Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey

Abstract

The primary goal of this investigation was to identify the areas of the brainstem and cerebellum that provide afferent projections to the nucleus prepositus hypoglossi in primates. After horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was injected into the prepositus in squirrel monkeys (Saimiri sciureus), the largest populations of retrogradely labeled neurons were found in the vestibular nuclei, the contralateral perihypoglossal nuclei, and the medullary and pontine reticular formation. Unlike the cat, the prepositus in Saimiri received substantial projections from the nucleus raphe dorsalis and the central mesencephalic reticular formation, whereas few or no labeled cells were found in the cerebellar cortex, the superior colliculus, or the nucleus reticularis tegmenti pontis. By comparing the afferents to the prepositus with those to the abducens nucleus, we found that all regions projecting to the abducens also projected to the prepositus, without exception. Anterogradely transported WGA-HRP showed that the major brainstem recipients of prepositus efferents were the vestibular and perihypoglossal nuclei, the inferior olive, the medullary reticular formation, and the extraocular motor nuclei. In the cerebellar cortex, the prepositus projected to restricted regions of crura I and II as well as the caudal vermis and vestibulocerebellum. The many parts of the oculomotor system receiving input from the prepositus and the parallel innervation of the prepositus and the abducens by a large number of premotor centers lend support to the hypothesis that the prepositus may distribute an efference copy of motor activity, and may also play an important role in the process of neural integration.

Neural Correlates of Forward and Inverse Models for Eye Movements: Evidence from Three-Dimensional Kinematics

“…Because motoneurons are restricted to innervating the extraocular muscles, the output of the inverse model is also represented in BT cells, which like motoneurons discharge during both saccades and slow eye movements. BT neurons also receive similar neuroanatomical projections as motoneurons (McCrea and Baker, 1985;Belknap and McCrea, 1988), making it possible that they construct a replica of the motoneuron signal. In addition, BT cells project to multiple targets, including motoneurons, the cerebellum, and the thalamus (Langer et al, 1985;McCrea and Baker, 1985).…”

“…Despite significant anatomical and physiological evidence that BT neurons are ideally suited for carrying a duplicate of the motoneuron signal, this notion was not explicitly made (but see Belknap and McCrea, 1988) until very recently when BT neurons during pursuit and the VOR were shown to have identical dynamics to those of extraocular motoneurons (Green et al, 2007). In particular, by comparing the high-frequency responses of horizontal BT neurons and motoneurons, Green et al (2007) proposed that, like motoneurons, BT cells represent the output of the inverse model.…”

“…Superb candidates for this role are burst-tonic (BT) neurons that are found throughout the brainstem, including the nucleus prepositus hypoglossi (McFarland and Fuchs, 1992), vestibular nuclei (Scudder and Fuchs, 1992), interstitial nucleus of Cajal (Fukushima, 1991), and paramedian tract (Buttner-Ennever and Horn, 1996;Nakamagoe et al, 2000). The notion that BT cells represent the output of the inverse model was first introduced by Belknap and McCrea (1988) and recently resurrected, because BT cell dynamics during pursuit and the vestibulo-ocular reflex (VOR) are identical to those of extraocular motoneurons (Green et al, 2007).…”

Squirrel Monkeys and Space Motion Sickness

“…In this connection, the afferent projections from the brain stem and the cerebellum to the PPH were studied in detail with the WGA-HRP method [55]. The vestibular nuclei, the contralateral PPH, and the medullary and pontine reticular formation intensively projected fibers to the PPH.…”

“…The PPH also projected to the restricted regions of crus I and II, the caudal vermis, and the vestibular cerebellum. In consideration of the parallel innervation of afferents and efferents of the PPH and the abducens nucleus, it is proposed that the PPH distribute an efferent copy of motor activity [55].…”