Create diagrams (of the cells) and graphs (of the potentials) to illustrate the two different situations described below in a multipolar neuron with a threshold voltage of 15 mV above resting potential. In each, indicate decrement of graded potentials (by drawing the same GP at different points as it spreads along the neuron), as well as summation.

Question

Situation 1: There are three simultaneously active synapses on the multipolar neuron, two producing EPSPs
and one an IPSP. At least one of the EPSPs is larger than 15 mV at the synapse. The neuron does NOT
generate an action potential.
Situation 2: There are two simultaneously active synapses, one producing an EPSP and the other an IPSP.
The neuron generates an action potential.

Answer 1

(Note to instructor: It is best to have demonstrated this in class or on a handout at some time prior to the exam.) Diagrams and graphs should resemble those in Figure 8.7 in the chapter, except that the number of synaptic inputs shown will be greater (see Fig. 8.24), and a set of graphs should be drawn for each input. Diagrams of a multipolar neuron with the different synapses indicated should be produced. There should be a total of three to five synapses, depending on whether or not the student assumes the three synapses described for situation 1 are different from
those for situation 2. The graph of summation should resemble Figure 8.24c,d. In situation 1, the student should choose amplitudes that sum to a value below +15 mV (the PSP that was 15 mV at the synapse is smaller when it reaches the trigger zone). In situation 2, the student should choose amplitudes that sum to a value equal to or above +15 mV.