(EPP) reversal potential homework assignment

need questions 4,8 and 10 done, ignore the rest!

Neuroscience3600 Problem Set 4

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Please write neatly and give complete, but concise, answers. Point credit for each problem shown to

the left (total of 20 pts).

1. (6 pts) What’s the difference between an EPSP and an action potential? Compare and contrast

these two potentials. Consider the ions involved, types of channels, duration, amplitude,

triggering events, and mechanisms of depolarization.

2. (1 pt) Do EPSPs show an after-hyperpolarization? Why or why not?

3. (0.5 pt) True of False: The key determinant of whether a synapse is excitatory or inhibitory is

the identity of the neurotransmitter. Briefly explain your answer.

4. (3 pts) Measurements used to determine the reversal potential of a particular synapse are

shown below. Separate experiments confirm that changing Cl- and Ca
2
+ concentrations have

no effect on these results.

a) Plot the results on the I-V plot at right (approximate values are sufficient) and identify the

reversal potential.

b) What can you say about the relative contribution of each permeant ion to the reversal

potential? Explain your reasoning.

c) Describe a manipulation that would shift the reversal potential downward to around 0 mV.

5. (1 pt) The medial prefrontal cortex (mPFC) and striatum both receive a dopaminergic

projection from the ventral tegmental area (VTA). You hypothesize that the same VTA cells that

innervate the mPFC also send a parallel projection (also called a collateral axon) to the striatum.

The alternative hypothesis is that there are separate sub-populations in VTA, with one group of

Neuroscience 3600 Problem Set 4

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cells projecting solely to mPFC and a separate group projecting to NA. How would you test to

discriminate between these two hypotheses?

6. (1 pt) The following electron micrograph shows a close-up of an axon (A) and two dendrites,

labeled D1 and D2. Is there anything unusual about these dendrites vis-à-vis their ability to

communicate? If so, what? Describe any evidence you use to reach your conclusion.

7. (1.5) Describe three mechanisms whereby an drug or toxin can enhance neurotransmission. For

each mechanism, cite a specific example (the drug and the affected neurotransmitter) and

briefly describe its mechanism of action.

8. (3 pts) The membrane potential, Em, of a post-synaptic cell can be calculated from the following

equation:

where grest is the leakage current which maintains the resting potential, Erest is the normal

resting potential, gE is the conductance due to excitatory post-synaptic channels (e.g., AMPA

channels), ErevE is the reversal potential for the excitatory channels, gI is the conductance due to

inhibitory post-synaptic channels (e.g., GABA channels) and ErevI is the reversal potential for the

inhibitory channels (this equation comes from the equivalence circuit discussed in Lecture 13 &

14, slide 21). Assume grest = 1 milliSiemens, Erest = -65 mV, ErevE = -10 mV, ErevI = -70 mV.

a) If gE at the peak of the EPSP is 2 milliSiemens, what is the height of the EPSP without an IPSP

(i.e., what is the change in potential from resting)?

b) If gI at the peak of the IPSP is 1 milliSiemens, what is the change in potential from resting?

c) What is the sum of the peak IPSP voltage change and the peak EPSP voltage change?

d) What is the net change in synaptic potential when an EPSP and IPSP occur simultaneously?

Why is your answer different from the simple sum of potentials in part c?

Neuroscience 3600 Problem Set 4

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9. (1 pt) A cell receives two synaptic inputs at adjacent locations on a dendritic branch. Each

synapse, when activated by a single pre-synaptic spike, produces a depolarizing EPSP. From

resting potential, these EPSPs create a peak voltage change of +18 mV. If the resting potential is

-80 mV and threshold for spike generation is -40 mV, can these two synapses ever cause a spike

in the post-synaptic cell? Explain.

10. (2 pts) The left two columns in the figure below (Fig 5.20 from your book) show ion flow and net

current during an EPSP at different clamp voltages in an ACh channel that is equally permeable

to both Na+ and K+. Now, assume that Na+ concentration inside is equal to the outside Na+

concentration. In the right two columns, show ion flow and net current during an EPSP under

these new conditions.