Nervous density

September 22, 2005 4:54 AM

Some questions about nerve endings...

This has been bugging me for a while and I can't find any easy to follow answers on googling...

Do we have a fixed number per person or per square centimeter of flesh?

Does someone with a bigger body feel more pain? Does a large penis have less sensation than a small one? I've often heard it said that big breasts have less sensation - is this just because the nerves are more widely spaced? If so do they become more sensative as they shrink back with weight loss?

posted by twine42 to science & nature (8 comments total)

Would it not depend? If a girl was to eat a lot, she'd put on a lot of weight and her breasts would soon enough balloon to massive proportions, but that would all be fat -- no new nerve endings would be created in the process. I'm sure the number of nerve endings vary from person to person, however I do not know how much.

I will grab my dad's Gray's Anatomy and see if I can find out. :)
posted by PuGZ at 5:26 AM on September 22, 2005

This question is complex, because the spatial resolution of your tactile sense is due partly to receptors in your skin and partly to the configuration of your somatosensory cortex.

I don't know whether you get more receptors if you get fatter. I DO know, though, that you can gain effective resolution by using them a lot---witness people who learn to read Braille having much more sensitive fingertips. A recording study was done in monkeys to determine the receptive field size (i.e. the size of a finger "pixel") of some of the cortical cells mapped to the monkey's finger. At first they were fairly large, but after an extended period where the monkey's hand was kept placed on a rotating disk with texture on its surface, they shrank, effectively becoming more discerning.

No citation, naturally, but I was told so by a Professor.

(In a Neuroscience class, not, say, basket weaving.)
posted by tss at 6:52 AM on September 22, 2005

If I recall neuropsych classes correctly, the perception of sensations is controlled by very specific mapping of your sensory cortex to various body parts. Some areas, although small in size, have proportionately large areas of the cortex devoted to them. This is often referred to as your somatosensory homunculus, with homunculus meaning "little man". You can draw all sorts of weird and interesting conclusions about humankind just by looking at the map. (For example, I had a professor who was fond of pointing out the side-by-side placement of your genitalia and your feet, and theorizing that foot fetishes develop from a couple crossed wires.) The homunculus suggests that more endings/larger size wouldn't get you anything in the way of increased sensitivity, since the mapped areas don't correspond to size in the first place. You may also want to look at this little flash presentation for another view.

By the way, you also have a motor cortex homunculus, which explains why some parts of our bodies are more controllable/capable of fine movements than others.
posted by donnagirl at 9:54 AM on September 22, 2005

Playing around with one sense or another won't increase the number of nerve endings. What it will do is recruit "unused" or underused cortical cells from neighboring areas. If you use your fingers more, then more neurons in your sensory cortex will be devoted to interpreting those signals (at the expense of a small loss in resolution of surrounding senses, most likely, although I don't think anyone has studied it).

The cool thing about sensory cortex is that it turns into whatever it needs to be based on input. If you experimentally re-route visual pathways to the cortex that normally interprets sound, for example, it develops into visual cortex and makes the right connections. I saw a great presentation on this, with a ferret that could see with what was supposed to be auditory cortex and hear with what would normally be visual cortex. The brain never ceases to amaze.

This also explains why you get "phantom limbs" after amputations: the cortex devoted to that area is picked up by surrounding senses, but because it is still hooked in to your brain as "left arm" for example, an itch in your left shoulder might get misinterpreted as (missing) left elbow, if the former elbow nerves have been recruited for use by the shoulder.

(Note for position of authority here that although I am a neuroscientist, my field of study was neural mechanisms underlying behavior, not sensation, and my "hands on" experience is pretty much limited to the thalamus and hypothalamus, not the sensory cortex.)
posted by caution live frogs at 12:50 PM on September 22, 2005

Here is a fun experiment for you. Get two pencils, not too sharp. Get a volunteer. Tell them to close their eyes, and tell you whether they feel one prick or two. Than, start exploring their body by poking them (lightly!) with both pencils close to each other, in carious spots. You will probably want a volunteer that you can get very up close and personal with.

What you will be doing is determining the resolution of skin sensitivity (which as pointed out above is a combination of actual mechanical receptors feeding into neurons, and the wiring of the cortex). You will typically find that a person's finger can tell one poke apart from two even if they are very very close - as close as you will be able to get to using pencils. On their back, you will likely have them reporting two pokes within a centimeter of each other as one. That's because there is not enough spatial resolution to tell them apart. This is not, however, proof enough to say that a square cm of back skin has only 1 neuron going to it - the true "resolution" is more complicated and I won't pretend I even know all the factors that go into figuring it out.
posted by blindcarboncopy at 7:48 PM on September 22, 2005

Everyone who's posted above is correct, and certainly cortical representation and recruitment is more important than nerve fiber density in terms of the sensory experience that a person has.

But no one's answered the OP's original question, which is a different question, and also interesting.

I don't know the answer either. I'll try to dig it out.
posted by ikkyu2 at 12:03 AM on September 23, 2005

The answer is found on page 436 of Principles of Neural Science, Kandel, Schwartz, and Jessel, 4th ed. (McGraw-Hill 2000, if you care.)

A section title on that page reads: The Spatial Resolution of Stimuli on the Skin Varies Throughout the Body Because the Density of Mechanoreceptors Varies.

There are many different types of skin-based sensory receptors, and the different types are predominant in different locations. Also, the density of a single given type of receptor varies depending on where you are looking in the body.

Interestingly, because from 10-25 receptors are connected to a single afferent nerve fiber, the afferent nerve fibers behave differently depending on the location where their receptors are found. In places of low receptor density, for example, the individual fibers have larger "receptive fields."

If you were interested in learning more about this, Kandel, Schwartz, and Jessel is considered a definitive reference and is wonderfully well written and illustrated.
posted by ikkyu2 at 12:35 PM on September 24, 2005

Does someone with a bigger body feel more pain? Does a large penis have less sensation than a small one? I've often heard it said that big breasts have less sensation - is this just because the nerves are more widely spaced?

All of these questions immediately run into a philosophical conundrum: how do you compare subjective sensory intensities from person to person? The answer is that this cannot now be done in any kind of meaningful way that would satisfy our wish for scientific rigor.

Peripheral nerve structures in skin die off and regenerate all the time, too; and memory of sensations may not be accurate. So, asking about sensations in someone whose boobs shrank over time is also fraught with issues of validity.

Still, I'd be interested in hearing from boob-shrinky ladies, but I guess no one wants to admit that their boobs shrank on AskMe.
posted by ikkyu2 at 12:38 PM on September 24, 2005