Innervation of extraocular muscles in the rabbit.

The innervation of extraocular muscles in the rabbit was studied by using the methods of horseradish peroxidase (HRP) histochemistry, gross dissection, and quantitative morphology. Subdivisions of the oculomotor complex that innervate the superior rectus, inferior rectus, medial rectus, and inferior oblique and levator palpebrae are described, and our results are in agreement with previous accounts of the projections of this nucleus. Our analysis of the innervation of the lateral rectus and retractor bulbi muscles, however, differs from previous descriptions. The axons of approximately 80% of neurons in the abducens nucleus are in the VIth nerve and innervate the lateral rectus muscle, and approximately 15-20% are internuclear neurons both surrounding and intermingling with the motor neurons of the abducens nucleus. The interneurons project to the medial rectus subdivision of the contralateral oculomotor complex via the medial longitudinal fasciculus (MLF). Neurons in both the abducens and the accessory abducens nucleus innervate the retractor bulbi muscles via the VIth nerve. All neurons in the accessory abducens nucleus innervate the retractor bulbi muscles, but gross dissection revealed that the retractor bulbi is also innervated by the IIIrd nerve. The bases for differences between our data and previously published descriptions are discussed. The trochlear nucleus of the rabbit has not been previously studied by methods of axonal transport. The body of the nucleus, its caudal tail, the trajectories of axons entering the trochlear nerve, and soma size distributions are described. The trochlear nucleus contains approximately 900 neurons; most are motoneurons the axons of which travel in the trochlear nerve and decussate in the anterior medullary velum. Approximately 3% of trochlear motor neurons innervate the ipsilateral superior oblique muscle. Their soma size is significantly smaller than that of contralaterally projecting neurons. For comparative purposes, the innervation of extraocular muscles by the trochlear nerve was also investigated in several rodents and carnivores. In all animals studied, the percentage of trochlear neurons innervating the ipsilateral superior oblique muscle was strikingly uniform (2-4%). Gross dissection of the extraocular muscles revealed in the rabbit a muscle, innervated by the trochlear nerve, for which we propose the name "tensor trochleae." In the rabbit, this muscle is innervated by approximately one-third of the trochlear motor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)