Figure 2.

Varying the membrane potential reveals that primary and secondary EPSPs differ in magnitude and strength. (A) Example of a neuron in which primary (left) and secondary (right) EPSP amplitudes increased as the driving force on nicotinic currents was increased by injecting hyperpolarizing current through the recording electrode (noted at the left of each trace). This cell was typical of most recordings in that strong hyperpolarization was unable to block firing of an action potential by the primary EPSP. (B) Example of a neuron where the primary synaptic response was recruited with relatively low stimulus strength and obscured the ability to resolve secondary EPSPs with stronger stimuli. In this cell, hyperpolarization was able to block firing and revealed a 20-mV primary EPSP.