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Summation of postsynaptic potentials at the axon hillock

The figure below shows a single postsynaptic neuron that has synapses with four other neurons via their synaptic terminals. Two of the presynaptic neurons produce EPSPs on the postsynaptic cell (green, E1 and E2), and the other two produce IPSPs (red, I1 and I2).


The graph below shows the membrane potential measured at the axon hillock of the postsynaptic neuron. Each target indicates a change in the membrane potential at the axon hillock caused by a postsynaptic potential at one of the four synapses. Summation occurs when two or more postsynaptic potentials overlap in time; that is, one postsynaptic potential begins before the membrane has returned to resting potential after a previous postsynaptic potential. When such overlap occurs, the membrane potential measured at the axon hillock is the sum of the two (or more) overlapping postsynaptic potentials.


Drag the labels onto the graph to indicate which presynaptic neuron produced each change in the axon hillock’s membrane potential. Assume that the same number of ligand-gated ion channels opens at each synapse. (To review the effect of distance between the synapse and the axon hillock, see Hint 2.) Labels can be used once, more than once, or not at all.

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