The efferent fibers of the cerebellar cortex, which consist essentially of the axons of Purkinje cells in both humans and animals, project onto the deep cerebellar nuclei (see later). The projections from Purkinje cells are inhibitory whereas those from the nuclei are excitatory on other parts of the motor nervous system. According to the scheme of Jansen and Brodal, cells of the vermis project mainly to the fastigial nucleus; those of the intermediate zone, to the globose and emboliform nuclei (that are combined in humans as the interpositus, or “interposed” nucleus); and those of the lateral zone, to the dentate nucleus.
The deep cerebellar nuclei, in turn, project to certain thalamic and brainstem nuclei via two main pathways: fibers from the dentate and interposed nuclei form the superior cerebellar peduncle, enter the upper pontine tegmentum as the brachium conjunctivum, decussate at the level of the inferior colliculus, and ascend to the ventrolateral nucleus of the thalamus and, to a lesser extent, to the intralaminar thalamic nuclei (Fig. 5-3). Some of the ascending fibers, soon after their decussation, synapse in the red nucleus, but most of them traverse this nucleus without terminating, and pass on to the thalamus. Ventral thalamic nuclear groups that receive these ascending efferent fibers project to the supplementary motor cortex of that side. Since the pathway from the cerebellar nuclei to the thalamus and then on to the motor cortex is crossed, and the connection from the motor cortex through the corticospinal tract is again crossed, the effects of a lesion in one cerebellar hemisphere are manifest by signs on the ipsilateral side of the body.
One special pathway forms a loop, called the Guillain-Mollaret triangle that is of clinical interest. A small group of fibers of the superior cerebellar peduncle, following their decussation, synapse in the red nucleus and then descend in the ventromedial tegmentum of the brainstem via the central tegmental fasciculus, terminating in the inferior olivary nuclei of the medulla (as well as in reticular nuclei of the pons, but these are not part of the feedback loop of the triangle). The olivary nuclei, in turn, project via the inferior cerebellar peduncle back to the cerebellum, mainly the anterior lobe, thus completing a cerebellar–reticular–cerebellar feedback system (Fig. 5-4). The clinical syndrome of oculopalatal tremor results from lesions in the central tegmental fasciculus component of the triangle.
The fastigial nucleus sends fibers to the vestibular nuclei of both sides and, to a lesser extent, to other nuclei of the reticular formation of the pons and medulla. There are also direct fiber connections with the alpha and gamma motor neurons of the spinal cord. The inferior olivary nuclei project via the restiform body (inferior cerebellar peduncle) to the contralateral cerebellar cortex and corresponding parts of the deep cerebellar nuclei. Thus the cerebellum influences motor activity through its connections with the motor cortex and brainstem nuclei and their descending motor pathways (see Evarts and Thach). Chapter 4 details the integration of basal ganglionic influences with those of the cerebellum by their confluence in the anterior thalamic nuclei.