Nerve impulses can be transmitted along a myelinated motor neurone to a neuromuscular junction at fast speeds of up to 100ms⁻¹. Outline how a transmission speed of 100ms⁻¹ is achieved by the neurone.

Mark scheme

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

1. [3] (a) any three from : 1 myelin sheath / Schwann cells, acts as an insulator / prevents movement of ions ; 2 depolarisation / action potential, only occurs at the nodes; I impulses 3 action potentials jump from node to node / saltatory conduction ; I impulses 4 long(er) local circuits ; 5 AVP; e.g. wide axon diameter
2. [4] (b) any four from : 1 ACh binds to receptors on sarcolemma ; 2 (ligand-gated) Na+ channels open or Na+ enters, muscle cell / sarcoplasm ; 3 sarcolemma depolarised ; 4 depolarisation / action potential, spreads, to / down, T-tubules ; 5 voltage-gated Ca²+ channels open ; 6 Ca2+, diffuse / move, out of sarcoplasmic reticulum (and bind to troponin);
3. [3] (c) any three from : 1 (decreased rate of respiration so), no / less, ATP produced ; 2 no / less, Ca2+ pumped back into sarcoplasmic reticulum ; 3 no/less, release of myosin heads from actin / cross bridges broken or no / less, hydrolysis of ATP by myosin head / return of myosin head to original position; AW 4 sarcomere, permanently contracted / cannot relax; A paralysis 5 slower sarcomere contraction / less sarcomere shortening / weaker power stroke ;