Jul 032010
 

I’ve been brushing up on my neuroanatomy lately, and I’m surprised by this thing I haven’t been able to find out. Maybe one of you can tell me.

All the cortical homunculi I’ve ever seen have been male. Like this guy:

somatosensory homonculus

And this slightly more controversial guy:
somatosensory homunculus

I’ve never seen a female somatosensory homonculus.

Why?

I *suspect* it’s because a female somatosensory homunculus would have a clitoris bigger than its head – nearly TWICE the size of the homologous penis – and the scientists responsible for these things just can’t quite cope with that.

But. Would it have that massive clitoris?

It’s true that the clitoris has something approaching twice the number of nerve endings as the penis. Not quite, but nearly – which is pretty impressive in something roughly 1/8th the size of its male homologue – but that’s the PERIPHERAL nervous system.

What about its CORTICAL sensory representation? These cortical somatosensory homunculi (*so* fun to say!) are representations of how much sensory cortex is taken up by each body part. You’ll notice the penis is fairly large in one and QUITE large in the other; this is because the penis – along with the hand, toes, and lips – takes up a lot of sensory BRAIN space.

So is the peripheral hyperenervation of the clitoris reflected in its cortical representation?

I’ll ask that again in a different way, for people who have never thought seriously about the nervous system:

Body parts have nerves endings that detect sensation – some (like your hands, feet, lips, and genitals) have a LOT. They bundle together in the spine and make their way north to the brain. Those nerves in your body are your peripheral nervous system (PNS). The nerves in your spine and brain are your central nervous system (CNS).

Your brain, in turn, is organized in layers. The outermost layer, with all the wrinkly bits and the split down the middle like an unbaked loaf of French bread, that’s your cortex. An adult’s cortex is organized into modules (for want of a better word). The part in the front does decision-making and reasoning. The part in the very back processes visual information. Over on the right side you’ve got motor coordination, sensation, and visuospatial things like music and math, and over on the left you’ve got language comprehension and production.

(Cortical neuroanatomy in one paragraph. Obviously, this is not the whole story.)

Now. What I’ve said about the clitoris having all these nerve endings describes the relevant bit of the PERIPHERAL nervous system. Those picture up there of the dudes with massive hands is a representation of the relevant module of the CENTRAL nervous system.

What I want to know is: is the difference in PERIPHERAL enervation of the clitoris reflected in the CENTRAL representation of the clitoris?

To put it plainly: would a female somatosensory homonculus have a clitoris nearly twice the size of this penis?

I can not find the answer. No one I know seems to know either.

I’m putting out a call to the internet. Please, someone, tell me if the representation of the clitoris in primary somatosensory cortex is as disproportionate as its representation in the peripheral nervous system?

emily nagoski

  22 Responses to “the clitoris in your head”

Comments (21) Pingbacks (1)
  1. That is one hell of a title!

  2. Adult cortex is not organised in modules.

  3. I thought the clit that one can see is merely the tip of it and the entire/actual clit is a lot larger but almost completely buried/hidden from view?

    • in a homonculus of this type, the interior structure of the clitoris wouldn’t matter, at least for surface dimensions of the figure, no?

  4. Interesting question! I don’t know the answer, but I have a related anecdote to share:
    My neuroscience class based a video skit for our annual retreat on a similar question. Some grad students went around to all the professors with a version of pin-the-tail-on-the-donkey game — they could choose between penises of varying sizes and pin one on the map of the cortical somatosensory homunculus. (At the time, I did not even think of the gender dichotomy!) Most of the professors treated it as the joke it was and chose the largest penis and pinned it on near the representation of the abdominal wall.** One prof had an interesting point, though. He chose a smallish penis. He pointed out that although the penis has high sensitivity, it has a low two-point discrimination threshold (I don’t know if this is really true). The way he put it was “There’s a reason blind men don’t read braille with their penises” (although I can think of other reasons) … He argued that the cortical representation has more to do with the calculation of fine discrimination than sensitivity thresholds, but I don’t know if this is true. You could imagine that it would be, though. Genitalia have lots of nerve endings, but they may overlap in a way that makes fine point discrimination difficult, no? I’m not really sure. Is there really evidence for the “more controversial” figure you depict?
    ** Interesting side note. In early experiments, the representation of the penis in the somatosensory cortes was determined to be located near the upper legs, buttocks, etc. More recent experiments using fMRI have relocated it next to the toe/foot area. People think this may have something to do with sensory experience in the fetal position in the womb. The previous results may have to do with people preferring to self-report stimulation of the thighs/buttocks rather than the genitalia during brain mapping experiments.

