Vestibulospinal effects on hindlimb motoneurons of the frog

Summary Field and intracellular potentials were recorded in the lumbar spinal cord of the frog following stimulation of the anterior branch of the vestibular nerve and vestibular nucleus. The field potential recorded in the motoneuron pool after VIIIth nerve stimulation consisted of two presynaptic positive-negative potentials (latencies 1.7 and 2.6 msec) followed by a slow negative wave. The latency of the first presynaptic field potential was only 0.6 msec longer than the presynaptic field potential evoked by stimulation of the vestibular nucleus; it is suggested that electrotonic coupling in the vestibular nuclei is responsible for the fast vestibulospinal transmission.Whereas VIIIth nerve stimulation produced EPSPs in both flexor (peroneal) and extensor (tibial) motoneurons, IPSPs were found only in extensor motoneurons. The functional implication of these findings was discussed. Comparison of PSP latencies with the extracellular presynaptic field potentials generated by VIIIth nerve or nucleus stimulation indicated that EPSPs were produced by the excitatory action of vestibulospinal axons on motoneurons. The longer latencies of the vestibular induced IPSPs suggested that they were generated indirectly by inhibitory spinal interneurons. Preliminary experiments on the interaction of segmental and vestibular induced PSPs suggest that the latter are generated close to the soma of motoneurons.