Study Questions on Post-synaptic Potentials

Create a plot of the amplitude of an EPSP in mV (y-axis) as a function of the membrane potential (x-axis), using a reversal potential of -10mV. On the y-axis, depolarizing amplitudes should be plotted upward, as positive values. This plot illustrates the original results of Takeuchi & Takeuchi for the nicotinic ACh receptor, which is roughly equal in permeability to Na and K ions. Now, draw additional plots on the same axis to illustrate the behavior of the EPSP under each of the following conditions:

(a) Double the number of ACh receptors activated.

(b) Half the number of resting channels.

(c) Mutated ACh receptor permeable only to Na ions

The control plot shows Vrev = -10 and is linear, with depolarizing EPSP amplitudes negative to Vrev, hyperpolarizing positive to Vrev.

Doubling the number of ACh receptors activated or reducing by half the number of resting channels each will double the EPSP amplitude at each potential without changing Vrev.

Plot c, where we made the ACh receptor permeable only to Na ions, shows Vrev = VNa, which I assumed to be +30mV in this cell. The slope depends on your assumptions. If you assume the same total conductance, the slope should stay the same, as I have drawn it above. Another reasonable assumption would be that you removed the K conductance leaving the Na conductance intact, in which case the slope would be reduced by the fraction of total conductance originally contributed by Na. So, for

example, if the conductance ratio originally were 1.8:1 Na:K, then the slope of line c should be 1.8/2.8 of the original slope (but still +30mV Vrev.

Now do the exact same set of plots, but this time plot the current underlying the EPSP, as measured in voltage clamp. Plot inward current down (negative) on the y-axis. Some of the conditions (a)-(c) change the voltage plots in Question 1, but do not change the current plots in Question 2. Do you understand why?

Voltage clamp plots of EPSP current are inverted, because current is inward for a depolarizing EPSP, and inward currents are plotted as negative number (convention). The control plot and plot (b) are the same, because changing R (reducing resting channels) does not change EPSP current. Plot a is double control, because twice as many receptors activated means twice as much current at each potential. All have the same Vrev. The plot of c has the same assumptions as in the first graph, and its slope would change with a new assumption, just like in the first graph.