The static sensitivity of tendon organs during recovery from nerve injury.

An investigation has been carried out into the physiological properties of tendon organs and their interactions with motor units following two types of nerve injury: nerve crush and nerve transection followed by suture repair. Recovery from nerve crush was very successful: 6 weeks after the injury 60% of the tendon organ-motor unit interactions (n = 62) evoked normal or near-normal patterns of afferent discharge but with reduced firing rates. After 10 weeks of recovery 81% of the interactions (n = 43) were normal. The main abnormality observed was a phasic-only pattern of discharge. The overall reductions in firing rate during early recovery may be attributable to the lower contractile forces generated by the reinnervated muscle units, while the phasic-only responses may also represent immaturity of the transduction mechanism of the regenerated afferent axons. Following nerve transection the quality of recovery was much lower and a range of abnormal, as well as normal patterns of response were observed. For all the afferents studied, both types of response were recorded, suggesting that although there may be changes in the sensitivity of the afferents to muscle contraction, the abnormal responses more probably reflect changes in the form of the mechanical input rather than abnormalities of the transduction process.