A alpha fastest, then A beta, A gamma, A delta, B and C slowest
Mnemonic
Two overlapping systems. The A, B, C classification runs by decreasing diameter and speed:
- A alpha - proprioception, motor to skeletal muscle. Fastest, myelinated
- A beta - touch, pressure
- A gamma - motor to muscle spindles
- A delta - fast, sharp, localised pain and cold
- B - preganglionic autonomic
- C - slow, dull, poorly localised pain, warmth, postganglionic autonomic. Unmyelinated, slowest
“A delta is the first pain, C is the second pain.” The sharp prick you feel immediately travels in A delta; the aching that follows travels in C fibres.
Susceptibility runs opposite to size: local anaesthetic blocks the small fibres first, so pain and temperature go before touch and motor, while pressure and ischaemia block the large fibres first, so motor and proprioception go before pain, which is why a limb goes numb and weak before it hurts under a tourniquet.
Expansion
| Type | Myelin | Velocity | Function |
|---|---|---|---|
| A alpha | Heavy | 70 to 120 m/s | Motor, proprioception |
| A beta | Heavy | 30 to 70 | Touch, pressure, vibration |
| A gamma | Medium | 15 to 30 | Muscle spindle motor |
| A delta | Thin | 12 to 30 | Fast, sharp pain, cold |
| B | Thin | 3 to 15 | Preganglionic autonomic |
| C | None | 0.5 to 2 | Slow, dull pain, warmth, postganglionic autonomic |
Conduction velocity rises with diameter and with myelination, which allows saltatory conduction between nodes of Ranvier.
Two clinically useful sequences:
- Local anaesthetic blocks small fibres first: B (autonomic, hence hypotension in spinal anaesthesia), then C and A delta (pain and temperature), then A beta (touch), then A alpha (motor). This is why a patient can feel pressure but not pain
- Compression does the reverse, affecting large myelinated fibres first, which is why a limb “goes to sleep” with loss of touch and power before pain