Which ion channel property best explains why myelinated C fibres mediating pain are more readily blocked by local anaesthetics than myelinated A-alpha fibres mediating motor function?

• A C fibres express more NMDA receptors
• B A-alpha fibres express fewer sodium channels per unit length
• C C fibres are smaller with closer nodes of Ranvier allowing block of three consecutive nodes
• D C fibres have a higher frequency of firing causing more use-dependent sodium channel block ✓

Explanation

Local anaesthetics exhibit use-dependent (phasic) block — sodium channels blocked more effectively at higher firing frequencies because the LA binds the open and inactivated channel states preferentially. C fibres firing at high frequencies during pain transmission are more susceptible. Additionally, for blockade of myelinated fibres, at least 3 consecutive nodes of Ranvier must be blocked; small myelinated fibres (Aδ) have closer nodes than large Aα fibres, so smaller fibre concentrations achieve blockade more easily. The concept of 'differential blockade' explains why sympathetic, then pain, then temperature, then touch, then pressure, then motor functions are sequentially lost.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.