The operational genius of this spinal cord circuitry lies in a fundamental physical metric: conduction velocity. Human peripheral nerves are not built equal. Their cross-sectional diameter and the presence of an insulating myelin sheath determine the rate at which action potentials travel from an injured extremity to the spinal axis.
| Fiber Classification | Axon Diameter & Myelination | Conduction Velocity | Primary Modality | Gate Mechanism Impact |
|---|---|---|---|---|
| A-beta (Aβ) | Large (6, 12 µm), heavily myelinated | 30, 70 m/s | Light touch, vibration, deep tactile pressure | Closes the Gate: Strongly excites inhibitory interneurons in lamina II. |
| A-delta (Aδ) | Medium (1, 5 µm), thinly myelinated | 5, 30 m/s | Acute pricking pain, cold temperature | Partial Open: Fast localized warning; weakly bypasses local inhibition. |
| C Fibers | Small (0.2, 1.5 µm), completely unmyelinated | 0.5, 2 m/s | Burning sensation, chronic ache, dull soreness | Opens the Gate: Inhibits lamina II interneurons, maximizing transmission. |
Because A-beta fibers conduct signals at rates upwards of 30 to 70 meters per second, their tactile messages arrive at the dorsal horn substantially earlier than the sluggish C-fiber volleys, which crawl along at 0.5 to 2 meters per second.
When you strike your knee against a table corner, sharp A-delta signals arrive first, followed seconds later by a sickening, throbbing ache driven by unmyelinated C fibers. By immediately rubbing your skin, you flood the dorsal horn with rapid-fire A-beta action potentials. These tactile signals recruit inhibitory interneurons, dampening the incoming C-fiber waves before they cross the wide dynamic range neuron.
First isolated electrophysiologically by Mendell in 1966, the wide dynamic range neuron sits in lamina V of the dorsal horn. It acts as an integration engine. Unlike nociceptive-specific neurons that fire only during overt physical damage, the wide dynamic range neuron receives structural inputs from both delicate mechanical fibers and destructive nociceptors. It grades its output frequency based on net synaptic input. If A-beta collateral firing successfully activates local inhibitory cells, the wide dynamic range neuron scales down its firing rate, dropping the intensity of the signal ascending to the conscious brain.
