The cerebellum is best known as a motor part of the brain, serving to maintain equilibrium and coordinate muscle contractions. The cerebellum makes a special contribution to synergy of muscle action (i.e., to the synchronized contractions and relaxations of different muscles that make up a useful movement). The cerebellum ensures that contraction of the proper muscles occurs at the appropriate time, each with the correct force. There is reason to believe that the cerebellum participates in learning patterns of neuronal activity needed for carrying out movements and in the execution of the encoded instructions.
Despite their complexity, the activities of the cerebellum have long been thought to occur without conscious awareness because cerebellar diseases cause disturbed motor function without voluntary paralysis. This traditional viewpoint may not be entirely correct: imagined movements are accompanied by an increase in cerebellar blood flow that is larger than the increase detected in the motor areas of the cerebral cortex. Evidence also suggests
that the cerebellum has sensory and cognitive functions.
The cerebellum consists of a cortex, or surface layer, of gray matter contained in transverse folds or folia plus a central body of white matter. Four pairs of central nuclei are embedded in the cerebellar white matter. Three pairs of cerebellar peduncles, composed of myelinated axons, connect the cerebellum with the brain stem.
The superior cerebellar surface conforms to the dural reflection or tentorium, which forms a roof for the posterior cranial fossa. The inferior surface is deeply grooved in the midline; the remainder of this surface is convex on each side and rests on the floor of the posterior cranial fossa (Fig. 10-1).
FIGURE 10-1 The cerebellum. (A) Superior surface. (B) Inferior surface.
Certain terms are useful to identify regions of the cerebellar surface. The region in and near the midline is known as the vermis, and the remainder is known as the hemispheres.The superior vermis blends into the hemispheres, but the inferior vermis lies in a deep depression (the vallecula) and is well delineated. The paravermal zone is the medial parts of the hemispheres for 1 to 2 cm on either side of the vermis.
Three major regions or lobes are recognized in the horizontal plane (see Fig. 10-1). The flocculonodular lobe (or lobule) is a small component that lies at the rostral edge of the inferior surface. If the cerebellum were unrolled, this would be its most caudal part. The
nodule is the end portion of the inferior vermis, and the flocculi are irregularly shaped masses on each side. Several transverse fissures indent the cerebellum. The dorsolateral (or posterolateral) fissure is the first of these to appear during embryonic development; it demarcates the flocculonodular lobe. The main mass of the cerebellum (all but the flocculonodular lobe) consists of anterior and posterior lobes. The anterior lobe is the part of the superior surface rostral to the primary fissure. The remainder of the cerebellum on both surfaces constitutes the large posterior lobe.
The roof of the rostral part of the fourth ventricle is formed by the superior cerebellar peduncles and by the superior medullary velum that bridges the interval between them (Fig. 10-2; see also Fig. 7-10). The remainder of the roof consists of the thin inferior medullary velum, formed by pia mater and ependyma. This membrane (see Fig. 6-4) commonly adheres to the inferior vermis. The three pairs of peduncles are attached to the cerebellum in the interval between the flocculonodular and anterior lobes.
Other fissures outline further subdivisions or lobules, especially in the posterior lobe. Figure 10-3 is provided for reference if smaller subdivisions of the cerebellum need to be identified. The position of the tonsils is clinically significant because these parts of the cerebellar hemispheres are close to the medulla and can compress this vital part of the brain stem if the contents of the posterior fossa of the skull are displaced downward into the foramen magnum. The tonsil is also an angiographic landmark, associated with a characteristic curve in the course of the posterior inferior cerebellar artery.
FIGURE 10-2 The cerebellum viewed from in front and below, showing the cut surfaces of the cerebellar peduncles.
The cerebellar surface is folded into many narrow folia, with 85% of the cortical surface concealed in the intervening sulci. The cortical area is about three-quarters the size of the cerebral cortex.
Three layers are seen in sections (Fig. 10-4). The Purkinje cell layer consists of a single row of bodies of Purkinje cells, the large principal cells of the cerebellar cortex. Superficial to these is the molecular layer, which is a synaptic zone, containing the dendrites of the Purkinje cells, which branch profusely in a plane perpendicular to the long axis of the folium. The granule cell
layer, deep to the layer of Purkinje cells, contains closely packed interneurons with axons that extend into the molecular layer. Other cerebellar interneurons (Fig. 10-5) have their cell bodies in the molecular and granular layers.
FIGURE 10-3 Anatomical names of parts of the cerebellum. (The lingula, not seen in these figures, is a small, flattened portion of the superior vermis beneath the central lobule and adherent to the superior medullary velum; see Fig. 10-2.)
