Neurophysiology
Practice questions with explanations

D. Smaller, less myelinated pain and autonomic fibres are blocked at lower concentrations than larger motor fibres

Differential blockade allows labour analgesia that preserves the ability to move and push, using dilute local anaesthetic often combined with an opioid. The same principle explains why sympathetic blockade extends higher than sensory blockade, producing hypotension at levels where the patient still feels touch.

A. Sodium influx through voltage-gated sodium channels

Once threshold is reached, voltage-gated sodium channels open in a regenerative cycle and sodium rushes in down its electrochemical gradient. They then inactivate, and potassium efflux repolarises the membrane. Local anaesthetics and many antiepileptics act by blocking these sodium channels, preferentially in rapidly firing neurones, which is the basis of their selectivity.

C. Increased receptor sensitivity together with reduced clearance and a larger volume of distribution for lipophilic drugs

Older brains respond more to the same concentration, and pharmacokinetic changes compound it: phase I metabolism declines, and lipophilic agents such as diazepam accumulate in an expanded fat compartment with half-lives measured in days. Agents cleared by glucuronidation, such as oxazepam and temazepam, are less affected, which is why they are preferred when a benzodiazepine is necessary.