Effects of sensory inputs on the excitability of the crossed extension reflex

Crossed extension reflexes (CERs) recorded from the right quadriceps femoris in acutely decerebrated cats were consistently evoked by single stimuli applied to the left sciatic nerve if the cats were placed with the right body side on the table surface. In contrast, CERs were observed to follow repetitive stimulation only if the left body side was on the table. Sustained CERs, evoked by repetitive stimulation, were inhibited by (i) rotating the head to the left, (ii) flexing the left forelimb, and (iii) placing a weighted surface on the left body side. The reflexes were facilitated by rotation of the head to the right. Ventriflexion, dorsiflexion, or lateral flexion of the neck had no effect on the CER. These data are not consistent with several earlier reports that the CER can be evoked only by repetitive stimulation in the acutely decerebrated preparation. The changes in excitability of the CER described here are all consistent with involvement of the right quadriceps group in righting and postural reflexes induced by various proprioceptive and other mechanoreceptive stimuli. Such effects on the CER were associated only with such stimuli that have opposite actions on extensor neurons supplying the two hind limbs.