Changes of Unit Discharge of the Medullary Respiratory Neurons during Apnea

Changes of the neuronal discharge of 128 medullary respiratory unitswere recorded and studied during the period of expiratory apnea induced reflexlyby intracarotid sinus injection of sodium citrate in rabbits.Generally,theneuronal discharge of inspiratory units began,stopped and recovered at the sametime with those of the phrenic nerve.But,about 5% the phase-spanninginspiratory units near the obex showed a different time course with the dischargeof the phrenic nerve.They fired continuously in a low frequency while thephrenic nerve was quiet.When increasing progressively and approaching to acertain level,the firing rate increased abruptly and at the same time phrenic nervebegan to fire.So it seemed that they acted as the pacemaker of inspiration.Comparison of the cycle-triggered histograms(CTH)of these inspiratory unitswith those of phrenic nerve showed clearly the above mentioned phasicrelationship.They started firing before the phrenic nerve,but they reached theirmaximal rate and then declined and stopped quite in accordance with the phrenicnerve.It is,therefore,reasonable to assume that the central mechanism of theswitch from expiratory apnea to inspiration may originate from this kind ofneurons.Most of the expiratory units show tonic discharges during the period of apneawith a higher discharge rate than normal and then the rate decreases just beforerecovery of phrenic firing.In addition,small portion of the expiratory units weredepressed as the phrenic discharge ceased.The function of these two differentkinds of neurons in the mechanism of development of respiratory rhythm is,apparently,different.

Changes of Unit Discharge of the Medullary Respiratory Neurons during Apnea

Changes of the neuronal discharge of 128 medullary respiratory units were recorded and studied during the period of expiratory apnea induced reflexly by intracarotid sinus injection of sodium citrate in rabbits.Generally,the neuronal discharge of inspiratory units began,stopped and recovered at the same time with those of the phrenic nerve.But,about 5% the phase-spanning inspiratory units near the obex showed a different time course with the discharge of the phrenic nerve.They fired continuously in a low frequency while the phrenic nerve was quiet.When increasing progressively and approaching to a certain level,the firing rate increased abruptly and at the same time phrenic nerve began to fire.So it seemed that they acted as the pacemaker of inspiration. Comparison of the cycle-triggered histograms(CTH)of these inspiratory units with those of phrenic nerve showed clearly the above mentioned phasic relationship.They started firing before the phrenic nerve,but they reached their maximal rate and then declined and stopped quite in accordance with the phrenic nerve.It is,therefore,reasonable to assume that the central mechanism of the switch from expiratory apnea to inspiration may originate from this kind of neurons. Most of the expiratory units show tonic discharges during the period of apnea with a higher discharge rate than normal and then the rate decreases just before recovery of phrenic firing.In addition,small portion of the expiratory units were depressed as the phrenic discharge ceased.The function of these two different kinds of neurons in the mechanism of development of respiratory rhythm is, apparently,different.