Reciprocal inhibition during the tonic stretch reflex in the decerebrate cat.

Publisher Website
Google Scholar
Abstract
Post-synaptic reciprocal Ia inhibition during the stretch reflex was investigated; particularly the extent to which an increased Ia excitation of the Ia inhibitory interneurons will be counteracted by recurrent inhibition from motor axon collaterals. Depression of monosynaptic test reflexes to antagonist flexors (reciprocal inhibition) during static stretch of quadriceps or triceps surae in unanesthetized decerebrate cats was investigated. With increasing stretch of the extensor muscle there was first a linear augmentation of reciprocal inhibition, but along with the stretch reflex in the extensor a plateau appeared in the inhibition of the flexors, although the extensor stretch reflex (judged by EMG [electromyography]) increased with further stretching. Within the range of stretching of triceps surae which gave increased stretch reflexes the plateau in the reciprocal inhibition was usually maintained, while during stretching of quadriceps a 2nd phase of augmenting reciprocal inhibition often appeared. Stretch beyond the level which increased the strech reflex activity gave augmenting reciprocal inhibition both in the quadriceps and triceps surae. Excitability measurements from central terminals of Ia afferents revealed that the increasing reciprocal inhibition during increasing stretch reflex activity in quadriceps was associated with a primary afferent depolarization in knee flexor Ia afferents. The primary afferent depolarization evoked in knee flexor Ia afferents by electrical nerve stimulation was then compared with the presynaptic inhibition of knee flexor monosynaptic test reflexes produced by the same stimuli. The 2nd phase of increasing reciprocal inhibition in knee flexors is probably due to presynaptic inhibition and the depth of post-synaptic reciprocal inhibition remains constant at different degrees of stretch reflex activity in both knee and ankle extensors. During increasing stretch reflex activity the increment in Ia excitation and recurrent inhibition on to the Ia inhibitory interneurons almost exactly balance each other. Recurrent inhibition of Ia inhibitory interneurons may serve as a segmental autoregulatory mechanism to keep .alpha.-.gamma.-linked reciprocal inhibition at a constant depth during different levels of agonist activity.