Figure 5. Functional responses of cardiac vagal ganglion principal cells to cardio-respiratory afferent stimulation.

All responses shown were obtained from the same neurone.A, activation of the peripheral chemoreflex (NaCN 0.03%i.a.) triggered an increase in the discharge frequency of the principal cell along with the tachypnoea. The pattern of action potential discharge was also altered with a spread of firing into the expiratory phase. Both the increase in firing rate and the phase spread recovered as the respiratory response to the chemoreflex activation subsided.B, ramp increases in perfusion pressure evoked a barrage of action potential discharge in principal cells which preceded the fall in heart rate. This increase in firing was seen irrespective of the respiratory phase; in this example the discharge is seen to occur predominantly in expiration. Also noteworthy was the summation of the AHPs seen during the activation which caused progressive membrane hyperpolarisation.C, the diving response was evoked by the application of cold saline to the snout. This stimulation produced a robust discharge of action potentials from the principal cell. This burst commenced with the onset of the transient apnoea. As with the baroreflex activation, the principal cell was again seen to hyperpolarise during the response. Abbreviations:V_m– membrane potential; PNA – phrenic nerve activity; HR – heart rate; PP – perfusion pressure. (Note the monopolar phrenic nerve recording also includes the ECG signal.)