Neuro Question: How the brain knows the origin of touch senses

[pizza100]

---

Members don't see this ad.

Hi. Do you guys know how the brain knows where you're being touched? If every inch of your body has a sensory neuron that goes directly to the brain, then their axons would clog up the space inside the backbone due to the massive number of sensory neurons in the periphery. The peripheral sense neurons must use some sort of common highway along the spinal cord to transfer their signals to the brain. That means many peripheral sensory neurons converging onto 1 neuron in the spinal cord, and that 1 neuron in the spinal cord would carry that info to the brain.

So, if they use a common neuron "or highway" in the spinal cord to carry sensory info for a bunch of other sensory neurons in the periphery, then how does the brain know where is the origin of that info? Basically, how does the brain know that you're touching my 4th finger rather than my 5th finger?

 

[munchymanRX]

Hi. Do you guys know how the brain knows where you're being touched? If every inch of your body has a sensory neuron that goes directly to the brain, then their axons would clog up the space inside the backbone due to the massive number of sensory neurons in the periphery. The peripheral sense neurons must use some sort of common highway along the spinal cord to transfer their signals to the brain. That means many peripheral sensory neurons converging onto 1 neuron in the spinal cord, and that 1 neuron in the spinal cord would carry that info to the brain.

So, if they use a common neuron "or highway" in the spinal cord to carry sensory info for a bunch of other sensory neurons in the periphery, then how does the brain know where is the origin of that info? Basically, how does the brain know that you're touching my 4th finger rather than my 5th finger?

I think where you're going wrong is that you're assuming that once the sensation gets to the spinal cord, it is transmitted onto 1 neuron. This is not the case. Peripheral touch-sensing neurons can synapse onto many neurons in the spinal ganglia, which in turn send axons up through the dorsal spinal cord in distinct tracts such as the fasciculus gracilis and fasciculus cuneatus. These axons ascend up to the thalamus (on the opposite side for pain/temp and same side for touch/position), where they are routed to the somatosensory cortex of the brain. Different areas of this cortex are responsible for processing sensation for different parts of the body. To illustrate, ever seen http://en.wikipedia.org/wiki/Cortical_homunculus that picture?

 

[calvnandhobbs68]

Haha have you taken neuro yet or are you taking it and are confused?

What you're talking about would travel through the Dorsal Column Medial Lemniscus pathway (DCML). http://en.wikipedia.org/wiki/Posterior_column-medial_lemniscus_pathway

It would also depend on the individual neuron receptive fields. http://en.wikipedia.org/wiki/Receptive_field

If you read through those two articles you should have a pretty decent idea of the answer for your question.

 

[calvnandhobbs68]

I think where you're going wrong is that you're assuming that once the sensation gets to the spinal cord, it is transmitted onto 1 neuron. This is not the case. Peripheral touch-sensing neurons can synapse onto many neurons in the spinal ganglia, which in turn send axons up through the dorsal spinal cord in distinct tracts such as the fasciculus gracilis and fasciculus cuneatus. These axons ascend up to the thalamus (on the opposite side for pain/temp and same side for touch/position), where they are routed to the somatosensory cortex of the brain. Different areas of this cortex are responsible for processing sensation for different parts of the body. To illustrate, ever seen http://en.wikipedia.org/wiki/Cortical_homunculus that picture?

Well no, nothing actually synapses in the dorsal root ganglia. Remember this is where the bodies of primary afferents are. In fact, the first synapse in the DCML pathway occurs when the primary afferent synapses with the dorsal column nuclei in the thalamus.

 

[pizza100]

Thanks everyone for helping me out. I'm 2 weeks into my Neuro course.

I read the articles that calvnandhobbs68 pointed me to. I understand the concept of receptive fields through the following line "many sensory receptors all form synapses with a single cell further up, they collectively form the receptive field of that cell".

I take the above quote to mean that, in the three sensory neurons system, many first neurons probably synapse with one second neuron, and many second neurons synapse with one third neuron. Am I correct in that assumption? It seems like the more senses that get combined into one neuron further up, the less precise the location of those senses become to the brain.