NEURONAL ACTIVITIES UNDERLYING INSPIRATORY TERMINATION BY PNEUMOTAXICMECHANISMS

The purpose was to identify and characterize the discharge patterns of pontile neurons which are responsible for the termination of inspirat ory activity. Phrenic discharge is prolonged following destruction of neurons at the junction of mesencephalon and pens by neurotoxins. Neur onal activities were recorded in this region in decerebrate, vagotomiz ed, paralyzed and ventilated cats. At normocapnia, neurons had tonic d ischarge patterns, most of which were linked to phasic periods of phre nic activity. Peak activities occurred in late neural inspiration or e arly expiration. In hypercapnia, neuronal discharge frequencies did no t increase, rather activity became more concentrated during one portio n of the respiratory cycle. In severe hypoxia, neuronal activities dim inished in parallel with the prolongations of phrenic discharge and es tablishment of apneusis. During recovery, some neurons transiently acq uired phasic, respiratory-modulated discharge patterns. Neuronal activ ities from neighboring regions did not exhibit comparable changes in h ypercapnia or hypoxia. We conclude that rostral pontile neuronal activ ities are a primary determinant of the reversible and irreversible ter minations of eupneic inspiratory activity.