20 Cerebellum
The cerebellum helps coordinate movement by sampling most kinds of sensory information, comparing current movements with intended movements, and issuing planning or correcting signals. The comparisons are made in a uniform, precisely organized, cerebellar cortex and the planning or correcting signals are issued through a set of deep cerebellar nuclei. Because its output is concerned with coordination of movement and not with perception, cerebellar lesions cause incoordination but no sensory changes.
The Cerebellum Can Be Divided into Transverse and Longitudinal Zones
In a gross anatomical sense, the primary fissure divides the bulk of the cerebellum into anterior and posterior lobes, and another deep fissure separates the flocculus and nodulus (together forming the flocculonodular lobe) from the body of the cerebellum (Fig. 20-1). Assorted exotic names are sometimes applied to various parts of the anterior and posterior lobes, but most are of limited clinical utility. One worth remembering is the tonsil, the part of the posterior lobe nearest to the flocculus (THB6 Figure 20-2D, p. 496). The tonsil is one of the most inferior parts of the cerebellum, and expanding masses in the posterior fossa can cause it to herniate through the foramen magnum, compressing the medulla (THB6 Figure 4-19D, p. 96).
Deep Nuclei Are Embedded in the Cerebellar White Matter
The fundamental building plan of the cerebellum as a whole involves afferents that reach the cerebellar cortex, which in turn projects to deep nuclei embedded in the cerebellar white matter. The deep nuclei then give rise to the output of the cerebellum. There are a series of three deep nuclei on each side, arranged in a medial to lateral array: the fastigial (most medial), interposed, and dentate (most lateral) nuclei (THB6 Figure 20-7, p. 501).
Three Peduncles Convey the Input and Output of Each Half of the Cerebellum
Three peduncles containing the cerebellar afferents and efferents attach the cerebellum to the brainstem (Fig. 20-2). The superior cerebellar peduncle is the major output route from its side of the cerebellum, carrying all the efferents from the dentate and interposed nuclei and some of the efferents from the fastigial nucleus. The middle cerebellar peduncle is the input route for information from the cerebral cortex, carrying the fibers from contralateral pontine nuclei. By elimination then, the inferior cerebellar peduncle is a complex bundle, carrying most of the remaining cerebellar afferents (including climbing fibers, as described a little later), as well as the remaining cerebellar efferents.
