ALTERATION IN RATE MODULATION OF REFLEXES TO LUMBAR MOTONEURONS AFTERMIDTHORACIC SPINAL-CORD INJURY IN THE RAT - I - CONTUSION INJURY

This study investigated the regulation of reflex excitability in norma l and midthoracic contusion-injured animals. Recent observations revea led that rate depression, a rate-modulatory process that decreases ref lex excitability, was significantly decreased following experimental m idthoracic contusion injury. The present experiments were performed to extend those studies and to determine if posttetanic potentiation (PT P), a rate-modulatory process that increases reflex excitability, also was altered in lumbar monosynaptic reflexes (MSRs) following midthora cic contusion injury. In normal animals, a mean PTP of 160% of the pre tetanus control was observed at 30 sec following tetanus of the tibial MSR. The decay of the PTP in normal animals followed a rapid initial, then a more gradual pattern, before returning to pretetanus values by 5 min posttetanus. Following midthoracic contusion injury, the maxima l (unpotentiated) MSRs were significantly increased in amplitude, wher eas the percent potentiation of the PTP of the tibial MSRs was signifi cantly decreased. PTP decay in postcontusion animals was significantly more gradual than observed in normal animals and followed a single de cay process. Further analysis of rate depression of tibial MSRs in nor mal animals revealed that the attenuation pattern produced by stimulat ion within the lower range of test frequencies was different from that produced by stimulation at the higher test frequencies. Following con tusion, rate depression of tibial MSRs was significantly reduced at al l test frequencies. These physiological changes in the stretch reflex neural pathway are discussed relative to the development of spasticity .