How do nerves differentiate between hot, cold, sharp, smooth, rough, soft, etc.?
Are there different nerves for these different sensations? If not, is the signal being sent to the brain somehow different for hot than it is for cold (different voltage levels or pulses or frequency or amplitude)? If it is different nerves, how many different kinds of nerves are there for the different sensations?
Along those same lines, it would seem any sensation can become “pain” if taken to an extreme. Is this just an interpretation by the brain of how much stimulus is present? Or are there different nerves for pain?
In the case of hot and cold, how, physically, does the body translate the physical property of temperature into an electrical signal?
Based on answers to the above, I have a bunch more questions if someone would be willing to help explain this to me!
Roughly, yes, there is a broad diversity of various “touch” receptors within your skin.
For instance, there are separate receptors for “heat,” and “noxious heat,” which actually depolarize (fire) at different temperatures. There are also touch receptors that specialize in receiving vibratory sensation, proprioceptice (how is your body oriented), and a generic touch receptor for “ouch,” that fires when tissue has been damaged or disrupted slightly.
I read a week or two ago (perhaps a small news note in the latest Scientific American) that vibration sensing nerves work with your fingerprints to sense texture. As you drag your fingers over something, the vibrations of your swirls and whorls bouncing over surface details create sensations in these nerves that you use to interpret small scale surface geometry of a surface.
I think I read somewhere else (not so sure about this one though) that cold and pain are sensed with the same nerves. Presumeably some other difference in the signal, like the OP asks about, tells us which it is.
Yes, there are particular nerves for hot, and cold (different ones for hot and cold), and pain, and pressure [though what is different is not so much the nerve, but (1) the type of receptor organ that it ends at in your skin, and (2) where it connects to at the other end, in your brain], but it is not all like that.
Things like rough and smooth are sensed from the temporal pattern of the signals produced as you move your fingers over a surface, and certain pressure receptors get stimulated. If you are moving your fingertip over sandpaper the nerve signal is going to fire a burst of impulses as the receptor cell hits the top edge of a grain, and then go off again as the cell moves over the gap between grains, and so on. Moving your finger over a smooth surface is going to give a more continuous rate of nerve signaling.
[That is simplifying things quite a bit, as (1) we humans have several types of pressure receptor in our skin, and (2) all nerve signal (well, the long distance ones that matter here) go in bursts with gaps in between (so moving over some thing smooth it is something like on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on-off-on whereas moving over something rough is more on-off-on-off-on-off-on-off-on-off-off-off-off-off-off-off-off-off-off-off-off-off-off-off-off-off-off-off-on-off-on-off-on-off-on-off), and (3) a bunch of other complications.]
So the short answer: some of it is a matter of what the nerve is connected to at each end, and some of it is to do with the pattern in the signal (generally in relation to how you are moving your sense organs in relation to what you are sensing).
Now don’t get me started on how it works with visual and auditory sensations. Those are waaaaaaaay more complicated! (Let alone what then happens in your brain.) :eek:
In a very, very simplified way I can actually picture how optic and auditory perception might work, especially since those senses are working in just one “type” of sensation.
I’m sorry about the peas, njtt, but I do appreciate the answer. It helped quite a bit.
The visual is especially interesting. It is more accurate to think of the retinas and optic nerve as parts of the brain rather than sensor and connecting cable. The retina does a lot of processing on its own. For example it is the retina that identifies some linear and angular shapes and some simple patterns. The retina is also very well understood, as neural processing systems go. If you want to study part of the central nervous system that does some kind of simple thinking on a level that can be logically sorted out, the retina may be your best choice.