Floccular efferents in the rhesus macaque as revealed by autoradiography and horseradish peroxidase

Abstract

To fulfill its putative role in short- and long-term modification of the vestibulo-ocular reflex, the flocculus of the cerebellum must send efferents to brainstem nuclei involved in the control of eye movements. In order to reveal the sites of these interactions, we determined the projections of the flocculus by autoradiography and orthograde transport of horseradish peroxidase in five rhesus macaques. Anterogradely labeled axons collected at the base of the injected folia and coursed caudally and medially between the middle cerebellar peduncle and the flocculus. They swept medially over the caudal surface of the middle cerebellar peduncle, over the dorsal surface of the cochlear nuclei, and then caudally along the lateral surface of the inferior cerebellar peduncle to pass over its dorsal surface in the cerebellopontine angle and terminate exclusively in the ipsilateral vestibular nuclei. Three contingents of axons could be differentiated. The axons of one group flowed caudally and medially into the y-group, which clearly received the densest floccular projection. Other, notably thicker, axons of this group continued rostrally and medially to terminate chiefly in the large-cell core of the superior vestibular nucleus. A second large contingent of thin axons streamed caudal and ventral to the y-group to form a compact tract adjacent to the lateral angle of the fourth ventricle and dorsal to the medial vestibular nucleus. Fibers from this tract (the angular bundle of Löwy) supplied a sizable projection to the rostral part of the medial vestibular nucleus and modest projection to the ventrolateral vestibular nucleus. A final group of fibers extended caudally and medially from the y-group in a plexus ventral to the dentate and interposed nuclei to terminate in the basal interstitial nucleus of the cerebellum (Langer, '85), a broadly distributed cerebellar nucleus on the roof of the fourth ventricle. The flocculus can affect vestibulo-ocular behavior only through these efferents to the vestibular nuclei and the basal interstitial nucleus of the cerebellum.

Chronic Changes in Inputs to Dorsal Y Neurons Accompany VOR Motor Learning

“…Namely, our previous report on vertical Purkinje cells (Blazquez et al 2003) (Blazquez et al 2000;Langer et al 1985;Partsalis et al 1995b). To calculate G FTN_vestib , we utilize a particular property found in the Purkinje cell population and graphically described in Fig.…”

“…The only known source of eye velocity to Y neurons arrives from the flocculus (Partsalis et al 1995b;Rambold et al 2002). The head signal, however, arrives to Y neurons via two pathways: the flocculus, which sends an upward head velocity signal (inhibitory synapses convert the downward head velocity of Purkinje cells to upward head velocity) (Langer et al 1985;Partsalis et al 1995b), and interneurons with either up or down head-velocity signal are located in the superior vestibular nucleus (SVN) (Blazquez et al 2000;Sato and Kawasaki 1987). In the normal gain or naïve animal, Y neuron head-velocity sensitivity arrives almost exclusively from the flocculus because inputs from up-head and down-head velocity interneurons in the superior vestibular nucleus cancel each other at the level of the dorsal Y group, as suggested by pharmacological inactivation of the cerebellar flocculus (Partsalis et al 1995b).…”

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

“…That study found that the region at the border of PH and VMN, the site of our dense patch of calbindin-labeled fibers, was free of calbindin-labeled fibers in those animals. Several studies suggest the most likely source of cerebellar projections to the regions of dense calbindin fiber label is the flocculus (Sato et al 1982;Langer et al 1985a;Shojaku et al 1987;Umetani 1992;De Zeeuw et al 1994;Tan et al 1995). The calbindin-dense patches thus define input compartments.…”

Brain Stem Pursuit Pathways: Dissociating Visual, Vestibular, and Proprioceptive Inputs During Combined Eye-Head Gaze Tracking

“…A number of parallel and interconnected pathways are involved in initiating and maintaining smooth-pursuit eye movements (for review, see Keller and Heinen 1991); however, particular importance has been assigned to the cortico-ponto-cerebellar pathway arising from the medial superior temporal sulcus (MST) of the extrastriate cortex. This pathway accesses the brain stem circuitry via inhibitory projections from the ipsilateral cerebellar flocculus and ventral paraflocculus, herein referred to as the floccular lobe (Balaban et al 1981;Dow 1937;Gerrits and Voogd 1989;Langer et al 1985). Under more natural conditions (i.e., when the head is not restrained), humans and primates use coordinated movements of the head as well as the eyes, referred to as gaze pursuit, to align their axis of gaze (gaze ϭ eye-in-head position ϩ headin-space position) with a moving target.…”