SHORT-LATENCY EXCITATION OF HINDLIMB MOTONEURONS INDUCED BY ELECTRICAL-STIMULATION OF THE PONTOMESENCEPHALIC TEGMENTUM IN THE RAT

The monosynaptic reflex response evoked by stimulating the dorsal root L6 was greatly facilitated when a low intensity conditioning stimulus was applied to the pontomesencephalic tegmentum (PT) 1-2 ms in advance. When increasing the stimulus strength or the number of stimuli, motor discharges were recorded in the ventral roots and in nerves innervating hindlimb muscles. The lowest threshold site for reflex facilitation was found in a region just ventral to the superior colliculus. A descending volley was recorded from the medulla midline, in the region of the medial longitudinal fascicle (MLF) and from the spinal cord surface at thoracic and lumbar level. The latency of the descending volley and of the motor responses indicates that excitation of hindlimb motoneurons was due to activation of a disynaptic pathway having a relay in the lower brainstem. All spinal and peripheral responses evoked by PT stimulation disappeared when a small electrolytic lesion was placed in the MLF 1-2 mm rostral to the obex. The results show that in the rat the PT region may exert a powerful facilitatory action on hindlimb motoneurons.