Labyrinthine and vestibulospinal effects on spinal motoneurons in the pigeon

Experiments were performed on pigeons to investigate the effects of labyrinthine stimulation on motoneurons innervating neck and limb muscles. Intracellular recordings from the ventral horn of the spinal cord in the neck revealed that stimulation of the ipsilateral labyrinth with only a single shock evokes prominent EPSPs and/or IPSPs in the majority of neck motoneurons at thresholds less than 3--4 times the threshold for the labyrinth-evoked N1 potential recorded in the ipsilateral vestibular nuclei. In many neck motoneurons, the latencies of the PSPs were short enough so that no more than two synapses could be involved in their transmission. Recordings were also obtained from identified motoneurons which innervate muscles of the wing or the leg. Stimulation of the labyrinth, even with multiple stimuli, failed to elicit any observable PSPs in limb motoneurons. These data were consistent with other observations that in awake pigeons stimulation of the labyrinth with weak shocks evoked a pronounced turning of the head to the contralateral side, while even strong or multiple stimuli failed to produce any observable movement of wings or legs. Thus in the pigeon the association of the labyrinth with limb muscles is insignificant when compared to the association of the labyrinth with neck muscles. In addition, supraspinal descending fibers in the pigeon may make monosynaptic excitatory and inhibitory contact with spinal motoneurons.