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

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Abstract
Efferent intercostal [alpha] and gamma 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. Low threshold intercostal responses were elicited from lobuli IV and V of the anterior lobe of the cerebellum. The existence of 2 functionally different types of intercostal gamma neurones has been confirmed. These are the "rhythmic" or "specifically respiratory" [gamma] neurones, and the "tonic" [gamma] neurones. In response to cerebellar stimulation, facilitatory, inhibitory and diphasic tetanic and post-tetanic effects were obtained from [alpha] and the 2 types of [gamma] fibers in both external and internal intercostal nerve branches. Generally both insplratory and expiratory [alpha] and [gamma] activity was facilitated in response to tetanic stimulation at contralateral stimulus sites, and inhibited in response to stimulation of ipsilateral sites. ''Rhythmic'' [gamma] activity appeared to be rather closely linked to the respiratory [alpha] activity but the balance between ''rhythmic'' [gamma] and [gamma] was often changed in response to cerebellar stimulation, as indicated by the responses of primary muscle spindle afferents. The ''tonic'' [gamma] neurones were as a rule more responsive to cerebellar stimulation than were the [alpha] and ''rhythmic'' [gamma] neurones. Long-lasting post-tetanic effects were much more prominent in the''tonic'' [gamma] fibers that in the [alpha] or ''rhythmic'' fibers. ''Rhythmic'' [gamma] activity was abolished after cervical transections of the cord. ''Tonic'' [gamma] activity remained in the spinal preparations although usually at a different discharge rate. ''Tonic'' [gamma] neurones were more responsive than the ''rhythmic'' [gamma] neurones to the proprioceptive [gamma] reflex elicited by passive movements of the chest wall as well as to other spinal supraspinal reflexes. Both ''dynamic'' and ''static'' fibers seem to be represented in the group of '' tonic'' intercostal neurones. The significance of the results are discussed with regard to main points: integration of respiratory and postural functions of the intercostal muscle; cerebellar control of the [alpha]-[gamma] balance in active contractions and passive relaxations.