But we might think of everything we feel with our skin as one sense – touch – these are several separate sense. We feel pressure. We feel changes in temperature, and and different neurons handle warmth and chills.
And we feel pain.
While Wilder Penfield published the famous maps of the somatosensory cortex over 60 years ago, it hasn’t been clear if the neurons that we use to pick up pain from tissue damage, nociceptors, make maps in the brain the way other sense do. There are fewer nociceptors in the skin than other sensory neurons.
A new paper by Mancini and colleagues set out to test this. They gave their volunteers either innocuous little puffs of air on their hands, or...
This hurt. Not much, but enough to set off the nociceptors in the volunteers’s fingers. The authors describe it as “pinprick.”
While they were doing this to the hands, Mancini and company were taking brain scans using functional magnetic resonance imaging (fMRI).
If you look at your hand, the middle finger is well, in the middle, flanked on either side by the ring and index fingers.
If there’s a map of nociceptors in the cortex, you should find that same order in the parts of the brain that respond to being shot with lasers. Using the colour scheme above, the blue should always be flanked by red on the one side and green on the other.
And that’s what you see. Check the area surrounded by the dotted white line in the picture:
The team also shows that the responses for the control puffs of air also map out in the same way.
Strictly speaking, the authors only show that there’s a map of the nociceptors of the fingers. Now, to assert that this means there is a full map of the sort that gets shown in textbooks is sort of like saying that because you have a decent map of the Mediterranean, you also have a decent map of Australia. That’s plausible, though strictly speaking, they haven’t mapped the entire nociceptive globe, so to speak.
It’s a nice demonstration that these neurons follow some of the same patterns of organization as other sensory systems. Which does lead to a bigger question: why does the nervous system tend to make these maps instead of some other form of organization?
Mancini F, Haggard P, Iannetti GD, Longo MR, Sereno MI. 2012. Fine-grained nociceptive maps in primary somatosensory cortex. The Journal of Neuroscience 32(48): 17155-17162. DOI: 10.1523/JNEUROSCI.3059-12.2012
Classic graphics #3: The somatosensory cortex