FIGURE 10-4 Transverse section of cerebellar folia showing the three layers of the cortex and the underlying white matter (stained with cresyl violet).
Of the afferent fibers to the cortex, climbing fibers originate in the inferior olivary complex of nuclei and synapse with the proximal parts of the dendritic trees of Purkinje cells. Cerebellar afferents from other sources end as mossy fibers, each synapsing with the neurons in the granular layer in a formation known as a glomerulus (Fig. 10-6). The axons of the granule cells have branches known as parallel fibers that run in the long axis of the folium in the molecular layer. Whereas each Purkinje cell is contacted by a single climbing fiber, parallel fibers are much more numerous, with each one contacting many Purkinje cells. (Noradrenergic and serotonergic projections to the cerebellum from the brain stem are also present; these are mentioned in Chapter 9 but are not discussed here.) The only axons that leave the cortex are those of the Purkinje cells. These terminate in central nuclei of the cerebellum, with the exception of some fibers from the cortex of the flocculonodular lobe that proceed to the brain stem.
The cerebellar cortex was one of the first regions of the brain to be thoroughly studied with microelectrodes to determine whether synapses between specific types of neurons produced excitatory (EPSP) or inhibitory (IPSP) postsynaptic potentials. The observations have since been supplemented by immunohistochemical and pharmacological studies of neurotransmitters and their receptors.
FIGURE 10-5 Neurons in the cerebellar cortex, showing excitatory and inhibitory synapses. The diagram represents a longitudinally sectioned folium, with an edge-on view of the dendritic tree of the Purkinje cell. Glutamatergic (excitatory) neurons are red; GABA-ergic (inhibitory) neurons are blue.
The axons afferent to the cerebellum all make excitatory connections. Before reaching the cortex, all afferent axons give off collateral branches that contact the neurons in the cerebellar nuclei. The granule cells also make excitatory synapses with the Purkinje cells. The excitatory transmitter is glutamate. All the other cerebellar neurons make inhibitory synapses, with gamma-aminobutyric acid (GABA) as the transmitter. The excitatory input to the cortex is thereby modified by intracortical circuits that inhibit Purkinje cells and suppress transmission from the cortex to central nuclei. The granule cells are the most numerous cerebellar interneurons; others are the Golgi cells and basket cells shown in Figure 10-5. For example, activation of parallel fibers elicits EPSPs in basket cells, but synapses between basket cells and Purkinje cells cause IPSPs. Parallel fibers also excite Golgi cells, which inhibit granule cells. Whereas each parallel fiber contacts the dendrites of many Purkinje cells along the
length of a folium, the axon of each basket cell contacts several Purkinje cells across the width of a folium (see Figs. 10-5 and 10-7). Inhibitory circuits, which include more synapses than do the excitatory relays, serve to limit the area of cortex excited and the degree of excitation resulting from a volley of impulses delivered by a mossy fiber.
FIGURE 10-6 Ultrastructure of a synaptic glomerulus in the granule cell layer. The astrocyte processes (yellow) prevent diffusion of neurotransmitters to adjacent synapses.
Four pairs of nuclei are embedded deep in the cerebellar white matter; in a medial to lateral direction, they are the fastigial, globose, emboliform, and dentate nuclei (Fig. 10-8).
The fastigial nucleus is close to the midline, almost in contact with the roof of the fourth ventricle. The interposed nucleus (comprising two cell clusters, the globose and the emboliform nuclei) is situated between the fastigial and dentate nuclei. The prominent dentate nucleus has the irregular shape of a crumpled purse, similar to that of the inferior olivary nucleus, with the hilus facing medially. Its efferent fibers occupy the interior of the nucleus and leave through the hilus.
The input to the cerebellar nuclei is from (a) sources outside the cerebellum and (b) the Purkinje cells of the cortex. The extrinsic input consists of pontocerebellar, spinocerebellar, and olivocerebellar fibers, together with fibers from the precerebellar reticular nuclei. Most of these afferents are collateral branches of fibers proceeding to the cerebellar cortex. A few rubrocerebellar fibers end in the interposed nucleus, and the fastigial nucleus receives afferents from the vestibular nerve and nuclei. Whereas the fastigial nucleus projects to the brain stem through the inferior cerebellar peduncle, efferents from the other nuclei leave the cerebellum through the superior peduncle and end in the brain stem and thalamus.
FIGURE 10-7 Cell body of a Purkinje cell situated between the molecular layer (above) and the granule cell layer of the cerebellar cortex. Most of the fibers surrounding the Purkinje cell are preterminal branches of basket cell axons. (Stained by one of Cajal's silver nitrate methods.)
