Point
2
Intro
In the discussion of
the dorsal column system, I mentioned that the entering
alpha-beta fibers course within the medial division
of the dorsal root. As I hope you recall, alpha-betas are
relatively large, myelinated and fast-conducting. In
contrast, the fibers in the lateral portion of the
dorsal root are thinner and consist of both myelinated and
unmyelinated axons.
There are two types of dorsal root processes in the lateral division. One is called a delta fiber. This axon measures 1-5um in diameter (compare with 6-12um for alpha-betas) and conveys information regarding the sense of cooling and what is referred to as pricking or first pain. Delta fibers also convey what is referred to as crude touch. This contrasts with discriminative touch carried in the dorsal columns and is the sense of contact. You know that you are being contacted by something but have difficulty localizing the stimulus.
The thinner of the two fibers in the lateral division of the dorsal root is called a C fiber. This fiber is unmyelinated (slower conducting than a delta) and measures 0.2-1.5um in diameter. C fibers carry information regarding the sense of warming and slow or burning pain. The difference between first and second pain can be felt when you touch a hot pan. At first (no pun intended!!) there is a shooting pain (delta fibers which conduct faster than Cs) and then a slow agonizing, burning, lasting pain (C fibers). Most of the receptors for pain, temperature and crude touch are naked nerve endings (compare with the elaborate receptors associated with alpha-betas).