The graphs compare the changes in membrane potential that result from a combination of stimuli. Graph A shows two excitatory post-synaptic potentials (E₁ and E₂) acting on a neuron. Graph B shows one excitatory (E₁) and one inhibitory (I₁) post-synaptic potential, both acting on a neuron.

With respect to the graphs, explain what is meant by summation.

More than one presynaptic neuron can form a synapse with the same postsynaptic neuron

Summation involves combining the effects of «excitatory and inhibitory» neurotransmitters/potentials
OR
action potentials form depending on the balance of signals of excitatory and inhibitory signals

E₁ and E₂/two excitatory potentials/effects are added

«E₁ and E₂/two excitatory potentials» depolarizes membrane

«Membrane potential» goes above threshold
OR
Generate action potential

Effect of inhibitory neurotransmitter/potential/ I₁ cancels effect of excitatory neurotransmitter/E₁
OR
Effect of inhibitory neurotransmitter/potential/ I₁ prevents threshold being reached when E₁ applied

This question was quite discriminating with one or 2 marks awarded to many candidates. Although summation appeared to be fairly well understood, candidates struggled to express themselves clearly. Many struggled to explain summation using the graphs provided so only the better candidates received 3 marks.