Axon's diameter size and their speed

Question

I know that the Axon with largest diameter is the fastest.

For the '' feeling the position of limbs in space'' this neuron and axon is used.

And the axon with a very small diameter, is very slow. when Twisting the ankle, this is the neuron that sends the signal.

Shouldn't it be opposite? Shouldn't I feel the pain in a second?

Answer 1

That is a good question and a very interesting way to begin to understand our body's nervous systems! There are essentially 2 types of neurons that signal pain back to our brain. These are called alpha fibers and c fibers. They roughly can be broken down in this manner: alpha fibers carry fast/quick pain signals (such as touching your hand to a hot stove), c fibers carry the slow/dull pain that result from existing tissue irritation that serve to remind us that something is still going awry in our bodies. While the axonal sizing rule still applies to speed, in this situation it is somewhat overruled by our bodies system to perceive pain.

Answer 2

My best answer is that you have to consider the survival aspects of the situation.

According to this article (https://www.khanacademy.org/test-prep/mcat/organ-systems/neuron-membrane-potentials/a/action-potential-velocity), pain was one of the first and most important things to feel, so it developed when organism were smaller and the nerves were simpler. So it moved relatively slowly.

But we know that the slowest organisms get eaten first! This article, much less user-friendly ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3820973/), suggests that increased predation pressure forced the evolution of faster nerve messages in order to escape becoming someone's dinner. The trade off was more energy had to be used to get the message across.

So you will feel pain from a cut on your finger quickly so that you can remove your hand from immediate danger. And you will not feel the pain of internal damage in your ankle right away so that you can keep running and escape the danger.

Answer 3

Good question! There are 2 different types of neurons that carry pain signals. Alpha delta axons are the thick myelinated axons -- similar to proprioception (the fancy term for "feeling the position of limbs in space"). This is referred to as "sharp pain" and has a very rapid, and intense, onset. The other pain fiber is type c. They are small and not myelinated. This carries the "dull, throbbing" pain that comes after sharp pain. It's not fully understood why there are two types of pain systems, but many suspect that the fast and rapid pain system signifies urgent bodily injury (or potential thereof), and the slow, dull pain, is just a chronic reminder to not injure oneself further. Hope that helps!