| Date | May 2016 | Marks available | 7 | Reference code | 16M.2.SL.TZ0.5 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Explain | Question number | 5 | Adapted from | N/A |
Question
Explain the propagation of electrical impulses along a neuron including the role of myelin.
Markscheme
Neurotransmitter attaches to receptor site, initiating transmission
Nerve impulses are action potentials propagated along the axons of neurons
Resting potential is more negative inside/–70 mV/ more positive outside the membrane
OR
a resting potential has greater concentration of Na ions outside than K ions inside the axon
«volted gated» channels open and Na ions diffuse in
Causes depolarization of the membrane/–70 mV to +40 mV
Local currents affect adjacent channels/cause action potential
Depolarization is followed by repolarization of the neuron
«voltage gated» channels open and K ions diffuse out/repolarize the membrane
Na-K pumps restore Na/K balance/resting potential
Myelin around the neuron insulates the axon
OR
speeds the transmission
Myelin permits saltatory conduction
OR
permits jumping from node to node
Award [6 max] if no mention of the role of myelin.
Examiners report
Nerve impulses – Most candidates knew the purpose of myelin. Unfortunately many could explain little else. Well prepared candidates gave very clear answers. The confusions between sodium and potassium and diffusion and pumps were rife.
