Nerve Fiber Classification

Core Principle: Conduction Speed ∝ Diameter & Myelination.
A-α fibers
- Structure: Largest diameter, most myelinated. Fastest.
- Function: Proprioception (muscle spindles, Golgi tendons) & somatic motor (α-motor neurons).
  - Golgi tendons
    - Senses: Active muscle TENSION / FORCE.
    - Location: At muscle-tendon junction, in series with muscle fibers.
    - Afferent Fiber: Type Ib.
    - Reflex Arc: Activates an inhibitory interneuron in the spinal cord.
    - Result: Inhibits the alpha motor neuron, causing relaxation of the contracting muscle.
    - Purpose: Protective; prevents damage from excessive force.
    - Contrast: Muscle spindle senses STRETCH (via Ia/II fibers) and causes CONTRACTION.
A-β fibers
- Structure: Large, myelinated. Fast.
- Function: Fine touch, vibration, pressure.
A-δ fibers
- Structure: Small, thinly myelinated. Slower.
- Function: Fast, sharp pain & cold temperature.
B fibers
- Structure: Lightly myelinated.
- Function: Preganglionic autonomic fibers.
C fibers
- Structure: Smallest diameter, unmyelinated. Slowest.
- Function: Slow, dull, burning pain, warmth, & postganglionic autonomic fibers.

Clinical Correlations

Speed of Sensation: Touch/Pressure (A-β) > Sharp Pain (A-δ) > Dull/Aching Pain (C).
Susceptibility to Blockade:
- Pressure/Ischemia: Large fibers are most sensitive (A > B > C). Leads to numbness first.
- Local Anesthetics: Small fibers are most sensitive (C > B > A). Blocks pain first.