Figure 5.

Resetting of spontaneous firing by excitatory and inhibitory synaptic stimulation. A. (top) Superimposition of 50 trials applying a strong stimulation of the internal capsule at the arrow, in the presence of picrotoxin (150 μM) and CGP-55845 (2 μM) to block GABA_A and GABA_B receptors respectively. The resulting EPSP triggers action potentials on nearly every trial, but with variable latency. The evoked action potential is followed by resumption of spontaneous firing at its normal interspike interval, thus resetting rhythmic firing. (bottom) Three trials are superimposed, showing the phase-sensitivity of firing latency of in response to the stimulus. A trial in which the EPSP arrives near the end of the interspike interval (red trace) results in an almost immediate action potential, whereas a trial in which the stimulus arrives near the beginning of the interspike interval (blue trace ) produces a longer-latency response. A trial in which the stimulus falls at an intermediate latency (black trace) produces a response at intermediate latency. These phase relationships are preserved in the next cycle of spontaneous firing. B. (top) Superimposition of 50 trials applying a similar strong stimulus, but in the presence of DNQX (20 μM) and APV (50 μM) to block AMPA and NMDA receptors respectively. A strong IPSP hyperpolarizes the neuron on each trial, and produces a pause in firing approximately the same duration as the spontaneous interspike interval. (bottom) Three trials are superimposed, showing the phase sensitivity of the interspike interval following inhibition. A trial in which the IPSP arrives almost at the end of the interspike interval (red trace) produces the smallest delay in the resumption of firing. A trial in which the stimulus arrives near the beginning of the interspike interval (blue trace) produces a longer subsequent interspike interval. An intermediate stimulus arrival time (black) produces an intermediate result. These phase relationships will be preserved on subsequent cycles.