A fundamental requirement for any pain medicine board exam is the ability to classify peripheral nerve fibers based on their physical characteristics and the specific types of sensory information they carry. The “speed” of a pain signal is determined by the diameter of the fiber and the presence of a myelin sheath.

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1. Classification Overview

Peripheral nerves are composed of various fiber types categorized by the Erlanger-Gasser classification. For pain specialists, the focus is primarily on the A-beta, A-delta, and C-fibers.

Fiber Type	Myelination	Diameter (μm)	Conduction Velocity (m/s)	Sensory Function
A-alpha	Heavy	12–20	70–120	Proprioception, Somatic Motor
A-beta	Heavy	5–12	30–70	Touch, Pressure, Vibration
A-delta	Thin	2–5	5–30	“Fast” Pain, Temperature
C	None	0.4–1.2	0.5–2	“Slow” Pain, Temperature, Itch

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2. A-beta Fibers: The “Non-Nociceptors”

These are large, heavily myelinated fibers. Under normal physiological conditions, they do not transmit pain.

• Role in Pain: They transmit innocuous stimuli like light touch.
• Gate Control Theory: These fibers are the stars of the Gate Control Theory. Activating A-beta fibers (via massage, TENS, or SCS) can inhibit the transmission of pain signals from A-delta and C-fibers in the dorsal horn.
• Allodynia: In chronic pain states, “crosstalk” or central sensitization can cause A-beta fibers to be perceived as painful (Mechanical Allodynia).

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3. A-delta Fibers: The “First Pain”

A-delta fibers are the larger of the two nociceptive fibers and are thinly myelinated.

• Sensation: They are responsible for “First Pain”—the sharp, stinging, or pricking sensation that occurs immediately after an injury (e.g., a needle stick).
• Reflexes: Because of their relatively high conduction velocity, they trigger the rapid withdrawal reflex.
• Anatomy: They primarily terminate in Rexed Laminae I and V of the spinal cord.

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4. C-fibers: The “Second Pain”

C-fibers are small, unmyelinated, and make up about 70% of all nociceptive fibers.

• Sensation: They are responsible for “Second Pain”—the dull, aching, burning, or throbbing sensation that follows the initial sharp pain.
• Polymodal: They are often polymodal, meaning they can respond to mechanical, thermal, and chemical stimuli.
• Anatomy: They primarily terminate in Rexed Lamina II (Substantia Gelatinosa).
• Wind-up: Repetitive stimulation of C-fibers leads to the NMDA-mediated “wind-up” phenomenon, a key component of central sensitization.

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5. High-Yield Board “Fast Facts”

• Conduction Velocity: C-fibers are the slowest ($< 2 m/s$) because they lack myelin.
• First vs. Second Pain: A-delta = First (Sharp); C = Second (Aching).
• Substantia Gelatinosa: The “home” of C-fiber termination in the dorsal horn.
• Local Anesthetics: Small, unmyelinated C-fibers and thinly myelinated A-delta fibers are typically blocked before the larger A-alpha motor fibers.
• Differential Block: This explains why a patient can lose pain sensation during an epidural while still being able to move their legs.