Origin of cerebellar projections to the region of the oculomotor complex, medial pontine reticular formation, and superior colliculus in new world monkeys: A retrograde horseradish peroxidase study

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Abstract
Cerebellar projections to oculomotor‐related brainstem regions were studied in four groups of New World (capuchin, squirrel) monkeys by using the retrograde transport of horseradish peroxidase (HRP) to determine the origin of the principal cerebellar influence on eye movement. Group A monkeys had HRP injections or transcannular HRP gel implants into the oculomotor complex (OMC), the largest of which involved adjacent paraoculomotor nuclei (e.g., ventral periaqueductal gray, PAG; nucleus of Darkschewitsch, ND; medial accessory nucleus of Bechterew, MAB; dorsomedial parvicellular red nucleus, dmPRN). All of these cases contained large numbers of retrogradely labeled cells in cell group Y. Whereas the smallest OMC injection only labeled a few cells in the dentate nucleus (DN), injections involving paraoculomotor nuclei produced labeling in all of the cerebellar nuclei except the basal interstitial nucleus (BIN). Injections extending into the ND and MAB produced particularly heavy labeling within the interposed nuclei. Group B monkeys had injections/implants into the medial pontine tegmentum and dorsomedial basilar pons. The pontine tegmental cases contained labeled cells in all cerebellar nuclei, but the DN was the most heavily labeled when the implant involved the nucleus reticularis tegmenti pontis (NRTP). Cases with injections into the caudal medial pontine tegmentum (nucleus reticularis pontis caudalis, NRPC), including the physiological paramedian pontine reticular formation (PPRF), but not NRTP, contained the largest number of labeled cells in the fastigial nucleus (FN) and lacked retrograde labeling in the DN. Dorsomedial basilar pontine cases contained almost no labeled cells in the FN, anterior interpositus nucleus (AIN), and posterior interpositus nucleus (PIN) but did contain DN labeling when the injection involved the NRTP. Two dorsomedial pontine tegmental cases and one dorsomedial basilar pontine case had more labeled cells in the BIN than in other cases. Tegmental cases also contained a few labeled cells in cell group Y. Group C monkeys had injections into the parvicellular red nucleus (PRN) and had their heaviest labeling in the DN, although the AIN and PIN also contained labeled cells. The FN, BIN, and cell group Y, on the other hand, contained almost no labeling. Group D consisted of monkeys which had injections into the intermediate and deep superior colliculus (SC). These cases contained thelargest numbers of labeled cells in the PIN and a lesser number in the ventrolateral FN. The DN, AIN, BIN, and cell group Y lacked labeled neurons in these cases. The retrograde labeling in the FN raised important issues with regard to oculomotor function. The FN projected to the supraoculomotor part of the ventral PAG, known to contain preoculomotor neurons, to the caudal medial pontine reticular formation (including PPRF), and toa lesser degree to the deep SC. Considering its other well‐established projections to the perihypo‐glossal complex and vestibular nuclei, the FN probably has a more significant influence on the oculomotorsystem than other deep cerebellar nuclei. The possible convergence of frontal eye field and cerebellar projections in paraoculomotor nuclei, PPRF, NRTP, SC, and perihypoglossal complex is also discussed.