Midbrain evoked potential produced by stimulation of the muscle branch of the oculomotor nerve.

Evoked potential was recorded by needle electrode from the oculomotor nucleus in decerebrate cats, following electrical stimulation of the inferior oblique nerve. The evoked potential was usually triphasic, but complex potential with the second and third negative leaves, or with long latency negative waves, was incidentally recorded depending upon the position of the recording electrode. In the present paper, an attempt was made to clarify in detail, the physiological properties of the evoked complex potential. By changing the intensity or frequency of the inferior oblique nerve stimulation, the complsx potentials could be classified into 2 types: (1) those produced by firing of the motoneurons by antidromically conducted impulses on the efferent fibers, and (2) those produced by transsynaptic firing of the motoneurons by orthodromically conducted impulses on the afferent fibers. As shown by the results of repeated stimulation of the trigeminal nerve and severance of the trigeminal nerve trunk in the middle cranial fossa, the inhibitory effect of the trigeminal nerve on the oculomotor nucleus xoas demonstrated. Also, the possible role of the trigeminal nerve in exerting an inhibitory force on the oculomotor neurons through the internuncial neurons was discussed.