Distribution of Intersegmental Interneurones That Can Reset the Respiratory Rhythm of the Locust

Interneurones in the respiratory rhythm generator of the locust were identified by means of intracellular recording and staining techniques. A description is made of the properties and structures of nine intersegmental neurones which reset the respiratory rhythm when injected with current pulses. All but one of these neurones discharged in phase with expiration. The injection of constant depolarizing current into these interneurones altered the respiratory rate (increase for six, decrease for three). The respiratory rhythm generator extends more posteriorly within the ventral nerve cord than the metathoracic ganglion. In the first fused abdominal ganglion, four individual interneurones were identified descending into the unfused abdominal ganglia. In the first unfused abdominal ganglion an intemeurone which reset the respiratory rhythm was found ascending into the metathoracic ganglion. The respiratory rhythm generator also extends more anteriorly within the ventral nerve cord than the metathoracic ganglion. Two interneurones influencing the respiratory rhythm send their axons from the first fused abdominal ganglion into the metaand mesothoracic ganglia. One of these directly excited a mesothoracic intemeurone which also influenced the respiratory rhythm when injected with current. In the suboesophageal ganglion another intemeurone was found which, although capable of resetting the respiratory rhythm, was not alway active during respiration. We conclude that the respiratory rhythm generator is distributed over abdominal, thoracic and suboesophageal ganglia. At least one part of the respiratory rhythm generator (in the suboesophageal ganglion) is not always active and can be recruited during vigorous respiration. Thus the number of active components in the respiratory rhythm generator is variable and additional elements can be recruited depending on the behavioural situation.