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Abstract
Attempts were made to determine brainstem and cerebellar afferent and efferent projections
of the superior vestibular nucleus (SVN) and cell group 'y' ('y') in the cat using
axoplasmic tracers. Injections of HRP, WGA-HRP and [3H]amino acids were made into
SVN and 'y' using two different infratentorial stereotaxic approaches. Controls were
provided by unilateral HRP injections involving the oculomotor nuclear complex (OMC),
the interstitial nucleus of Cajal (INC) and the deep cerebellar nuclei (DCN). Large
injections of SVN almost invariably involved 'y' and dorsal parts of the lateral vestibular
nucleus (LVN). Smaller injections involved central and ventral peripheral parts of
SVN. Discrete injections of 'y' involved small dorsal parts of LVN. Afferents to SVN
are derived mainly from the vestibular nuclei (VN) and parts of the vestibulocerebellum.
SVN receives afferents: bilaterally from caudal portions of the medial (MVN) and inferior
(IVN) vestibular nuclei and 'y'; contralaterally from ventral and lateral parts of
SVN and rostral MVN; and ipsilaterally from the nodulus, uvula and medial parts of
the flocculus. Purkinje cells (PC) in medial parts of the flocculus project to central
regions of SVN, while PC in the nodulus and uvula appear to project mainly to dorsal
peripheral regions of SVN. SVN receives sparse projections from the ipsilateral INC,
the contralateral central cervical nucleus (CCN) and virtually no projections from
the reticular formation. SVN projects via the medial longitudinal fasciculus (MLF)
to the ipsilateral trochlear nucleus (TN), the inferior rectus subdivision of the
OMC, the INC, the nucleus of Darkschewitsch (ND) and the rostral interstitial nucleus
of the MLF (RiMLF). Contralateral projections of SVN cross in the ventral tegmentum
caudal to most of the decussating fibers of the superior cerebellar peduncle and terminate
in the dorsal rim of the TN and the superior rectus and inferior oblique subdivisions
of the OMC; sparse crossed projections enter the INC and the ND. Cerebellar projections
of SVN end as mossy fibers in the ipsilateral nodulus, uvula and in medial parts of
the flocculus bilaterally. Retrograde transport from unilateral injections of the
OMC indicate that afferents from SVN arise ipsilaterally from central and dorsal regions
and contralaterally from dorsal peripheral regions. Ventral cell group 'y' receives
small numbers of afferent fibers from caudal central parts of the ipsilateral flocculus.
No fibers from ventral 'y' could be traced to other vestibular nuclei, the OMC or
the cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)