A note on the development of the vestibulo-ocular pathway in the chicken

The vestibulo-ocular pathways have been examined in embryonic chicks using horseradish peroxidase or diI as retrograde and anterograde tracers. The vestibular neurons project to the rostral, external eye motor nuclei over one or the other of three separate pathways; the ipsilateral and contralateral medial longitudinal fascicle and the contralateral brachium conjunctivum. The brachium conjunctivum component originates dorsally in the superior vestibular region and projects to the contralateral inferior oblique and superior rectus motor nuclei. An ipsilateral component of the medial longitudinal fascicle is labeled from more ventral sites in the vestibulo-cerebellar process and terminates in the ipsilateral superior oblique and inferior rectus nuclei. The contralateral medial longitudinal fascicle component originates still more ventrally and terminates in the contralateral superior oblique and inferior rectus motor nuclei. Accordingly, the vestibulo-ocular pathways in chickens operate predominantly on synergistic pairs of external eye muscles. These selective terminal fields are established within a day or two after the first terminals invade the eye motor nuclei during embryogenesis.