The nerve center of the rat extrinsic ocular muscles as studied using horseradish peroxidase.

1) The author established the representation of the lower motor center of all the extrinsic ocular muscles by means of HRP labeling in the rat. 2) The muscles which are supplied by the neurons on both sides of the oculomotor nuclei are the levator palpebrae superioris, superior rectus, medial rectus, inferior oblique, inferior rectus, superior oblique and lateral rectus. 3) The muscles which are supplied by the neurons on both sides of the trochlear nuclei are the levator palpebrae superioris, inferior rectus and superior oblique. 4) The muscles which are supplied by the neurons on both sides of the abducens nuclei are the levator palpebrae superioris, superior rectus and lateral rectus, while lightly stained neurons were observed in specimens from the inferior oblique, inferior rectus and superior oblique. 5) The inferior oblique showed a few neurons originating in the contralateral trochlear nucleus but none in the ipsilateral. The lateral rectus revealed its originating neurons mostly in the ipsilateral abducens nucleus. 6) Although it is true that a lower motor center for each muscle does exist, our results indicate that there can be no lower motor center consisting of originating neurons which supply only one muscle in the rat.