David Linden - Touch

For some women, a lot of neural pathways originate in the clitoris, and these women’s vaginas will be less “innervated”—less dense with nerves. A woman in this group may like clitoral stimulation a lot, and not get as much from penetration. Some women have lots of innervation in their vaginas, and climax easily from penetration alone. Another woman may have a lot of neural pathway terminations in the perineal or anal area; she may like anal sex and even be able to have an orgasm from it, while it may leave a differently wired woman completely cold, or even in pain. Some women’s pelvic neural wiring will be closer to the surface, making it easier for them to reach orgasm; other women’s neural wiring may be more submerged in their bodies, driving them and their partners to need to be more patient and inventive, as they must seek a more elusive climax. Culture and upbringing definitely have a role in how you climax and can affect whether you climax easily or not, but that is not all there is to it. This discourse heaps vast unnecessary guilt and shame on millions of women or, conversely, depending on their tastes, leads them to feel slightly perverted…. Whatever it is you like and need in bed—as a woman, with all that variability—these preferences may just be due to your physical wiring. 9

This is a reasonable proposition: Sensory nerves in this region of the body do transmit sexual sensation, and there is individual variation in the fine structure of these nerves. So it is plausible that subtle differences in the configuration of the nerves of the pelvis might underlie at least some portion of the individual variation in sexual experience and therefore preferences for certain types or styles of sexual act. However, several caveats must be raised. Most important, there is no evidence to prove that normal variation in the fine structure of genital sensory nerves underlies differences in sexual sensation or sexual preferences. The anatomical variation involved here is below the resolution limit of today’s medical scanning machines, and the knowledge we do have of the fine structure of the relevant sensory nerves and their endings in the skin and other tissues comes from cadavers and biopsy samples, which we can cut into thin slices and examine under a microscope, not from healthy, intact living people whom we can interview about their sexual experiences. 10

Sexual sensation and sexual desire require an ongoing dialogue between the body (particularly the genitals) and the brain. When considering potential biological substrates for individual differences in sexual experience, we should also include variation in those parts of the brain activated by sexual touch. Crucially, in both the skin and the brain, that variation might not be only structural in nature. The most significant individual differences might be a result of the electrical or chemical signaling functions of neurons—features that do not necessarily involve changes in the shape of neurons or their wiring diagram. Differences in the properties of ion channels or neurotransmitter receptors can have a profound effect on the function of a neuron in a sexual touch circuit, but these differences cannot be identified by structural measurement, even with the most powerful microscope. So when we read Wolf’s concluding phrase, “these [sexual] preferences may just be due to your physical wiring,” we should remember that the word “may” is key, as the causal link remains unproven. And we should also think of “wiring” in the broadest sense of the term: not just the wiring of nerves in the genital and perigenital regions but also the wiring of neurons in the relevant parts of the brain. Additionally, we should also include individual variation that changes the electrical and chemical signals of neurons but does not change the neuronal wiring diagram (in either the brain or the skin) at all. 11

What can be learned by mapping the pattern of brain activation in the somatosensory cortex resulting from genital stimulation? Touch signals from the pelvis are carried to the brain along different pathways in the spine and the brain stem, depending upon the touch modality (fine discriminative touch, caress, temperature, etc.). Like other regions of the body, pelvic tactile signals form a relay connection in the thalamus and then arrive at the neocortex, where they are represented in a body map in the primary somatosensory region (figure 2.8). As we discussed previously, the genitals occupy a fairly small section of the overall body map, consistent with their low density of mechanosensors. In one study, undertaken by Barry Komisaruk and his colleagues at Rutgers University, women in a brain scanner were given a handheld dildo and asked to self-stimulate various genital regions—the external clitoris, the vagina, and the cervix. 12When examining the pattern of activation in the primary somatosensory cortex, each of these three genital regions activated a discrete patch of brain tissue, but these patches were adjacent to one another, and in some cases partially overlapped (figure 4.4). Importantly, and consistent with Penfield’s maps derived from brain stimulation, there is some individual variation in the size and precise location of these patches. Might the size of the patches be determined by individual variation in the fine structure of sensory nerve endings in the pelvis? If a woman has an unusually dense sensory innervation of the vaginal walls or a man has a particularly dense innervation of the scrotum, will those regions be enlarged in the body map? And what about the effects of experience? If one regularly engages in anal sex, will that expand the cortical representation of the anus and rectum in much the same way that daily violin practice expands the sensory map of the fingering hand? These questions remain to be addressed.

Consistent with Penfield’s classical body map (figure 2.8), the stimulation of the female genitals activated an area detached from the contiguous portion of the map, beyond the representation of toes. However, examining figure 4.4 you can also see adjacent, semioverlapping patches of activation just where you might expect, at the groin portion of the map at the intersection of the thighs and the abdomen. Why are the female genitals represented in two places on the map? It turns out that this is not only a female phenomenon, as men have dual representation of the genitals as well. Figure 4.4 shows activity in both locations, but the authors of this study claim that the patches near the toes are the real genital activation sites and the patches at the groin site result from incidental activation of the adjacent tissue surrounding the genitals. 13

Interestingly, when women in this experiment stimulated their nipples, two different patches of activation were likewise seen in the brain scanner: one at the chest site on the body map, and another detached one, located beyond the toes. In fact, the patch activated by nipple stimulation significantly overlapped those of the clitoris, vagina, and cervix. This may explain why nipple stimulation is sexually exciting to many women. More generally, it raises the issue of whether this particular area of the primary somatosensory cortex (called the mesial surface of the postcentral gyrus) has a special role in sexual touch. 14If you were to examine a microscope slide prepared from postmortem tissue of this area of the brain, you’d find nothing unusual about it. The individual neurons and glial cells and their overall layered structure look almost the same as those in regions processing somatosensory information from less erotic parts of the body. It may turn out that this region has unusually strong connections with other brain regions involved in pleasure and fear, but to date this has not been investigated.