Re: Are touch receptors evenly distributed on arm and legs of humans?

Hi Jon --

This is one of the coolest topics in neuroscience (I think) ...

Touch receptor density is not uniform across the body surface of humans (or 
other animals, for that matter).  Receptor density is highest where fine 
discrimination is most important -- places like your lips and your 
fingertips -- and lowest where fine discrimination is pretty unimportant -- 
for instance, across your back.

You can test receptor density by doing a 2-point discrimination test.  Have 
a friend take two sharp pencils, and hold them a small distance apart.  
While your eyes are closed, have your friend touch the pencil tips 
simultaneously to a part of your body (try your arm, your back, and your 
index finger, for starters).  You may feel the two tips as one point; if so, 
ask your friend to move the tips a bit further apart until you can 
distinguish the two points. You may be surprised by how far apart the tips 
need to be for you to discriminate them separately!

The area of your brain which senses stimuli to the body surface is called 
the somatosensory cortex; it's a stripe of brain that runs vertically, 
roughly around your ears.  What's cool is that body surfaces with a high 
density of receptors also get a big portion of area in this brain region.  
Conversely, areas of the body with a low density of receptors get a really 
small area of the somatosensory cortex.  Most neuroscience textbooks have a 
picture of the "homunculus," which represents the body with respect to the 
amount of brain area devoted to sensing touch there.  The homunculus has 
HUGE lips, tongue, and hands, but a pretty small body.

One of the coolest areas of neuroscience has been the study of brain 
plasticity.  A series of studies have looked at the allocation of 
somatosensory cortex area to the hand.  If a particular finger is used 
heavily, its brain area increases; if it is underused, the brain area 
decreases.  When a few fingers are repeatedly used together, their 
representations become fused together -- touching any one finger kind of 
feels like touching all of them.  Some amazing recent work has looked at the 
brains of violin players and people who read Braille, to determine what 
kinds of changes have taken place with intense finger activity.

Hope this helps,

Amanda Kahn
amandak@phy.ucsf.edu

**********
Here are some references you might want to check out:
 http://faculty.washington.edu/chudler/twopti.html  -- a guide to 
two-point discrimination experiments.
 http://faculty.washington.edu/chudler/brainsize.html -- scroll down to see 
the homunculus.  Another good place to see a homunculus is in any college-
level Neuroscience textbook.  Two books to look for are Eric Kandel's and 
Dale Purves', both of which should be at any University library.
 http://www.jhu.edu/~jhumag/996web/touch.html -- a good article on touch

***********

If you have access to a University library, you may try to look up these 
papers (most of them are not on the web).  They're written at an advanced 
level, but the data is fascinating:

**Classical papers on somatosensory cortex plasticity in primates:

Merzenich, MM, Nelson, RJ, Stryker, MP, Cynader, MS, Schoppmann, A, and 
Zook, JM. Somatosensory cortical map changes following digit amputation in 
adult monkeys. Journal of Comparative Neurology 224 (1984): 591-605.

Allard, T., Clark, SA, Jenkins, WM, and Merzenich, MM.  Reorganization of 
somatosensory area 3b representations in adult owl monkeys after digital 
syndactyly.  Journal of Neurophysiology 66 (1991), 1048-1058.  (what happens 
when you surgically fuse fingers, so that they're always stimulated 
simultaneously).

Pons TP, Garraghty PE, Ommaya AK, Kaas H, Taub E, Mishkin M (1991) Massive 
cortical reorganization after sensory deafferentation in adult macaques.  
Science 252: 1857-1860. (shows that big cortical changes can occur even in 
adult brains)

*** Recent papers on plasticity in humans:

from http://www.sciencemag.org

Elbert T, Pantev C, Wienbruch C, Rockstroh B, Taub E (1995) Increased 
cortical representation of the fingers of the left hand in string players. 
Science 270:305-307. 
Magnetic source imaging revealed that the cortical representation of the 
digits of the left hand of string players was larger
than that in controls. The effect was smallest for the left thumb, and no 
such differences were observed for the
representations of the right hand digits. The amount of cortical 
reorganization in the representation of the fingering digits
was correlated with the age at which the person had begun to play. These 
results suggest that the representation of
different parts of the body in the primary somatosensory cortex of humans 
depends on use and changes to conform to the
current needs and experiences of the individual. 

From http://www.nature.com

Sterr A, Muller MM, Elbert T, Rockstroh B, Pantev C, Taub E (1998) Changed 
perceptions in Braille readers. Nature 391: 134-135. 
© Macmillan Publishers Ltd.
The mature mammalian nervous system has a striking capacity for plastic 
remodelling in response to environmental changes,
but little is known about the perceptual and behavioural relevance of this 
phenomenon. Using magnetic source imaging we
show that the cortical somatosensory representation of the fingers is 
topographically disordered in blind Braille readers who
use three fingers on both hands to read. In addition, they frequently 
misperceive which of these fingers is being touched. This
correlation is suggestive of a functional role for cortical reorganization 
in the perceptual experience of these individuals.