David Linden - Touch

Figure 4.1The same odor can be perceived as foul or pleasant, depending upon context and expectation. This is a general feature of perception that also applies to tactile sensations. Comic by Julia Wertz; used with permission.

Still, while giving armpits, ears, and eyebrows their due in erotic potential, the genitals are unique. In the right context almost everyone finds stimulation of the external portion of the clitoris or the glans of the penis to be sexually exciting. Why is that the case? Is there something special about the structure of the skin in those locations? Like the lips and the fingertips, the skin of the external clitoris and the glans penis is glabrous, lacking even the finest hairs and their associated nerve fibers. As we’ve seen, the lips and fingertips are richly endowed with mechanoreceptors that allow for fine discriminative touch, but both the clitoris and glans penis have very few of these. What these tissues do have in abundance are free nerve endings that transduce heat, cold, pain, and inflammation. 3In addition, they have a specialized type of nerve ending consisting of a coiled axon wrapped by a few nonneuronal encapsulating cells (figure 4.2). These have been called genital end bulbs (or, in the wonderful original German term, Genitalnervenkörperchen). 4

Figure 4.2Is this structure important for sexual touch? Two genital end bulbs in the skin of the human glans penis, located in the most superficial layer of the dermis. These genital end bulbs are also found in the glans clitoris at even higher density.

While there’s been some argument about their distribution over the years, it seems that the genital end bulbs are not entirely restricted to the genitals. In other tissues that are similarly coiled, loosely wrapped nerve endings are called mucocutaneous end-organs. They are also present in other types of glabrous skin that can be involved in sexual touch, such as the lips, the nipples, and the skin surrounding the anal opening. Perhaps the external clitoris and the glans penis play a prominent role in sexual sensation merely because they have a high density of this type of nerve ending. The external portion of the clitoris has the highest density of any tissue, and within the glans penis, the greatest density of genital end bulbs is found at the corona (the ridge marking the edge of the glans) and the frenulum (the elastic tissue on the underside of the glans penis). These are spots that men often report to be the very most sensitive locations for sexual stimulation. 5

But do mucocutaneous end-organs truly have a special role in sexual touch sensation? Unfortunately, we don’t know, as there are as of yet no drugs or genetic tricks that can selectively activate or inactivate them, in either humans or laboratory animals. We don’t even know if their electrical signals follow a unique path into the spinal cord or the brain. In fact, to my knowledge, their electrical signals have not even been recorded in isolation. For a structure that’s potentially so important to human life, it’s amazing how little we understand about it. 6

Touch sensations from the genital region pass through three different nerves on their way to the spinal cord and, ultimately, the brain (figure 4.3). The pudendal nerve is the most important for sexual sensation, carrying signals from the clitoris in women and the penis in men. Because the penis and the clitoris derive from the same undifferentiated bit of embryonic tissue (which ultimately takes its particular form in males and females based upon the influence of sex hormones during early development), it is not surprising that they are innervated by the same sensory nerve. Importantly, a single sensory nerve can carry information from several different parts of the crotch. In women, the pelvic nerve conveys touch signals from the labia minora, the vaginal walls, the anus, and the rectum. In men, the pudendal nerve carries information from the anus and the scrotum as well as the penis. In women, sensations from the cervix and the uterus can also be conveyed by the hypogastric nerve as well as the cranial nerve called the vagus nerve, which travels directly to the brain stem, thereby bypassing the spinal cord entirely.

This convergence and divergence of touch signals from the pelvic region have some important consequences for our experience of sex. Cross talk between adjacent genital and perigenital locations may help explain why stimulation of the anus, rectum, and perineum can sometimes elicit strong sexual sensations. While many people enjoy stimulation of these areas during sex, some find that perigenital erotic sensations are not entirely limited to sexual situations. For example, a man in the Netherlands reported that he had an orgasm every time he would defecate. 7

The wiring diagram of the sensory nerves also helps us understand how sexual sensation can be preserved in some cases of spinal cord injury. An injury resulting in complete transection of the spinal cord in the lower back, at the level of the second lumbar vertebra, will interrupt sensations from the penis and scrotum in men and the clitoris and labia minora in women, as carried by the pudendal and pelvic nerves. However, because the hypogastric and vagal nerve signals still have an intact path to the brain, this damage to the spinal cord would spare some sexual sensations from the cervix and uterus in women and the testicles and prostate in men. Indeed, some women with complete spinal cord transection even higher in the spinal column, well above the entrance of the hypogastric nerve, report strong sexual sensations from the cervix and uterus, which are thought to be carried by the vagus nerve. 8

It’s easy to look at schematic anatomical drawings like that in figure 4.3 and imagine that the wiring diagram for the nerves carrying sensations from the pelvic region is absolutely the same in every man and every woman. At the broadest level, this is true: Nearly every man will have sensation from the penis, and nearly every woman will have sensation from the clitoris, carried by the pudendal nerve. But when we look at the fine branching patterns of the nerves and the distribution of the nerve endings, individual variation becomes apparent. Comparing two women, one might have somewhat fewer genital end bulbs in her external clitoris but more in her labia minora and anus. Comparing two men, one might have more free nerve endings in his prostate but fewer in his scrotum.

Could the fine structure of the sensory nerves innervating the genital and perigenital regions underlie individual variation in preferred sexual activities? This idea has been suggested by anatomists for many years. 5More recently, feminist author Naomi Wolf has extended this argument. In her book Vagina: A Cultural History , she speculates that individual neural variation can explain differences in women’s sexual preferences:

Figure 4.3The organization of the sensory nerves innervating the pelvic region. Sensory information from the pelvis is carried to the brain by three pairs of spinal nerves—the pudendal, the pelvic, and the hypogastric—which enter at different levels along the spinal cord. Sensations from the uterus and cervix are also conveyed via a cranial nerve, called the vagus, which enters the brain stem directly. It is important to note that even a single nerve can convey information from a number of different skin sources. The male pudendal nerve, for example, carries sensations from the penis, anus, and scrotum. © 2013 Joan M. K. Tycko