A Afferent Fibers (Neurons)
These are types of sensory afferent nerve fibers that are myelinated (encased in a myelin sheath), and are classified according to their conduction velocity and sensory modality.
Aβ fibers are medium diameter afferent fibers with conduction velocities of 30–80 ms, and encode signals from non-noxious stimuli such as touch.
Aδ fibers are smaller caliber afferent fibers with conduction velocities of 5–30 ms, and principally encode signals from noxious stimuli. They are commonly thought to be responsible for the rapid sensation of ‘first pain’ following injury.
It is often difficult to precisely identify the different classes of A fibers during development, as growth in fiber diameter and myelination occur slowly, so the eventual fate of fibers is not necessarily obvious at earlier stages of development.