Cerebellum

Chapter 20 Cerebellum



Chapter Outline








Cerebellum means “little brain,” and in a real sense, it is: it accounts for only about 10% of the mass of the brain, but the cerebellum contains as many neurons as all the rest of the central nervous system (CNS) combined. This semidetached mass of neural tissue covers most of the posterior surface of the brainstem, tethered there by three pairs of fiber bundles called cerebellar peduncles. Sensory inputs of virtually every description find their way to the uniquely structured cortex of the cerebellum, which in turn projects (via a set of deep cerebellar nuclei) to various sites in the brainstem and thalamus. Although the cerebellum is extensively concerned with the processing of sensory information, and although it has few ways to influence motor neurons directly, it is considered part of the motor system because cerebellar damage results in abnormalities of equilibrium, postural control, and coordination of voluntary movements.



The Cerebellum Can Be Divided into Both Transverse and Longitudinal Zones


The outside of the cerebellum has a banded appearance, as though its surface were folded like an accordion (Figs. 20-1 and 20-2). This folding is a successful device for increasing the cerebellar surface area; by some estimates, if the cortex could be unfolded into a flat sheet, it would be more than 1 meter long (Fig. 20-3). Deep fissures, most easily seen in sagittal sections (Fig. 20-2E), indent the cerebellar surface. Smaller creases indent the walls of these deep fissures, with the result that the entire cerebellar surface is made up of cortical ridges called folia,*most of which are transversely oriented; prominent fissures are the basis of common systems of dividing the cerebellum into lobes and lobules. Beneath the cortex is a mass of white matter, the medullary center of the cerebellum, which is composed of fibers going to or coming from the cerebellar cortex.



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Figure 20-2 Gross anatomy and blood supply of the cerebellum, seen from above (A), behind (B), below (C), in front (D), and hemisected (E). The primary fissure, marking the division between the anterior and posterior lobes, is indicated by blue arrows. The horizontal fissure, which prominently subdivides the posterior lobe, is indicated by green arrows. In the hemisected view (E) the depth of many of the cerebellar fissures is apparent; these are used to divide the vermis into a number of lobules, commonly referred to by roman numerals. The same fissures continue laterally and divide up the cerebellar hemispheres (see also Fig. 20-4 and Table 20-1). Vermal lobules I and II are thin plates of cerebellar tissue (the lingula) that cover the roof of the rostral fourth ventricle; vermal lobule X is the nodulus. AICA, anterior inferior cerebellar artery; F, flocculus; I, M, and S, inferior, middle, and superior cerebellar peduncles; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; T, tonsil; V, vermis.


(Modified from Nolte J, Angevine JB Jr: The human brain in photographs and diagrams, ed 3, St. Louis, 2007, Mosby.)





Functional Connections Divide the Cerebellum into Longitudinal Zones


The cerebellum can also be divided into longitudinal zones, perpendicular to the fissures, which cut across the anterior, posterior, and flocculonodular lobes (see Fig. 20-4). The most medial zone, straddling the midline, is the vermis (Latin for “worm”). On either side of the vermis is a large cerebellar hemisphere. Each hemisphere is subdivided into a medial hemisphere, a longitudinal strip adjacent to the vermis (sometimes called the intermediate or paravermal zone), and a larger lateral hemisphere. The vermis is fairly clearly set off from the hemispheres on the inferior surface of the cerebellum* (Fig. 20-2C), but other longitudinal lines of separation are not as obvious from the outside (Fig. 20-2A). The demarcation into longitudinal zones is based primarily on patterns of connections and on the functional differences that result, as described shortly; cerebellar cortex has the same structure everywhere and is smoothly continuous from one hemisphere across the midline to the other.



The fissures that carve the cerebellum into lobules and folia are continuous across the midline during development, so each transverse wedge of cerebellum has a vermal portion and a more lateral portion. For example, the nodulus is the vermal portion of the flocculonodular lobe and continues laterally into the flocculus. The tonsils are the hemispheral portions just across the posterolateral fissure from the flocculi; appropriately enough, their vermal continuation is the uvula. An assortment of exotic names is applied to the lobules and the vermal areas of the corpus cerebelli (Table 20-1; Fig. 20-4), and a roman numeral system is commonly used as well; for the most part, however, these names and numbers are of limited utility in clinical settings. (One exception is the tonsil. Because this is the part of the cerebellum adjacent to the foramen magnum, expanding masses in the posterior fossa can cause tonsillar herniation and compression of the medulla [see Fig. 4-19D].)




