Midbrain electrical fields produced by stimulation of the muscle branches of the oculomotor nerve.

Evoked potentials were recorded from the oculomotor nucleus in decerebrate cats following electrical stimulation of individual muscle branches of the oculomotor nerve. These potentials were usually triphasic but other more complex potentials were evoked depending on the positions of the recording electrode. Equipotential contour maps of the evoked electrical fields were plotted at different time intervals following the stimulation of the individual nerve branches. These maps provided a dynamic picture of the electrical field changes in the nucleus. Potentials evoked from individual extraocular muscles predominated in different regions of the nucleus. Potentials from the inferior rectus muscle predominated in the rostral homolateral part of the nucleus, from the medial rectus in the middle part of the nucleus, and from the inferior oblique in the most caudal part of the nucleus. Maximum evoked potentials from the superior rectus were obtained in the contralateral caudal part of the nucleus, but many potentials were also recorded from the homolateral middle and caudal parts. It, therefore, appeared reasonable to assume that the superior rectus muscle was bilaterally innervated. This assumption toas further supported by retrograde degeneration studies in the oculomotor nucleus.