In addition, there is an occasional secondary activation in the dorsolateral paracentral lobule, indicative of groin stimulation.Ī “boxcar” experimental design was employed with each 5-minute trial consisting of 30 sec of rest, then 30 sec of stimulation, repeated 5 times in succession. In those studies, the clitoral stimulation was applied by the experimenter or the subject’s partner.Īs seen in the present findings, using fMRI, in which the women applied clitoral, vaginal or uterine cervical self-stimulation, there is clear evidence of activation of the medial region of the paracentral lobule, in the sensory genital region of the homuncular map of Penfield and Rasmussen. Two recent studies, using fMRI with electrical stimulation of the clitoris or using PET and mechanical stimulation of the clitoris by the subject’s partner, reported that clitoral stimulation activated the dorsolateral, rather than the medial, region of the paracentral lobule. While the basis for this discrepancy in the penile map is still not reconciled, it is possible that sensory activation of the more dorsolateral region of the paracentral lobule may result from inadvertent and incidental stimulation of the groin, on the basis that Penfield’s map and even Kell’s “revised” map both show the transition zone between upper thigh and trunk (i.e., “groin”) to be located on the dorsolateral region of the paracentral lobule.Ī parallel discrepancy in the genital map of women has now become evident. However, more recent studies using PET or fMRI, and another using penile evoked potentials, reported that a more dorsolateral portion of the paracentral lobule, rather than its medial region, was activated when direct penile stimulation was applied by the experimenter (using a toothbrush and recording fMRI ), when penile stimulation was applied by the subject’s partner (using manual stimulation and recording PET ), or in response to electrical stimulation of the penis. Similar findings were reported by Allison et al in response to electrical stimulation of the clitoris as well as the penis. Those studies confirmed the earlier homuncular map, for each reported that the evoked potentials were focused in the medial cortex, (in the medial region of the paracentral lobule), i.e., in the genital region as represented in the Penfield map. Subsequent studies used electrical stimulation of the dorsal nerve of the penis to measure the distribution of evoked potentials in the cortex. The relation of the paracentral lobule to the sensory cortical homunculus of Penfield and Rasmussen is shown by the lines connecting the corresponding regions. The equipment is generally available and the technique has been shown to be an accurate and repeatable quantitative test of anal sensation.Three views of the paracentral lobule, showing its relation to adjacent cortical regions (adapted from ). The technique involves the use of a specialized constant current generator and bipolar electrode probe inserted in the anal canal. Anal sensation may be quantitatively measured in response to electrical stimulation. The nerve pathway for anal canal sensation is via the inferior haemorrhoidal branches of the pudendal nerve to the sacral roots of S2, S3 and S4. 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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