During the course of an action potential, the potential difference across a neurone membrane changes. Table 10.1 shows the potential difference across the neurone membrane at three sequential time points, X, Y and Z, just before and during an action potential. [Table 10.1]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

1. [7] (a) causing X / –70 (mV) any three from: 1 active transport/sodium-potassium pump ; 2 moves three Na⁺ out of and two K+ in (to neurone) ; 3 more K+ diffuses out than Na+ diffuses in ; 4 makes outside of membrane (relatively) more positive; 5 maintaining / establishing, resting potential ; causing Y / +40 (mV) any two from: 6 depolarisation ; 7 Na+ / sodium ion, channels open ; 8 Na+, moves / diffuses, in (to neurone) ; causing Z / –90 (mV) any two from: 9 hyperpolarisation ; 10 (voltage-gated) K+ / potassium ion, channels open ; 11 K+, moves / diffuses, out (of neurone) ;
2. [2] (b) 1 local, circuit / current ; 2 from, depolarised / positive, area to, resting / negative, area ;
3. [3] (c) any three from: 1 speeds up (nervę) impulse / impulse transmission faster ; 2 (as) myelin sheath / Schwann cells, act(s) as an insulator ; 3 (as) local circuits flow between, nodes of Ranvier / gaps in sheath ; 4 saltatory conduction ; 5 fast(er), (described) response / muscle contraction;