Primary vestibular afferent projections to the cerebellum as demonstrated by retrograde axonal transport of horseradish peroxidase

Abstract

Afferents from the vestibular receptors reach the cerebellum as primary afferent fibers from axons of cells in the vestibular (Scarpa's) ganglion and as secondary fibers originating from the vestibular nuclei (for refs. see Brodal 3 and Walbergla). in experimental anatomical studies in mammals, the primary vestibular fibers have been traced to the flocculonodular lobe and the fastigial nucleus (for refs. see Alley et al. 1 and Larselll°,11). Because of this, the flocculonodular lobe is in the literature referred to as the 'vestibulocerebellum'. In the cat, Brodal and Hoivik 4 found that in addition the primary vestibular fibers also distributed to the caudal folia of the uvula, the ventral and dorsal paraflocculus, the lingula and the small-celled part (the so-called 'subnucleus lateralis parvocellularis') of the ipsilateral dentate nucleus. Based on these findings Brodal and Hoivik 4 and Brodal 2 have argued that the vestibulocerebellum includes more than the flocculonodular lobe. Recently, physiological studies have indicated that primary vestibular afferents may project also to the cerebellar vermisa,6aL During our studies on the afferent connections to the cerebellum by means of the method of retrograde axonal transport of horseradish peroxidase (HRP), consistent labeling of neurons in the vestibular ganglion was observed following HRP injections in areas of the cerebellum other than those mentioned above. The present communication describes these preliminary new findings. The experimental material includes 7 cats. Five cats served as controls. Details of the experimental technique for injection of HRP in the cerebellum have been given by Walberg et al. 1~ and Kotchabhakdi et al. s. After a survival time of 2-3 days the animals were anesthetized and killed by intracardiac infusion of a mixture of 0.4~i~ paraformaldehyde and 1.25'.~o glutaraldehyde in 0. I M phosphate buffer, pH 7.4. The brains and the vestibular ganglia were immediately dissected free and postfixed in the same fixative for 24 h at 4 °C. The brains were then cut into smaller blocks while the vestibular ganglia were dissected free from the surrounding bony labyrinth. No effort