Semicircular canal input to pigeon vestibulocerebellum

Plurisegmental vestibulocerebellar projections and other hindbrain cerebellar afferents in midterm chick embryos: biotinylated dextranamine experiments in vitro

The vestibular neuronal groups that project to the cerebellum were mapped in midterm chick embryos (10-11 days in "ovo") through "in vitro " retrograde tracing experiments. Massive unilateral deposits of biotin-dextranamine were placed at the basis of the cerebellum to label the cerebellar peduncles. Separate rostral and caudal vestibulo-cerebellar groups were identified, with predominance of contralateral neurons. We tentatively identified the rhombomeric location of both groups, as well as their topography within the conventional cytoarchitectonically-defined vestibular nuclei, by comparison with previously established segmental fate maps. The rostral group extended from rhombomeres 1-4 (r1-r4) and was restricted mainly to the superior vestibular nucleus. The caudal group stretched from r6 to pseudorhombomere "r8" and was related to the descending and medial vestibular nuclei. The less abundant ipsilateral vestibulocerebellar neurons had a similar topography. The crossing axons of the rostral vestibulocerebellar neurons formed a distinct rostral vestibulocerebellar decussation, restricted to the floorplate of rhombomere 2. The axons of the caudal vestibulocerebellar population mostly decussated associated to the deep cochlear commissure. The present results extend the "segmental hodological mosaic" of defined projection-neuron groups identified within the avian vestibular nuclear complex: The vestibulocerebellar projecting neurons as a type appear iterated from r1 to r4 and from r6 to pseudorhombomere "r8," albeit showing in their arrangement peculiarities related to single segmental domains, particularly rostrally. In contrast, the vestibulospinal groups are located more restrictedly in r4-r6, while the vestibulo-ocular projecting neurons extend from r1 to "r7." Only r4 and r6 contain elements of all three hodological types. The organization of the three vestibular projection populations studied to date seems comparable in chicken and frogs and may be a conserved feature in vertebrates.

Projections of the nucleus of the basal optic root in the pigeon: an autoradiographic and horseradish peroxidase study

The efferent projections of the nBOR complex, have been studied with both anterograde autoradiographic and retrograde horseradish peroxidase (HRP) techniques. The nBOR complex includes three distinct subdivisions: the nucleus of the basal optic root (nBOR), the nBOR pars dorsalis (nBORd) and the nBOR pars lateralis (nBOR1). Unilateral injections of 3H-leucine or 3H-proline/3H-leucine mixtures into the nBOR complex have demonstrated prominent bilateral projections upon (1) the vestibulocerebellum, (2) the inferior olivary complex, (3) the oculomotor nuclear complex, (4) the nucleus interstitialis, contralateral projections upon (5) the contralateral nBOR complex and ipsilateral projections upon (6) a pretectal nucleus, the nucleus lentiformis mesencephali, pars magnocellularis. Unilateral injections of HRP confined to folia IXc,d and paraflocculus of the cerebellum, the contralateral nBOR complex or the nucleus lentiformis mesencephali resulted in retrograde labeling of predominantly medium and large size cells within the entire nBOR complex. Unilateral injections of HRP within the inferior olive resulted in retrograde labeling of small, spindle-shaped cells within nBOR and nBORd. Unilateral injections of the oculomotor complex which included the trochlear nucleus resulted in retrograde labeling of small cells within the ipsilateral nBORd and predominantly medium and large cells in the contralateral nBOR. The displaced ganglion cells of the retina give rise to a prominent and distinct projection upon the nBOR complex (Karten et al., '77). The nBOR complex in turn projects upon the oculomotor nuclear complex, the nucleus interstitialis and the vestibulocerebellum, regions which have been implicated in oculomotor function. These findings strongly suggest that the displaced ganglion cells and the accessory optic system have a major influence upon oculomotor reflexes including eye and head movements.