How is information sent from the skin to the brain?

Question

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?

Answer 1

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

Answer 2

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.