Input from the spinal cord to the cerebellum conveys information from muscles and joints to influence muscle tone and posture. The primary spinal cord pathways with connections to the cerebellum include the anterior or ventral spinocerebellar and the posterior or dorsal spinocerebellar tracts. These tracts originate from joint and cutaneous mechanoreceptors and ascend through the spinal cord to enter the ipsilateral cerebellum primarily through the inferior cerebellar peduncle (eFig. 9.69).
A final source of cerebellar input arises from the vestibular nuclei and reticular formation in the brainstem. The connections are primarily involved in reflexive maintenance of balance. These nuclei send axonal projections to the ipsilateral cerebellum through the inferior cerebellar peduncle.
Output from the cerebellum originates from one of the four deep cerebellar nuclei. The largest collection of fibers leaving the cerebellum originates from the dentate nucleus. Axons from this nuclear complex project to the contralateral ventral nucleus of the thalamus after decussating in the superior cerebellar peduncle. From here, axons of the thalamic nuclei project to the motor cortex (eFig. 9.70). This pathway influences posture and movement.
The emboliform and globose nuclei, or interposed nuclei, have a similar course as the axons from the dentate, but with the addition of another synaptic target. Axons from the interposed nuclei decussate in the superior cerebellar peduncle to synapse on the contralateral ventral nucleus of the thalamus and the contralateral red nucleus in the midbrain (eFig. 9.70). Axons leaving the red nucleus descend to the inferior olivary nucleus in the medulla. The axonal projections from the interposed nuclei function in monitoring and correcting motor activity of the upper and lower extremities.
Axons from the fastigial nucleus project to the vestibular nuclei, reticular formation, contralateral ventral nucleus of the thalamus, and contralateral tectum. Vestibular axons pass through the inferior cerebellar peduncle to reach the ipsilateral vestibular nucleus and uncinate fasciculus to reach the contralateral vestibular nucleus (eFig. 9.70). Also going through the inferior cerebellar peduncle are axons heading to the reticular formation. Ascending in the superior cerebellar peduncles are axons that will synapse with the contralateral tectum and contralateral ventral nucleus of the thalamus.
The cerebellum is perfused by the vertebrobasilar system of arteries (eFig. 9.42). Before merging and forming the basilar artery, each vertebral artery gives rise to a PICA and posterior spinal artery and contributes to the formation of the anterior spinal artery (eFig. 9.71). The PICA perfuses the inferior portion of the cerebellum. At the level of the caudal pons, the AICA branches off the basilar artery and perfuses the anterior and lateral portion of the cerebellum, as well as the middle and inferior cerebellar peduncles (eFig. 9.71). Just before the midbrain, the superior cerebellar arteries branch off of the basilar artery and supply the superior cerebellar peduncles and superior portion of the cerebellar hemispheres (eFig. 9.71).
Part VIII: Visual System
The visual system is a complex special sensory system that begins in the eyeball and has neuronal connections to the thalamus, brainstem, primary visual cortex, and association cortices. In addition to mediating visual perception, these connections are also involved in higher visual functions, such as determining spatial relationships between objects and structural features of objects. In this section we will explore the primary or geniculate visual pathway from the retina to the primary visual cortex.