    • Yes indeed, penises (and indeed clitorises) have terrible two-point discrimination – it has to do with the TYPE of nerve endings, the names and other details of which I currently forget (Merkel… Meissner?), but phalluses have a dirth of the one that gives them protopathic sensation, whereas hands have that one in abundance.

      The idea, apparently, is that it takes more brain space to represent these fine-detail sensations because their “receptive fields” are so very very tiny – tiny on the body, that is, but receptive fields are all about the same size in the brain, so many small receptive fields on body = MASSIVE cortical resources.

      I didn’t know about the womb thing! So interesting! I’ll have to investigate further.

  5. Turns out if you Google “sensory cortext homunculus clitories” Google corrects your spolling and reveals the following link:

    http://neurocritic.blogspot.com/2009/08/new-clitoral-homunculus.html

    which summarises

    Michels, L., Mehnert, U., Boy, S., Schurch, B., & Kollias, S. (2009). The somatosensory representation of the human clitoris: An fMRI study NeuroImage. DOI: 10.1016/j.neuroimage.2009.07.024. http://dx.doi.org/10.1016/j.neuroimage.2009.07.024

    • And someone in the comments posted

      Allison, T., McCarthy, G., Luby, M., Puce, A. and Spencer, D. D. (1996). Localization of functional regions of human mesial cortex by somatosensory evoked potential recording and by cortical stimulation. Electroencephalography and clinical neurophysiology, 100(2):126-40.

    • Oooooh that’s interesting… it doesn’t seem quite to answer my question, but it’s good to know that someone is working on it.

      And I’d like to see YOU summarize the central nervous system and build some context for the idea of a somatosensory map of the body in cortex in one paragraph while not shortcutting with “modules”! Really – I’d like to see you do it so that I can just quote you from now on and not have to worry about it anymore!

      • I wouldn’t try to do any such thing, because it’s intrinsically the wrong way to think about the brain. Seriously, this is basically what you bang on about re:sex stuff here (getting precise, spelling things out) – why limit that to one domain? If you’re going to be a nerd, then you might as well commit.

        That 2009 reference should serve as a handy start for a lit search, anyway.

        • It definitely will – thanks! I’ll continue investigating and see where it takes me. (Quiet shock and horror that it’s so extremely recent!)

          Why limit it to one domain? That’s actually a really good question and I wonder sometimes, since I do take shortcuts about some of the brain stuff, some of the social construction stuff, some of the historical stuff, etc, why I sometimes feel it’s okay and sometimes not okay to take shortcuts. It’s something I’ve been thinking about a lot lately, as a matter of fact, since I’m preparing lectures for the first time in nearly 5 years, and I need to decide what’s important and what I can skip or skim over.

          Remember in “Partly Cloudy Patriot” when Sarah Vowell talks about nerds and how their extremely detailed focus on a tiny part of the puzzle helps to grow that little tiny part, that pixel in the picture?

          I think it’s like that. i am a nerd committed to growing the sex piece of the puzzle. Unfortunately both the particular piece of the puzzle that obsesses me (sex) and the particular way of looking at it that I have (extreme interdisciplinarity) means that exploring my puzzle piece demands MASSIVE integration of very diverse domains of knowledge.

          But how thorough need I be when laying a sort of groundwork for folks, so they can understand what it means for sex? Will the ladies and gentlemen out there find benefit to their sex lives if they understand about self-organizing dynamics of cortical development? Will they have better orgasms if they thoroughly thought through group selection as opposed to selfish gene theory? How much do they really need to understand of Foucault before they can let go of social constructs of sexuality in order to accept their own sexuality? In general, I tend to conclude, no.

