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Say you have a needle, and you poke a very specific area on your left thumb. A signal gets sent from that nerve up your spine and into your brain.

How does the brain know exactly where this signal has come from to such accuracy?

  • Does each nerve in every conceivable part of the body have a unique path that goes up the spine and into the brain, so that's how the brain knows where it came from?
  • Or does the nerve encode the signal as data which travels up a generic nerve, and the brain decodes the information, which contains the "address" of the particular spot that was stimulated? If this is the case, how are the data encoded? Binary?
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For a quick primer, see en.wikipedia.org/wiki/Receptive_field#Somatosensory_system, but hopefully I (or someone else) can come along and fill in the gaps in a bit. –  jonsca Jun 19 at 19:05
    
Ok so that explains the receptors and their surface area. How do these signals then reach the brain and how does the brain know exactly where the signal comes from? –  user2721465 Jun 19 at 21:14
    
    
@user2721465 I will try to address some of these questions in my answer. –  jonsca Jun 19 at 21:47

2 Answers 2

up vote 2 down vote accepted

In addition to what Spinoral has said, and in response to your comment above, I will add a bit more about the mechanisms.

Essentially, in some form or another it's receptive fields all the way up.

A brief outline of the anatomy: The cells from the spinal cord are pseudounipolar and have their cell bodies in the dorsal root ganglia in the spinal cord. The other "leg" of the sensory neuron projects to the medulla, which has cell bodies which send axons to the VPL thalamus, which then has cell bodies that project to the primary somatosensory cortex. There are points of decussation (crossing over the midline), but I'll ignore those for now.

The receptive fields:

The receptive fields of somatosensory neurons share much in common with the receptive fields of visual neurons. As for visual neurons, the somatosensory receptive fields comprise a restricted 2-dimensional region of space where a stimulus can evoke a neuronal response. In somatosensory neurons, however, space refers to a region of the body and the stimulus can be touch, vibration, temperature or pain

(from Scholarpedia)

Essentially, the somatosensory input is "filtered" at all of those stops along the way and refined in terms of things such as whether the sensation came from the center or the edges of the receptive field, etc.

Cells in the cortex (the waypoint for the sensory information) are organized in microcolumns, which are representative of a particular location on the body (and arranged topographically into the "homunculus", as the other answer mentions).

All of this follows what is called the "labeled line" theory, which is getting a bit outmoded due to new knowledge about ensemble coding in neurons, but for purposes of the somatosensory system it still holds up pretty well.

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This is a bit of a work in progress, but it should give you some terms to google, etc. –  jonsca Jun 19 at 22:12
    
Great, thanks this is exactly what I was looking for. So there's 2×10^8 minicolumns in the cortex corresponding to places on the body. Does that mean that there's roughly an equal amount of sensory neuron pathways in the spinal cord correlating to these minicolumns? –  user2721465 Jun 19 at 22:49
    
@user2721465 It's not necessarily a 1-1 relationship between skin receptive field and column in the cortex (there are higher level "abstractions" in the cortex such as orientation of the stimulus, etc.). Since the information is being "filtered", there are complex relationships between the thalamic cells, and there's some degree of consolidation that goes on. The bundles of axons are converging on the cord at different levels (see a map of the dermatomes for the mapping), but the tracts are quite thick anatomically, so there's many thousands of axons in one nerve bundle. –  jonsca Jun 19 at 23:08
    
(for example, the roots for the arms enter between the vertebrae of the cervical region -- en.wikipedia.org/wiki/Dermatome_%28anatomy%29, legs in the lumbar, etc.) –  jonsca Jun 19 at 23:10
    
If this is something that you are interested in, the rat whisker system is the model used for decoding some of these pathways. The earlier connections are different (it's a nerve that goes directly to the brainstem), but the cortical layout is similar. –  jonsca Jun 19 at 23:14

The accuracy of sensation is actually quite variable depending on the region of our body. Highly innervated areas such as our fingers have a higher degree of accuracy than sparsely innervated areas such as our legs.

There is a simple experiment you can do to illustrate this. Close your eyes and then get a friend to lightly and slowly run their finger from your wrist, up towards the bend in your elbow on the inside of your arm. Try to guess at what point the reach the joint of your elbow.

In terms of how the brain knows where it came from, there is part of your brain called the sensory cortex. This sensory cortex has a pattern that corresponds to areas of the body. This pattern is known as a the sensory homunculus (google images will find this). As signals pass up the various different spinal pathways and into the brain they are sent up to this sensory area and the appropriate region of the homunculus is stimulated thereby giving the sensation in the appropriate body area.

Interestingly in some brain surgery a patient may be kept conscious so that these areas can be directly stimulated by electric current and the patient will experience sensations.

There is also a matching area, called the motor cortex, which corresponds to movement rather than sensation.

The above is a simplified version of what goes on and there is lots more detail which would be covered in neuroanatomy books

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Ok, so the signal from the nerves travels up the spinal pathways into the sensory homonculus. Does the sensory homonculus know where the signal came from because a specific spinal pathway is used which is unique to each area of the body or is it because the signal is encoded in a particular and unique way? –  user2721465 Jun 19 at 21:38
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Due to specific spinal pathways, but my point with the experiment and the arm is that this is not as accurate as you may perceive it to be. –  Spinorial Jun 19 at 21:45

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