Three Peduncles Convey the Inputs and Outputs of Each Half of the Cerebellum


The cerebellum is attached to the brainstem by three substantial peduncles on each side (Figs. 20-5 and 20-6). The inferior cerebellar peduncle, composed mainly of afferents to the cerebellum from the spinal cord and brainstem, has two parts. Most of it is the restiform (“ropelike”) body, which ascends through the rostral medulla, growing larger as it accumulates cerebellar afferents. At the pontomedullary junction it turns dorsally toward the cerebellum, and additional fibers travel over its medial surface as the juxtarestiform body (see Fig. 15-6) interconnecting the cerebellum and vestibular nuclei. The middle cerebellar peduncle (or brachium pontis—the “arm of the pons”), the largest of the three, emerges laterally from the basal pons. It is composed virtually exclusively of afferents to the cerebellum from the pontine nuclei of the contralateral side. The superior cerebellar peduncle (or brachium conjunctivum*—the “joined-together arm,” named for its decussation; see Figs. 11-11, 11-13, 11-14, 11-16, 11-17, and 20-22) contains the major efferent pathways from the cerebellum to the red nucleus and thalamus.







All Parts of the Cerebellum Share Common Organizational Principles


The cerebellum as a whole has a straightforward organization (Fig. 20-8): inputs arrive at the cerebellar cortex, the cortex works its magic and projects to the deep nuclei, and the deep nuclei provide the cerebellar output. Cerebellar cortex has a systematic, strikingly regular structure (see Figs. 20-9 and 20-10). The uniformity of this organization suggests that all regions of the cerebellum perform the same fundamental operation and that distinctive inputs and outputs of different regions allow the cerebellum to be involved in multiple neural functions.






Inputs Reach the Cerebellar Cortex as Mossy and Climbing Fibers


The cortex of the cerebellum has a uniform and fairly simple three-layered structure (Fig. 20-9). The most superficial layer is the molecular layer, consisting mainly of the axons and dendrites of various cerebellar neurons. Deep to the molecular layer is a single layer of large neurons called Purkinje cells. Finally, adjacent to the medullary center is the granular layer, composed mainly of small granule cells arranged in a stratum many cells thick. The molecular and granular layers also contain characteristic types of interneurons (seeFigs. 20-10 and 20-14B), but the fundamental circuitry of the cerebellar cortex can be described in terms of Purkinje cells, granule cells, and the afferents to the cortex (see Fig. 20-14A).


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Figure 20-14 Connections of neurons in cerebellar cortex. Inhibitory connections are shown in red, excitatory in green. A, Schematic diagram of the principal circuit through the cerebellar cortex. Climbing fibers (cf) reach Purkinje cells (P) directly, and information from mossy fibers (mf) reaches them indirectly by way of granule cells (gc) and parallel fibers (pf). B, Additional interconnections in cerebellar cortex. One striking thing about the cerebellar cortex is the large amount of inhibition used in processing there: the mossy fiber–granule cell inputs and the climbing fiber inputs are excitatory, but everything else is inhibitory. Golgi cells (Go) make inhibitory feedback connections onto granule cells. Basket cells (B) and stellate cells (S) make inhibitory synapses on the cell bodies and dendrites, respectively, of Purkinje cells. Finally, synapses of Purkinje cells are all inhibitory. C, Schematic diagram of the general interconnections of cerebellar cortex and deep cerebellar nuclei. Climbing fibers and many mossy fibers send collaterals to cells of deep nuclei (N) before continuing to cerebellar cortex, where climbing fibers end directly on Purkinje cells and mossy fibers influence Purkinje cells indirectly through the granule cellrarr;parallel fiber pathway. Purkinje cells, in turn, end on cells of deep nuclei. The deep nuclei contain two populations of neurons. One sends mossy fibers to cerebellar cortex as well as massive numbers of fibers to extracerebellar sites in the brainstem and thalamus. The other is a collection of small neurons that send inhibitory (GABA) projections to the contralateral inferior olivary nucleus.


(Modified from Thach WT: Brain Res 40:89, 1972.)


