Slow touch and ultrafast pain fibres: Revisiting peripheral nerve classification

Area of Science:

• Neuroscience
• Peripheral Nervous System Physiology

Background:

• Erlanger and Gasser established a correlation between axon diameter and conduction velocity.
• Axon diameter was historically linked to specific sensory functions: large for touch, small for pain/temperature.

Purpose of the Study:

• To review recent evidence modifying the canonical classification of peripheral nerve axon function.
• To explore the roles of unmyelinated C-fibers in touch and large Aβ fibers in pain signaling.

Main Methods:

• Literature review of recent discoveries in neurophysiology.
• Analysis of evidence regarding conduction velocity and axonal functional roles.

Main Results:

• Unmyelinated (C) fibers can signal touch, contributing to affective tactile aspects.
• Large-diameter Aβ afferents can signal pain at high velocities, involved in withdrawal reflexes.
• The correlation between conduction velocity and functional role is less definitive than previously thought.

Conclusions:

• The traditional classification of axon function based solely on diameter and conduction velocity requires revision.
• Future peripheral nervous system taxonomy may integrate molecular expression and electrophysiological properties.