Studies on the synaptic interconnection between bulbar respiratory neurones of cats

In cats anaesthetized with pentobarbital, medullary respiratory neurones of both dorsal and ventral populations were recorded intracellularly with 1 mol.l-1 KCl-electrodes. The neurones were classified according to the projection of their axons to the spinal cord (bulbospinal neurones) or to the vagal nerves (vagal neurones). Those neurones which could not be activated antidromically (NAA-neurones) by either procedure were subdivided into (inspiratory) R beta-neurones, which were monosynaptically excited by lung stretch receptor afferents, and into inspiratory and expiratory NAA-neurones, which did not receive a direct synaptic input, from these afferents. All types of neurone investigated revealed postsynaptic activity during both inspiration and expiration. The periods when synaptic activity was minimal were the periods of transition between respiratory phases. The input resistance of most respiratory neurones varied in parallel with the respiratory cycle. A drastic fall of the input resistance during expiration was observed in R beta-neurones and in some inspiratory vagal neurones. This was not seen in inspiratory bulbospinal neurones. In stable intracellular recordings, periodic postsynaptic inhibition was demonstrated in 52 of 53 respiratory neurones by IPSP reversal following chloride injection. Maximal membrane potential then was generally reached during one of the periods of respiratory phase transition. Reasons for the failure of others to demonstrate these IPSPs are presented and discrepancies between other findings and these are discussed. It is concluded that reciprocal inhibition between bulbar respiratory neurones does exist and is a general phenomenon. It is argued that reciprocal inhibition is the fundamental mechanism underlying respiratory gating of afferent inputs. The probable existence of recurrent inhibition is inferred from the changes in the pattern of membrane depolarization during the active period of neurones.