SEGMENTAL REFLEX ACTION IN NORMAL AND DECEREBRATE CATS

The objective of this study was to evaluate the action of the stretch reflex on the ankle extensor muscles of normal and decerebrate cats. Experiments were performed on nine freely standing, unrestrained cats and repeated after decerebration at the premammillary level. The length, force, and electromyograph (EMG) of the soleus (SOL) and lateral gastrocnemius (LG) muscles were recorded with the use of implanted transducers and electrodes. The left ankle joint was unexpectedly and reproducibly dorsiflexed by briefly stimulating the common peroneal (CP) nerve with electrodes within an implanted nerve cuff. The ensuing twitch contractions of the ankle dorsiflexor muscles stretched the ankle extensor muscles by 0.3-2.0 mm. Lidocaine was infused into another nerve cuff proximal to the stimulation site, to reversibly block the central propagation of evoked volleys in the CP nerve. Reflex action before and after decerebration was measured from the responses to perturbations of similar amplitude and duration delivered at approximately matched background values of muscle length and force. In most cats the temperature of the hindlimb was monitored with an implanted thermistor and was restored to normal values with radiant heat after decerebration. A stretch imposed on the tonically active ankle extensor muscles immediately caused a considerable rise in the force recorded from the triceps tendon. Within 30-40 ms the triceps force peaked, reaching a value 10-20 N greater than background, and then rapidly declined while the extensor muscles were still lengthening. The initial rise in force preceded any change in triceps EMG. It was attributed to the intrinsic viscoelasticity of the stretched muscles and tendons. After decerebration the magnitude and timing of the initial force peak did not change. A short-latency reflex EMG burst was typically recorded from both the SOL and LG muscles, starting 11-17 ms after stimulus onset. After decerebration the area of the reflex EMG burst increased in all nine cats, typically by a factor of 2 or 3. After decerebration a second, smaller increase in force was typically observed starting 60-80 ms after onset of stretch. This later force rise, interpreted to be of reflex origin, was rarely apparent in normal cats. Decerebration introduced consistent modifications in postural behavior that were revealed by pushing down on the back of quietly standing cats. In normal cats, after brief pushes that stretched the ankle extensor muscles by 1-2 mm, the EMG, force, and length quickly stabilized near their initial values. After decerebration similar pushes initiated large, persistent, poorly damped oscillations in the ankle extensor force, length, and EMG. Deafferentation eliminated these oscillations, demonstrating that they were reflexly mediated. The findings from this study demonstrate that decerebration causes a two- to threefold increase in the amplitude of the short-latency reflex EMG response to stretch and subsequent, reflexly mediated oscillations in the background EMG. These two effects account for the mechanical instability characteristic of decerebrate posture. In normal cats, however, segmental reflex action appears appropriate for the rapid stabilization of posture in response to brief perturbations.