Purkinje cells are the only neurons whose axons leave the cerebellar cortex. They are, in addition, among the most anatomically distinctive neurons found in the nervous system. Each Purkinje cell has an intricate, extensive dendritic tree (see Fig. 1-4A) that is flattened out in a plane perpendicular to the long axis of the folium in which it resides (Fig. 20-10). Each granule cell (Fig. 20-11) sends its axon into the molecular layer, where it bifurcates to form a fine, unmyelinated parallel fiber (Fig. 20-12) that extends for about 5 mm along the long axis of the folium. In its course, each parallel fiber passes through and synapses on the dendritic trees of a succession of Purkinje cells (as many as 500 of them). Each of us is estimated to have an incredible 1011 granule cells—half the neurons in the CNS—and each of our 25 million Purkinje cells receives synapses from perhaps 105 of them.




The cerebellum receives its share of diffuse modulatory inputs from places such as the locus ceruleus and the raphe nuclei (see Figs. 11-24 and 11-27). There are also two distinctive sets of afferent fibers to the cerebellar cortex: climbing fibers and mossy fibers. A single climbing fiber ends directly on each Purkinje cell,winding around its dendrites like ivy climbing a trellis (Fig. 20-13); in the process, it makes tens of thousands of excitatory synapses, and collectively, these form the most powerful excitatory input in the nervous system. All these climbing fibers arise in the contralateral inferior olivary nucleus. All the rest of the afferents to the cerebellar cortex are mossy fibers. Mossy fibers end on the dendrites of granule cells, so this is a less direct route to the Purkinje cells (mossy fiber→granule cell→parallel fiber→aPurkinje cell).




Purkinje Cells of the Cerebellar Cortex Project to the Deep Nuclei


Although Purkinje cell axons are the only route out of the cerebellar cortex, few of them leave the cerebellum itself. Rather, they project to the deep nuclei, which in turn give rise to the cerebellar output (Fig. 20-8). However, the deep nuclei are not just simple relay stations (Fig. 20-14C). For example, climbing fibers and many mossy fibers send collateral branches to the deep nuclei. It has been suggested that these inputs provide a tonic excitatory drive to neurons of the deep nuclei, and that inhibitory projections from Purkinje cells then modulate the firing rates of these neurons. In addition to giving rise to axons that leave the cerebellum, the deep nuclei project back to the same areas of cerebellar cortex from which they receive Purkinje axons. The functional implications of these additional connections are not fully understood, but they make it less surprising that the consequences of cerebellar damage are much more severe and long lasting when the deep nuclei are included in the lesion.



One Side of the Cerebellum Affects the Ipsilateral Side of the Body


There are many crossings of the midline in the various circuits interconnecting the cerebellum and other parts of the CNS, ultimately forming the basis for the fact that one cerebral hemisphere controls skeletal muscle of the contralateral limbs, but one half of the cerebellum influences movements of the ipsilateral limbs (see Figs. 3-33 and 20-23). For example, pontine nuclei receive inputs from the ipsilateral cerebral cortex and project to the contralateral half of the cerebellum (see Fig. 20-16), and one half of the cerebellum projects to the contralateral thalamus (see Fig. 20-21).


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Figure 20-23 The principal output circuits through which the lateral and medial hemispheres of the cerebellum influence movement. Excitatory connections are indicated in green, inhibitory connections in red. A, The lateral hemispheres receive input via the pontine nuclei from widespread areas of cerebral cortex (not shown); then, via the dentate nucleus, superior cerebellar peduncle (SCP), and ventral lateral and ventral anterior (VL/VA) nuclei, they influence the output of motor and premotor cortex. Note that this cerebellar output crosses the midline in the decussation of the superior cerebellar peduncles (DSCP) and that the output from motor and premotor cortex recrosses the midline in the pyramidal decussation (PD). The result is that one lateral cerebellar hemisphere affects the ipsilateral side of the spinal cord and brainstem. (The projection from the dentate nucleus to the parvocellular red nucleus [RNp] and from there to the inferior olive [IO] is also indicated, although its role in influencing movement is not known.) B, Output connections of the medial hemisphere are similar in many respects to lateral hemisphere circuitry, but in this case the limb areas of primary motor cortex are the main targets; premotor cortex is less involved. However, the magnocellular red nucleus (RNm) and rubrospinal tract (RST) also have a minor role. Here again, there are compensating decussations: cerebellar output fibers cross in the decussation of the superior cerebellar peduncles, and rubrospinal fibers cross on their way to the spinal cord. The few fastigial fibers that reach the VL/VA nuclei behave in a similar manner, but their information is relayed to the trunk area of motor cortex.

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Jul 31, 2016 | Posted by in NEUROLOGY | Comments Off on Cerebellum
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