Touching the knee feels different than touching the foot for example.
Does each receptor have a direct connection to the brain or is it like a spider’s web? If it is a web, how can the brain differentiate between different signals? Does each part have a different signal voltage or such?
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The brain is indeed much like a spider’s web – more specifically, like the World Wide Web. Consider this: when each person powers up their computer and logs on to the internet, billions of pieces of data from millions of people are sent and received by millions of others. The same happens with the brain. Every part of the body – whether receptors, neurons, or nerves – “logs on” and sends a staggering amount of information to the brain. This includes data about stimuli, body signals, reactions to light, touch, hearing, smell, and so on. This complex interplay is how we interact with, respond to, and understand the world around us. It’s a mind-boggling amount of info, but the brain is like a supercomputer, expertly sorting and interpreting all this data in real-time
Something that is really telling about touch is how bad we are at localizing internal pain. It’s very common to feel pain somewhere else from where it originates, for instance getting kicked in the balls and feeling it in the stomach.
Much of the “whereness” of pain and touch comes from visual cues and proprioception (the sense of where your body is in the world). You learn to distinguish between different locations of touch partly because you can see when something touches you, partly because you have a model of where your body is, and partly because of the nerve path that the touch arrives from.
In fact, you can create tactile illusions where a person is convinced that they are out of their body by giving them conflicting information about touch and vision.
[https://www.discovermagazine.com/mind/virtual-reality-illusions-produce-out-of-body-experiences-in-the-lab](https://www.discovermagazine.com/mind/virtual-reality-illusions-produce-out-of-body-experiences-in-the-lab)
And there’s the famous “rubber hand” illusion:
[https://www.youtube.com/watch?v=sxwn1w7MJvk](https://www.youtube.com/watch?v=sxwn1w7MJvk)
The sensory cortex in the brain maps nerves from different parts of the body to specific locations on the cortex. The map is called a homunculus.
https://www.britannica.com/science/homunculus-biology
Its complex, but I have some pictures that are a simplification but may be helpful.
[The first one](https://ehealth.kcl.ac.uk/tel/NA1-HTML5/Pages/1_1_AscendingSomato/1_1_1/images/02-01_spino-thalmic-sag-sec.png) just crudely shows signals traveling from the spinal cord to the thalamus which can serve as a relay center and then on to the cortex of the brain. In reality there are 2 major spinal pathways for body sensation and an important structure in between thalamus and cortex called the internal capsule.
[The second one](https://www.researchgate.net/profile/Yin-Gang-2/publication/352375036/figure/fig5/AS:1118867166371859@1643770128205/The-mapping-skeleton-of-the-somatosensory-cortex-in-the-brain-and-the-human-body-The.png) is called a sensory cortex homunculus. It shows roughly where the signals get sent to the brain that originate from different parts of the body.
So in summary, instead of differentiating the signals based on something like voltage, they are organized “somatotopically”. These means that the sensory signal for different parts of the body occupy a different physical space. This organization occurs for pretty much the whole signal path (up to the primary cortex at least) and is organized through the spinal cord, brain stem, thalamus, internal capsule, and cortex. You can sort of draw a little homunculus for each of these, but he dances around a little bit (and falls over a few times).
Every nerve fiber in your body grew out of your vertebra in the womb and lengthened as you grew. Every single sensory nerve ending has a direct path to somewhere in your brain.
Yes, you are that complex!
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