Figure 7. Peripheral chemoreflex-evoked EPSP drive to principal cells.

A, activation of the peripheral chemoreflex (i.a. NaCN) evoked an initial strong increase in action potential discharge and also a clear increase in the frequency of large EPSPs. However during much of the response there was an excess of subthreshold EPSPs relative to action potentials, indicating a gearing of ganglionic throughput from pre- to postganglionic neurones.B, with the cell hyperpolarised (by current injection) to prevent action potential discharge, activation of the peripheral chemoreflex clearly evoked a striking increase in the frequency of EPSPs. These EPSPs were seen to occur across much of the respiratory cycle with only a brief pause during early inspiration suggesting strong gating of the central drive from the cardiac vagal preganglionic neurones during this phase of the cycle. The first EPSP in the burst was often the largest and this often coincided with the phase when action potential discharge was evident under basal conditions (in the immediate postinspiratory phase). There was weak temporal summation of the EPSPs during this stimulus (as shown on the expanded time base) offset again by the diminishing amplitude of the synaptic events during high frequency bursts. (Note the monopolar phrenic nerve recording also includes the ECG signal.)