The cerebellum is connected to the brain through three paired peduncles: the superior, which is efferent with the exception of the ventral spinocerebellar and tectocerebellar tracts; the middle, which contains the main input to the cerebellum from the pons; and the inferior, through which the vestibular and spinal input enters the cerebellum. The posterolateral walls of the fourth ventricle are bounded by these peduncles (Fig. 5-2 and Table 5-2).
The anatomic subdivision of the cerebellum corresponds roughly with its functional organization, based on the arrangement of its afferent fiber connections. The flocculonodular lobe receives special proprioceptive impulses from the vestibular nuclei and is therefore also referred to as the vestibulocerebellum; it is concerned essentially with equilibrium. The anterior vermis and part of the posterior vermis are referred to as the spinocerebellum, since projections to these parts derive to a large extent from the proprioceptors of muscles and tendons in the limbs and are conveyed to the cerebellum in the dorsal spinocerebellar tract (from the lower limbs) and the ventral spinocerebellar tract (upper limbs). The main influences of the spinocerebellum appear to be on posture and muscle tone. The neocerebellum derives its afferent fibers indirectly from the cerebral cortex via the pontine nuclei and through the middle cerebellar peduncles (brachium pontis). This portion of the cerebellum is concerned primarily with the coordination of skilled movements that are initiated at a cerebral cortical level.
Largely on the basis of ablation experiments in animals, three characteristic physiologic patterns corresponding to these major divisions of the cerebellum have been delineated. These constellations find some, but not perfect, similarities in the clinical syndromes that are observed when various parts of the cerebellum are damaged in patients. Lesions of the nodulus and flocculus (flocculonodular lobe) in animals have been associated with a disturbance of equilibrium and frequently with nystagmus; individual movements of the limbs are not affected. Anterior lobe ablation in primates results in increased shortening and lengthening reactions (the reactions of the muscle to passive flexion or extension at a joint), somewhat increased tendon reflexes, and an exaggeration of the postural reflexes, particularly the “positive supporting reflex,” which consists of extension of an animal’s limb in response to light pressure on the foot pad. Ablation of a cerebellar hemisphere in cats and dogs yields inconsistent results, but in monkeys it causes hypotonia and clumsiness of the ipsilateral limbs; if the dentate nucleus is included in the hemispheric ablation, these abnormalities are more enduring and the limbs also show an ataxic, or “intention” tremor. Again, these find only approximate similarities to clinical signs in patients with lesions of the cerebellum as discussed below.
The studies of Chambers and Sprague and of Jansen and Brodal have demonstrated that in respect to both its afferent and efferent projections in animals, the cerebellum is organized into longitudinal (sagittal) rather than transverse zones. There are three longitudinal zones—the vermian, paravermian or intermediate, and lateral—and there seems to be considerable overlap from one to another. Sprague and Chambers, on the basis of their investigations in cats, concluded that the vermian zone coordinates movements of the eyes and body with respect to gravity, and movement of the head in space. The intermediate zone, which receives both peripheral and central projections (from motor cortex), influences postural tone and also individual movements of the ipsilateral limbs. The lateral zone is concerned mainly with coordination of movements of the ipsilateral limbs but is also involved in other functions.