Discharge behaviour of feline gamma-motoneurones following induction of an artificial myositis

The study was undertaken to test the widely held hypothesis that a painful lesion of the skeleto-motor system leads to an increase in the neuromuscular component of muscle tone by activating gamma-motoneurones in the affected region. In chloralose-anaesthetized cats, artificial myositis was induced in the lateral gastrocnemius-soleus (LGS) muscle and several hours later the impulse activity was recorded from single gamma-motoaxons supplying the medial gastrocnemius (MG) muscle. Under the conditions of the study, the majority of the fusimotor neurones had a resting activity and could be readily excited by natural stimuli. In contrast to the assumptions of the working hypothesis, the gamma-motoneurones in the myositis animals were not activated but showed a strong inhibition; both resting activity and excitability by electrical and natural stimuli were decreased. Additional recordings from fusimotor neurones of a flexor muscle (tibialis anterior, TA) demonstrated that in the preparation used, the behaviour of the flexor gamma-motoneurones was different from extensor ones in that the former usually had no resting activity and did not respond to natural stimuli. The only discernible effect of a myositis of the LGS muscle on the TA gamma-motoneurones was a decrease in their electrical reflex threshold. The results of the study do not support the assumption that a painful muscle lesion is followed by an activation of the gamma-loop that leads to an increase in muscle tone. Instead, the data may offer an explanation for the weakness and--in chronic cases--the reflex atrophy of lesioned muscles.