Which statement correctly describes the mechanism of differential nerve block by local anaesthetics?
- A Larger myelinated fibres are blocked before smaller fibres due to higher local anaesthetic uptake
- B Motor fibres are always blocked before sensory fibres due to their higher membrane sodium channel density
- C All nerve fibres are equally sensitive to local anaesthetics when used in equivalent concentrations
- D Small unmyelinated C-fibres (pain/temperature) and small myelinated Aδ-fibres are blocked before large myelinated Aα/Aβ fibres (motor/proprioception) — allowing selective sensory block at appropriate concentrations ✓
Explanation
Local anaesthetics block sodium channels in a size- and myelination-dependent manner: small diameter and unmyelinated fibres are blocked at lower concentrations than large myelinated fibres. The order of block onset is: Aδ (sharp pain, temperature) → C fibres (dull pain, temperature) → Aβ (light touch, pressure) → Aγ (muscle spindle) → Aα (motor, proprioception). This explains why low concentrations of epidural bupivacaine can provide adequate labour analgesia (blocking Aδ and C fibres) while partially preserving motor function — the basis of 'walking epidurals' using low-concentration bupivacaine with opioid.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.