Muscle receptors in the cross-reinnervated soleus muscle of the cat.

1. Discharges have been recorded from afferents of the soleus muscle following reinnervation by the nerve of a fast twitch muscle, extensor digitorum longus. Recordings were made 227-449 d post-operatively.2. The gross afferent discharge from the cross-reinnervated soleus suggested the presence of fewer mechanosensitive receptors than in normal muscles, as judged by discharges seen during a maximal muscle twitch.3. A comparison of receptors in the cross-reinnervated muscle with afferents from a self-reinnervated muscle showed that many of the responses in the self-reinnervated muscle were also abnormal. It was concluded that much of the disruption resulted from the surgical interference and that rather less could be attributed to the foreign nerve.4. A detailed analysis of response characteristics of receptors in cross-reinnervated soleus muscles of five cats showed that afferent conduction velocities of identified spindles and tendon organs were generally lower than normal and responses to muscle stretch or vibration were often atypical. A large number of afferents which could not be classified as muscle spindles or tendon organs included a group called contraction receptors. These responded generally only during maximal muscle contractions and with a rather feeble discharge. A second group consisted of afferents in which impulses could be elicited by electrical stimulation of the nerve but not by any mechanical activity in the muscle.5. In a further five animals a detailed study was made of the motor supply of muscle spindles. A fusimotor innervation was common, but invariably stimulation of the gamma fibre had a static action on the spindle. No purely dynamic fusimotor fibres were encountered. There were many static beta fibres (skeletofusimotor) no dynamic betas and three axons conducting in the alpha range, which developed no tension, yet produced specific intrafusal effects. Two of these had a mixed static-dynamic action while the third was purely static.6. It was concluded that in the cross-reinnervated soleus muscle the majority of afferents were abnormal in one or other respect. The central action of such abnormal receptors would have to be taken into account when seeking explanations of the transformation of a muscle's mechanical properties following reinnervation by a foreign nerve.