endings: Meissner's corpuscles which record touch sensation, Krause end-bulbs which respond to thermal stimuli, Golgi-Mazzoni bodies and pacinian corpuscles which respond to changes in tension and pressure, and genital corpuscles which respond to friction. Touch, pain and temperature sensation exist in normal subjects. The modalities of anal sensation can be precisely defined. Three sensory thresholds are usually defined: constant sensation of fullness, urge to defecate, and maximum tolerated volume. The volumes at which these sensations are perceived are recorded. The balloon is progressively distended until particular sensations are perceived by the patient. The two main methods for quantifying rectal sensation are rectal balloon distension and mucosal electrosensitivity. The sensation of rectal distension travels with the parasympathetic system to S2, S3 and S4. However, myelinated and non-myelinated nerve fibres are seen adjacent to the rectal mucosa, but no intraepithelial fibres arise from these. ![]() No specific sensory receptors are seen on careful histological examination of the rectum in humans. It is, however, sensitive to distension by an experimental balloon introduced through the anus, though it is not known whether it is the stretching or reflex contraction of the gut wall, or the distortion of the mesentery and adjacent structures which induces the sensation. The rectum is insensitive to stimuli capable of causing pain and other sensations when applied to a somatic cutaneous surface. ![]()
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