Separate channels mediate prestimulus facilitation and suppression of rabbit nictitating membrane reflex

Neurophysiological mechanisms underlying prestimulus-induced facilitation and suppression of reflex amplitude were investigated by using the rabbit nictitating membrane reflex (NMR). Experiment 1 was conducted to investigate the retractor bulbi motoneuron pool in the region of the abducens nucleus. Direct electrical stimulation of the pool caused NM extension, and recordings from the motoneurons showed greatly increased activity under conditions of NMR amplitude facilitation. Decreased motoneuron activity was conspicuously absent under conditions of NMR amplitude suppression. In Experiment 2, the retractor palpebrae tertiae motoneuron pool of the nucleus centralis caudalis was investigated. Direct electrical stimulation of the pool caused NM retraction, and recordings from the motoneurons showed increased activity under conditions of NMR amplitude suppression, but not under conditions of NMR amplitude facilitation. A quantitative model constructed from these neurophysiological results was shown to generate predictions in close accord with directly measured NMR amplitudes under both suppressive and facilitative conditions. From these results it appears that the behaviorally independent processes of NMR amplitude facilitation and suppression are mediated by anatomically independent motoneuron pools and output pathways showing no evidence of central inhibition but opposing each other peripherally through simultaneous excitation of antagonistic muscles.