How do different sensory stimuli differ in nerve signal?

Question

Every day, we experience different types of sensory stimulus, like heat, pain, cold, etc. However, in each case, the transmission of the stimulus to the brain through the neurones is carried on in the same way: through propagated depolarisation and repolarisation in axons, while through the release of acetylcholine at the synapses.

So, how does the different sensory stimulus differ if they are transmitted the same way? (I mean to say that how does the nerve signals differ for two different kinds of stimulus?)

Let's put the question simply:

How do two nerve signals originated from two different stimuli differ? If they do not differ, then how does the brain differentiate between different nerve signals if they are all alike? How does it understand what nerve signal carries what message?

Answer 1

I think the short answer to your question is that the contents of different types of sensory stimulation are communicated to different populations of cells in the brain (note this transmission is mostly glutamatergic, not cholinergic). From there, the remaining associations and experiences derive from connectivity with other networks of brain neurons that result in different types of experience, based on the networks that those networks form and learned associations over development and a lifetime.

Computer analogies aren't always appropriate to understanding things about the brain, but maybe it could work out a bit here. How does a computer distinguish between input on your keyboard vs your mouse? On many modern computers, it is common for both to be connected with USB. The computer has some software that distinguishes between the different inputs - the brain has hardware that does the same.

Later on in the brain, or at a different stage in a computer program, those inputs can be mapped to functions that underlie experiences or functional outputs. Movement of the mouse can be converted to the movement of a screen cursor ("motor output"). Proprioception (the sensation of the relative position of the body from receptors in the muscles, tendons, and joints) can be combined with motor plans to create an error signal to move a particular muscle to match the intended movement.

Explaining any further than this probably requires entire textbooks of material, so I will leave it there.

Answer 2

This is exactly the area of study known as neural coding. The representation of information is not universal and differs for different modality, different stages of information processing, and what not. It's an active area of research.

That said, for the peripheral nervous system, the simplest code is pretty much rate code with label line. In other words, knowing which nerve fiber is firing what rate carries much of the sensory information. There are adaptation and some timing information if you look closer.