Cerebellar Efferent Pathways

The exception to the PC-DCN projection pattern throughout the cerebellum is the vestibulocerebellar cortex (vermal part of lobule IX [uvula], vermal [nodulus], and hemispheric parts [flocculus] of lobule X), which has direct reciprocal connections with vestibular nuclei. In addition, PCs in zone B of the anterior vermis commit their axons directly to the lateral vestibular nucleus, the source of the lateral vestibulospinal tract, by which the cerebellum regulates the activation of descending spinal motor systems (see Plates 8-11 and 8-12).

FASTIGIAL NUCLEUS

Corticonuclear projections from the cerebellar vermis are directed to the fastigial nucleus. It has rostral and caudal parts with different connections and functional significance. The rostral part of the fastigial nucleus sends efferents in the ipsilateral juxtarestiform body to the same side of the brainstem. Axons from its caudal part cross to the contralateral cerebellum in the hook bundle of Russell (the uncinate fasciculus) and project either to the contralateral brainstem in the juxtarestiform body or to the contralateral cerebral hemisphere in the superior cerebellar peduncle (SCP).

Both the rostral and caudal divisions of the fastigial nucleus project to nuclei of the pontomedullary reticular formation from which they receive inputs (see Reticular Afferents, Plate 8-9). They also both project to the vestibular nuclei; projections from the rostral fastigial nucleus are largely bilateral, those from the caudal fastigial nucleus are mostly contralateral. There are small, crossed projections from the caudal part of the fastigial nucleus to neurons in the posterolateral region of the basis pontis and to the medullary perihypoglossal nuclei. Crossed fastigiospinal projections terminate on motor neurons in the upper cervical spinal cord.

Crossed axons from the caudal division of the fastigial nucleus ascend in the superior cerebellar peduncle and terminate in the pretectal, superior colliculus, and posterior commissure midbrain nuclei concerned with oculomotor and visual control. Connections with periaqueductal gray, anterior tegmental, solitary tract, and interpeduncular as well as parabrachial nuclei impact autonomic, nociceptive, and limbic functions. Fastigial efferents also target the hypothalamus. Thalamic terminations occur in motor-related anterolateral/anterior posterolateral nuclei, the diffusely projecting midline nuclei, and the intralaminar nuclei (central lateral and centromedian). Earlier physiologic and anatomic studies pointed to fastigial nucleus connections with the septal region, hippocampus, and amygdala.

The fastigial nucleus connections with the contralateral inferior olivary nucleus are in the caudal part of the medial accessory olive.

Fastigial nucleus efferents influence multiple functional domains: axial and limb girdle musculature (medial motor system) via the vestibular and reticular nuclei; oculomotor systems, including vertical and horizontal gaze centers in the midbrain and pons; autonomic centers through connections with brainstem and hypothalamus; and emotional modulation through links with limbic-related circuits.

GLOBOSE AND EMBOLIFORM NUCLEI

These nuclei are referred to in lower mammals as the nucleus interpositus posterior (NIP) and nucleus interpositus anterior (NIA), respectively. They provide cerebellar efferents in the superior cerebellar peduncle from the predominantly motor-related spinocerebellum that receives proprioceptive and exteroceptive inputs from the spinal cord and brainstem, and sensorimotor information from the cerebral cortex. Fibers leave the interpositus nuclei and travel in the superior cerebellar peduncle (SCP), also known as the brachium conjunctivum, crossing to the contralateral side in the SCP decussation to course through the red nucleus, providing somatotopically arranged terminations in its caudal, magnocellular part. This red nucleus sector provides the origin for rubrospinal fibers that act on the spinal motor apparatus, particularly arm and hand flexor muscles.

Multiple other brainstem connections of the interpositus nuclei include (1) the lateral reticular nucleus and medullary reticular formation giving rise to reticulospinal tracts, (2) the vestibular nuclei as source for vestibulospinal tracts, (3) the superior colliculus giving rise to the tectospinal tract, (4) the oculomotor nuclei (prepositus hypoglossi, Darkschewitsch, and posterior commissures), (5) the sensory (lateral/external cuneate nucleus), and (6) the nociceptive systems (periaqueductal gray, medullary raphe). These rostrally directed fibers from the interpositus nuclei continue to the hypothalamus and zona incerta before reaching the thalamus. Here they provide heavy terminations to nuclei linked with the precentral motor cortex, notably the ventral posterolateral pars oralis (VPLo) and ventrolateral pars caudalis nuclei (VLc), and to the central lateral nucleus, which has widespread connections beyond motor areas. Efferents from the interpositus nuclei coursing within the descending limb of the SCP project back to the contralateral nucleus reticularis tegmenti pontis and to the dorsal and peduncular nuclei of the basis pontis.

The globose nucleus (NIP) is reciprocally linked with the rostral half of the contralateral medial accessory olive, and the emboliform nucleus (NIA) with the rostromedial part of the dorsal accessory olive.

DENTATE NUCLEUS

The large and multiply folded dentate nucleus is the most lateral of the four major DCN. It is divided into a dorsal part with closely packed folds (polymicrogyric) and a ventral part that is less folded (macrogyric). The dorsal part (paleodentate, because of its relationship to the paleo-, or spinocerebellum) is linked with motor regions of the cerebral cortex. The ventral part (neodentate, interconnected with the more recently evolved neocerebellum) is linked with cerebral association areas. Axons from dentate neurons course in the white matter hilum, enter the SCP, cross to the other side in the decussation of the brachium conjunctivum, and terminate in the thalamus. Dorsal dentate nucleus fibers terminate in motor-related thalamic nuclei, including the ventroposterolateral and ventral lateral nuclei, that then project to the primary motor and premotor cerebral cortex. Middle and caudal thirds of the dentate nucleus are linked via the ventral anterior nucleus of thalamus with the premotor cortex and with the frontal eye fields engaged in saccadic eye movements. Ventral and lateral parts of the dentate nucleus project via the dorsal sector of the ventral lateral nucleus and the medial dorsal nucleus to dorsolateral prefrontal, posterior parietal, and other cerebral association areas. Dentate nucleus projections to thalamic intralaminar nuclei provide widespread influence on cerebral cortical areas. These intralaminar nuclei also project to the striatum, providing an indirect link between cerebellum and basal ganglia. The dentate nucleus also projects to the small-celled (parvicellular) part of the red nucleus that feeds back through the central tegmental tract to the inferior olive, which, in turn, is linked with the cerebellum. Lesions in this triangle of Guillain and Mollaret result in palatal tremor. Dentate fibers in the descending limb of the superior cerebellar peduncle (SCP) terminate in reticular nuclei in the pons.

The rostral and dorsomedial parts of the dentate nucleus (the paleodentate) project to the dorsal lamina and bend of the principal olive. The ventral and caudal parts of the dentate nucleus (the neodentate) project to the ventral lamina of the principal olive.

The complex and varied destinations of the projections from the DCN and vestibular nuclei underscore the role of the cerebellum in multiple domains of neurologic function. Lesions of these different pathways produce a wide array of impairments, motor and otherwise. Damage to the DCN superimposed upon cerebellar cortical dysfunction appears to have adverse consequences on long-term recovery, as exemplified in patients with cerebellar stroke or tumor.

Only gold members can continue reading. Log In or Register to continue