Reflex and cerebellar influences on α and on ‘rhythmic’ and ‘tonic’ γ activity in the intercostal muscle

1. Efferent intercostal α and γ activity and afferent intercostal muscle spindle activity were studied in decerebrate cats in response to stimulation of the anterior lobe of the cerebellum and to postural and other reflexes. 2. Low threshold intercostal responses were elicited from lobuli IV and V of the anterior lobe of the cerebellum. 3. The existence of two functionally different types of intercostal γ neurones has been confirmed. These are the ‘rhythmic’ or ‘specifically respiratory’ γ neurones, and the ‘tonic’ γ neurones. 4. In response to cerebellar stimulation, facilitatory, inhibitory and diphasic tetanic and post‐tetanic effects were obtained from α and the two types of γ fibres in both external and internal intercostal nerve branches. 5. Generally both inspiratory and expiratory α and γ activity was facilitated in response to tetanic stimulation at contralateral stimulus sites, and inhibited in response to stimulation of ipsilateral sites. 6. ‘Rhythmic’ γ activity appeared to be rather closely linked to the respiratory α activity but the balance between ‘rhythmic’ γ and α was often changed in response to cerebellar stimulation, as indicated by the responses of primary muscle spindle afferents. 7. The ‘tonic’ γ neurones were as a rule more responsive to cerebellar stimulation than were the α and ‘rhythmic’ γ neurones. Long‐lasting post‐tetanic effects were much more prominent in the ‘tonic’ γ fibres than in the α or ‘rhythmic’ γ fibres. 8. ‘Rhythmic’ γ activity was abolished after cervical transections of the cord. ‘Tonic’ γ activity remained in the spinal preparations although usually at a different discharge rate. 9. ‘Tonic’ γ neurones were more responsive than the ‘rhythmic’ γ neurones to the proprioceptive γ reflex elicited by passive movements of the chest wall as well as to other spinal and supraspinal reflexes. 10. Both ‘dynamic’ and ‘static’ γ fibres seem to be represented in the group of ‘tonic’ intercostal γ neurones. 11. The significance of the results are discussed with regard to the following two main points: ( a ) integration of respiratory and postural functions of the intercostal muscles; ( b ) cerebellar control of the α—γ balance in active contractions and passive relaxations.