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Overview: Functions of the Cerebellum

Figure 5.1 Cerebellum

The cerebellum (“little brain”) is a structure that is located at the back of the brain, underlying the occipital and temporal lobes of the cerebral cortex (Figure 5.1). Although the cerebellum accounts for approximately 10% of the brain’s volume, it contains over 50% of the total number of neurons in the brain. Historically, the cerebellum has been considered a motor structure, because cerebellar damage leads to impairments in motor control and posture and because the majority of the cerebellum’s outputs are to parts of the motor system. Motor commands are not initiated in the cerebellum; rather, the cerebellum modifies the motor commands of the descending pathways to make movements more adaptive and accurate. The cerebellum is involved in the following functions:

Maintenance of balance and posture. The cerebellum is important for making postural adjustments in order to maintain balance. Through its input from vestibular receptors and proprioceptors, it modulates commands to motor neurons to compensate for shifts in body position or changes in load upon muscles. Patients with cerebellar damage suffer from balance disorders, and they often develop stereotyped postural strategies to compensate for this problem (e.g., a wide-based stance).

Coordination of voluntary movements. Most movements are composed of a number of different muscle groups acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the timing and force of these different muscle groups to produce fluid limb or body movements.

Motor learning. The cerebellum is important for motor learning. The cerebellum plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process (e.g., learning to hit a baseball).

Cognitive functions. Although the cerebellum is most understood in terms of its contributions to motor control, it is also involved in certain cognitive functions, such as language. Thus, like the basal ganglia, the cerebellum is historically considered as part of the motor system, but its functions extend beyond motor control in ways that are not yet well understood.

Cerebellar Gross Anatomy

The cerebellum consists of two major parts (Figure 5.2A). The cerebellar deep nuclei (or cerebellar nuclei) are the sole output structures of the cerebellum. These nuclei are encased by a highly convoluted sheet of tissue called the cerebellar cortex, which contains almost all of the neurons in the cerebellum. A cross-section through the cerebellum reveals the intricate pattern of folds and fissures that characterize the cerebellar cortex (Figure 5.3). Like the cerebral cortex, cerebellar gyri are reproducible across individuals and have been identified and named. We will only be concerned with some of the larger divisions of the cerebellar cortex.

	Figure 5.2 (A) Cerebellar deep nuclei and cerebellar cortex in an idealized brain section. (B) External morphology of the cerebellum.
	Figure 5.3 Midsagittal cross-section of cerebellum showing the three primary lobes of the cerebellum.

Divisions of the cerebellum. Two major fissures running mediolaterally divide the cerebellar cortex into three primary subdivisions (Figure 5.2B and Figure 5.3). The posterolateral fissure separates the flocculonodular lobe from the corpus cerebelli, and the primary fissure separates the corpus cerebelli into a posterior lobe and an anterior lobe (Figure 5.4). The cerebellum is also divided sagittally into three zones that run from medial to lateral (Fig. 5.4). The vermis (from the Latin word for worm) is located along the midsagittal plane of the cerebellum. Directly lateral to the vermis is the intermediate zone. Finally, the lateral hemispheres are located lateral to the intermediate zone (there are no clear morphological borders between the intermediate zone and the lateral hemisphere that are visible from a gross specimen).

Figure 5.4 Divisions of cerebellum. Click PLAY to see schematic “unfolding” of cerebellum.

Cerebellar nuclei. All outputs from the cerebellum originate from the cerebellar deep nuclei. Thus, a lesion to the cerebellar nuclei has the same effect as a complete lesion of the entire cerebellum. It is important to know the inputs, outputs, and anatomical relationships between the different cerebellar nuclei and the subdivisions of the cerebellum (Figure 5.5).

Figure 5.5 Input and output pathways of the cerebellum. Click on the names of each cerebellum functional subdivision (cerebrocerebellum, spinocerebellum, and vestibulocerebellum) to view each pathway in isolation. The cerebellar deep nuclei are the sole outputs of the cerebellum.

1. The fastigial nucleus is the most medially located of the cerebellar nuclei. It receives input from the vermis and from cerebellar afferents that carry vestibular, proximal somatosensory, auditory, and visual information. It projects to the vestibular nuclei and the reticular formation.
2. The interposed nuclei comprise the emboliform nucleus and the globose nucleus. They are situated lateral to the fastigial nucleus. They receive input from the intermediate zone and from cerebellar afferents that carry spinal, proximal somatosensory, auditory, and visual information. They project to the contralateral red nucleus (the origin of the rubrospinal tract).
   
3. The dentate nucleus is the largest of the cerebellar nuclei, located lateral to the interposed nuclei. It receives input from the lateral hemisphere and from cerebellar afferents that carry information from the cerebral cortex (via the pontine nuclei). It projects to the contralateral red nucleus and the ventrolateral (VL) thalamic nucleus.
   
4. The vestibular nuclei are located outside the cerebellum, in the medulla. Hence, they are not strictly cerebellar nuclei, but they are considered to be functionally equivalent to the cerebellar nuclei because their connectivity patterns are identical to the cerebellar nuclei. The vestibular nuclei receive input from the flocculonodular lobe and from the vestibular labyrinth. They project to various motor nuclei and originate the vestibulospinal tracts.

In addition to these inputs, all cerebellar nuclei and all regions of cerebellum get special inputs from the inferior olive of the medulla (discussed below).

It is convenient to remember that the anatomical locations of the cerebellar nuclei correspond to the cerebellar cortex regions from which they receive input. Thus, the medially located fastigial nucleus receives input from the medially located vermis; the slightly lateral interposed nuclei receive input from the slightly lateral intermediate zone; and the most lateral dentate nucleus receives input from the lateral hemispheres.

Cerebellar peduncles. Three fiber bundles carry the input and output of the cerebellum.

1. The inferior cerebellar peduncle (also called the restiform body) primarily contains afferent fibers from the medulla, as well as efferents to the vestibular nuclei.
   
2. The middle cerebellar peduncle (also called the brachium pontis) primarily contains afferents from the pontine nuclei.
   
3. The superior cerebellar peduncle (also called the brachium conjunctivum) primarily contains efferent fibers from the cerebellar nuclei, as well as some afferents from the spinocerebellar tract.
   

Thus, the inputs to the cerebellum are conveyed primarily through the inferior and middle cerebellar peduncles, whereas the outputs are conveyed primarily through the superior cerebellar peduncle. The inputs arise from the ipsilateral side of the body, and the outputs also go to the ipsilateral side of the body. Note that this is true even for the outputs to the contralateral red nucleus. Recall from the chapter on descending motor pathways that the rubrospinal tract immediately crosses the midline after the fibers leave the red nucleus. Thus, cerebellar output to the red nucleus affects the ipsilateral side of the body by a double-crossed pathway. Unlike the cerebral cortex, the cerebellum receives input from, and controls output to, the ipsilateral side of the body, and damage to the cerebellum therefore results in deficits to the ipsilateral side of the body.