Labyrinthine influence on cat forelimb motoneurons.

1. Intracellular responses in forelimb motoneurons to electrical stimulation of the whole labyrinth and of individual semicircular canal nerves were studied in decerebrated, unanesthetized cats. 2. Stimulation of the whole labyrinth typically produced EPSPs, usually bilaterally, in forelimb extensor (LON, LAT, MED) and shoulder (SI) motoneurons and bilateral IPSPs in forelimb flexor (BIC) motoneurons. 3. Latencies of PSPs indicated that most of those in extensor motoneurons were trisynaptic and many seen in flexor motoneurons may involve four synapses. 4. In the cells sampled, stimulation of the anterior, horizontal or posterior canal nerves often evoked EPSPs in extensor and IPSPs in flexor motoneurons, both ipsi- and contralaterally. Responses to canal stimulation were weaker and more variable than those to stimulation of the whole nerve. 5. Transection of the MLF in the lower medulla had no effect on PSPs evoked in forelimb motoneurons. Lesions in the medulla in the area of the LVST greatly reduced the occurrence of contralateral EPSPs in extensor and IPSPs in flexor motoneurons. The pathway linking labyrinths to forelimb motoneurons therefore appears to include the LVST. Hemisection shows that the pathway to contralateral motoneurons descends in the cord on the side of the stimulated labyrinth before crossing to influence these cells. 6. Labyrinthine control of forelimb motoneurons is less direct than control of neck and back motoneurons. It is suggested that the interneuron in the pathway to forelimb motoneurons is the site of integration of labyrinthine with other reflexes.