Differential Nerve Block — FRCA Primary MCQ
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Correct answer: D — Both C and D — small unmyelinated and small myelinated fibres are blocked before large fibres
Differential block follows a sequence based on fibre size and myelination: B fibres (preganglionic autonomic / small myelinated) and C fibres (pain / unmyelinated) are blocked first / followed by A-delta (pain / lightly myelinated) / then A-gamma and A-beta (touch / proprioception) / and finally A-alpha (motor / heavily myelinated). This is why sympathetic block extends further than sensory block which extends further than motor block in spinal/epidural anaesthesia.
Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care