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

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Abstract
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 gammamotoneurones 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 gammamotoaxons 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 musle tone. Instead, the data may offer an explanation for the weakness and — in chronic cases — the reflex atrophy of lesioned muscles.