Organization of vestibulo-oculomotor projections in the cat

Abstract

Summary Unilateral stereotaxic lesions were made in restricted portions of the cat vestibular nuclei, and the resulting axonal degeneration was stained by the Nauta 23 method. Vestibular projections to the extraocular motor nuclei arise only in the superior vestibular nucleus (SV) and in rostral portions of the medial vestibular nucleus (RMV), the regions which receive the central projections of the semi-circular canal ganglia. Fibers ascending from these two regions follow separate courses. From caudal portions of the SV, a few fibers pass medially to the ipsilateral VI nucleus. The majority of ascending SV fibers pass rostromedially into the ipsilateral MLF from which they enter the ipsilateral trochlear and III nuclei. Within caudal portions of the III nuclei, fibers decussate and turn caudally to distribute to the contralateral trochlear nucleus. Fibers passing to the contralateral extraocular motor nuclei cross the midline within the III nuclei. From the RMV, ascending fibers pass medially into the ipsilateral VI nucleus. The projections from rostral portions of the RMV decussate rostral to those from caudal portions of the RMV. A few fibers are distributed to the contralateral VI nuclei. Ascending fibers pass in the contralateral MLF to the EOM nuclei bilaterally. Both the SV and RMV project to the interstitial nuclei of Cajal and the nuclei of Darkschewitsch; from the SV, this projection is predominantly ipsilateral, while from the RMV it is predominantly contralateral. Extensive areas of overlap between the projections of the SV and RMV exist. Small regions within the SV and RMV have wide projection fields within the EOM nuclei. The projection fields of restricted regions of the SV and RMV are depicted in detail. This vast and beautiful intricacy of organization provides direct pathways over which a relatively few neurons within the superior and medial vestibular nuclei can influence the activity of large numbers of extraocular muscle motor neurons. Conversely, small groups of extraocular muscle motor neurons receive projections from large areas in the superior and medial vestibular nuclei. The delicate influences of the vestibular system on coordinated eye movements must depend on such exquisite organization.