This is an excerpt from Neurophysiological Basis of Motor Control-3rd Edition by Mark L. Latash & Tarkeshwar Singh.
As stated earlier, the cerebellum is divided into three areas: the cerebrocerebellum, spinocerebellum, and vestibulocerebellum. The cerebrocerebellum primarily receives information from the cerebral cortex. The cerebrocerebellum loop starts with the cortical projections to the pontine nuclei and proceeds through the middle cerebellar peduncle to the contralateral cerebrocerebellum, to the dentate nucleus, and back to the cortex of the large hemispheres via the thalamus.
The cerebrocerebellum has been implicated in motor planning of hand movements. It has also been implicated in the temporal control of movements, movement initiation, and the timing of different components of movements. These findings have led to the hypothesis that the cerebrocerebellum may be involved in the timing of motor and serial events (Ivry and Keele, 1989; Diedrichsen et al. 2007). Damage to the cerebrocerebellum affects patients’ ability to judge the passage of time and consequently impairs their ability to judge if one time interval was longer or shorter than another.
The spinocerebellum consists of the vermis and intermediate parts of the cerebellar hemisphere. The spinocerebellar tracts provide extensive somatosensory input from the spinal cord about limb position, touch, and pressure. The dorsal spinocerebellar tract conveys proprioceptive information from muscle and joint receptors to the spinocerebellum. This feedback provides the cerebellum with a continuous and real-time estimate of the sensory consequences of movement, irrespective of whether the movement is generated actively or passively. In contrast, the ventral spinocerebellar tract only provides sensory feedback during active movements. This has led to the idea that the cerebellum processes active and passive movements differently and that it may be involved in comparing the sensory consequences of the movement with the actual movement (Person 2019).
Purkinje neurons in the spinocerebellum project somatotopically to different descending motor pathways. First, the vermis sends axons to the fastigial nucleus, and the fastigial nucleus projects bilaterally to the brainstem, the reticular formation, and the vestibular nuclei. This way the vermis provides input to the medial descending motor pathways (see chapter 11) that primarily control postural (neck and trunk) muscles and proximal limb muscles. Purkinje neurons in the intermediate parts of the cerebellar hemispheres project to the interposed nucleus. These neurons exit through the superior cerebellar peduncle and decussate to terminate on the red nucleus (see section 11.4). Axons from the red nucleus decussate and descend to the spinal cord, forming the rubrospinal tract. The remaining axons from the interposed nucleus terminate in the thalamus. Thalamocortical neurons then project to the primary motor cortex (M1), where the corticospinal tracts originate. In this way, the intermediate cerebellum contributes to the control of distal limb muscles.
The vermis is also involved in regulation of two types of eye movements, saccades and smooth pursuits. Eye movements will be described in more detail in chapter 14. Briefly, saccades are rapid eye movements that shift gaze between targets, and smooth pursuit eye movements are used to track moving objects. Lesions in the vermis impair the accuracy of both types of eye movements.
The vestibulocerebellum receives sensory input from the vestibular system (i.e., the otolith organs and the semicircular canals that relay information on the head’s movement with respect to gravity). The otolith organs and semicircular canals project to the vestibular nuclei in the brainstem that provide input to the vestibulocerebellum. The output of the vestibulocerebellum bypasses the deep cerebellar nuclei and directly reaches the vestibular nuclei. Purkinje neurons in the medial parts of the vestibulocerebellum project to the lateral vestibular nucleus to modulate the lateral and medial vestibulospinal tracts. These tracts control head and neck muscles and limb extensors and primarily contribute to posture regulation. Purkinje neurons in the lateral parts of the vestibulocerebellum project to the medial vestibular nucleus that controls eye movements and coordination between head and eye movements (i.e., the vestibulo-ocular reflex). The vestibulo-ocular reflex (see chapter 14) is a gaze-stabilizing reflex: the vestibular system transforms sensory signals related to head movements into motor commands to generate compensatory eye movements in the opposite direction of the head movement, ensuring the stabilization of gaze direction. This reflex is critical to stabilizing our gaze on someone’s face while we are nodding our head in agreement.
A person is having difficulty reading road signs while walking. Which part of the cerebellum might be damaged?