          But they WILL have better sex lives if they actually understand the relationship between arousal and desire, the differences in selection pressure on human males and females, and the social construction (and medicalization) of women’s sexuality.

          I’m forced to conclude that sometimes the line I draw is arbitrary, sometimes it’s drawn by my own limitations of education, understanding, or interest, and sometimes it’s drawn by the way *I* benefit from the information. Occasionally it’s drawn where I think “Andrew will scold me if I say that.” :o)

          I do make an effort to be clear when I’m saying something that falls outside my domain or when I’m taking a big shortcut. But I’m going to take shortcuts. I want people to understand the kinds of problems sex researchers and educators deal with, and that means figuring out what details require precision and which can be rendered in broad strokes.

          (Am I wrong to doubt you’d object if I offered a one paragraph summary of Foucault? :o) )

        • WordPress doesn’t like it when I use a lower case “o” as a nose in my smiley faces. :(

      • Drawing lines is fine; you have to do it. But in a post about the sensory cortex homunculus it seems weird to summarise the brain as modular. I guess the line moves depending on what your talking about: knowing more about the brain might not help with orgasm but it might help in a post about the brain.

        Whenever I blog, I try not to write about something that includes a section I have to gloss over too much; I try to take the hint that if it’s important enough to come up in the discussion, it’s important enough to lay out properly. This comes from writing scientific articles, actually; it’s how you write a thorough piece.

        • Is it incorrect that some areas of cortex are responsible for some jobs, and other parts of cortex are responsible for others? If I called them “functional areas” rather than “modules” would it matter less that I had been so superficial in my description?

          And a blog post, to me, is a different thing from a scientific article; for me it’s much more like answering a question someone asks me at tea on Fridays at 4: they want an answer they can understand and use. And when a Women’s Studies major, who is smart but has never taken a psychology class in her life, asks, for reasons to do with social justice, what they guy with the giant hands and lips is and why there isn’t a female one, and if there were a female one what it would look like, what would YOU tell her about cortex – in about 500 words, that she’ll understand, and that will answer her question without boring her so much that she regrets ever asking?

          Really, what would you say? It’s a difficult problem, in my experience.

          I described cortex as a wrinkly unbaked loaf of French bread with a split down the middle, for heaven’s sake! And you’re worried that I used the word “modules”? Well, I suppose the bread thing is obviously metaphorical, whereas “module” has technical implications that I don’t actually mean. But honestly it plain old didn’t seem important enough to say more than I did.

          Good writing, good science, good advice. You see why it’s so rare to find all three? I’m moderately good at each of them, but in combination it’s really quite a challenge.

          So here’s what I propose: you write what I *should* have said in this post and we’ll put it up as a guest post on the blog.

  6. I’ve recently been thinking a lot lately, and have only to say that the clitoris is linked to the most inner most you in your brain. My theory is that it would be Jupiter/Earth-mammoth level proportions in that CNS somatosensory model of a human female.

    • and also, i think there is a significance in the difference of epicritic relays vs protopathic relays and where the electrical current is eventually dispersed in the brain. Are these differentiated?

  7. A quick question: I think I read somewhere that if you compare the nerve endings in the clitoris with the nerve endings in an uncircumcised penis, meaning that you include the innervation of the foreskin, that things are a little more balanced. Anybody know if that is true?

    • In “The Vagina Monologues,” Eve Ensler brings this up over and over again as a form of empowerment. When I first heard it, my question immediately was “are they comparing a circumcised penis to an unaltered clitoris?” Hearing this “fact” (sorry for the scare quotes, not sure the exact term to apply here) repeated again and again started making me angry – is she really gloating over those of us who’ve had our genitals desensitized at birth as a matter of course? I’m certain that was not Eve’s intention, but that’s how it came across once I went down that mental path.

  8. Emily, though I find Andrew’s insights peculiar yet interesting, I find HIM generally annoying! Your perspicuity is; however, refreshing and delectable and of real and prescient use to us lay folks, who delight in your well selected, succulent bits of info without having to go through so much excessive technical gobbeldygook! Thank You Emily- Andrew would do well to learn from Your example. Keep up the good work!!! :)

Sorry, the comment form is closed at this time.