Whereas the input to the central nuclei from outside the cerebellum is excitatory, the input from Purkinje cells, which use GABA as their transmitter, is inhibitory. Crudely processed information in the central nuclei is refined by the inhibitory signals received from the cortex. The combination of the two inputs maintains a tonic discharge from the central nuclei to
the brain stem and thalamus. This discharge changes constantly according to the afferent input to the cerebellum at any given time.
FIGURE 10-8 Central nuclei of the cerebellum, as seen in a transverse section that also passes through the open part of the medulla. ICP, inferior cerebellar peduncle; ML, medial lemniscus; MLF, medial longitudinal fasciculus; PY, pyramid.
The white matter in the region of the vermis produces a branching treelike pattern (the arbor vitae cerebelli) in a sagittal section (Fig. 10-9). Each hemisphere contains a large body of white matter in which the dentate nucleus is embedded (Fig. 10-10). The white matter consists of afferent and efferent fibers of the cortex and nuclei. The afferent and efferent systems are discussed in connection with the functional divisions of the cerebellum. They are identified at this point only as components of the cerebellar peduncles.
The inferior cerebellar peduncle consists mainly of fibers entering the cerebellum, with the largest contingent being from the contralateral inferior olivary complex of nuclei. The other components are the dorsal spinocerebellar tract and fibers from the vestibular nerve and nuclei and from various other nuclei of the medulla (Table 10-1). Efferent fibers in the inferior cerebellar peduncle proceed from the flocculonodular lobe and fastigial nucleus to the vestibular nuclei and to reticular formation of the medulla and pons.
The middle cerebellar peduncle consists of pontocerebellar fibers that originate in the contralateral pontine nuclei.
The superior cerebellar peduncle consists mainly of efferent fibers from the interposed and dentate nuclei. These axons end in the thalamus. Smaller contingents of fibers in the superior peduncle are summarized in Table 10-1.
Three divisions of the cerebellum are recognized on the basis of comparative anatomy. These are the archicerebellum, which is the only component of the cerebellum in fishes and in lower amphibians; the paleocerebellum, which is present in higher amphibians and is larger in reptiles and birds; and the neocerebellum, which is found only in mammals and is largest in humans. These phylogenetic divisions of the cerebellum (Fig. 10-11) correspond in large part with functional divisions (Fig. 10-12), based on the major
sources of afferent mossy fibers. (Olivocerebellar climbing fibers are distributed to all parts of the cortex.)
FIGURE 10-9 Midline structures of the brain stem and cerebellum, showing the arbor vitae cerebelli in the vermis. The cut surface of the specimen has been stained by a method that differentiates gray matter (dark) from white matter (light).
FIGURE 10-10 Section cut in a sagittal plane through a cerebellar hemisphere, stained to differentiate gray matter (dark) from white matter (light). The dentate nucleus is shown, embedded in the white matter of the hemisphere.
The functional divisions are as follows. The vestibulocerebellum is the flocculonodular lobe and receives input from the vestibular nerve
and nuclei. The spinocerebellum consists of the vermis of the anterior lobe together with the adjacent medial or paravermal zones of the hemispheres; the spinocerebellar tracts and cuneocerebellar fibers, which convey proprioceptive and other sensory information, terminate here. The pontocerebellum comprises the large lateral parts of the hemispheres and the superior vermis
in the posterior lobe; afferents are from the contralateral pontine nuclei. There is some overlapping of the divisions; for example, both spinocerebellar and pontocerebellar fibers terminate in the cortex of the paravermal zones.
TABLE 10-1 Composition of the Cerebellar Peduncles
FIGURE 10-11 Phylogenetic regions of the cerebellum. (A) Superior surface. (B) Inferior surface.
FIGURE 10-12 Functional regions of the cerebellum. (A) Superior surface. (B) Inferior surface
The vestibulocerebellum receives afferent fibers from the vestibular ganglion and from the vestibular nuclei of the same side (Fig. 10-13). Some of the afferent fibers from these sources terminate in the fastigial nucleus, which also receives collateral branches of the axons destined for the cortex of the vestibulocerebellum. The vestibulocerebellum also receives afferents from the contralateral accessory olivary nuclei. These fibers have collateral branches to the fastigial nucleus and end as climbing fibers in the cortex of the flocculonodular lobe.
FIGURE 10-13 Connections of the vestibulocerebellum and vestibular nuclei. Afferents to the cerebellum are blue, cerebellar efferents are red, and other neurons are black.
Some Purkinje cell axons from the vestibulocerebellar cortex proceed to the brain stem (an exception to the general rule that such fibers end in central nuclei), but most terminate in the fastigial nucleus. Fibers from the cortex and the fastigial nucleus traverse the inferior cerebellar peduncle to their termination in the vestibular nuclear complex and in the central group of reticular nuclei (see Fig. 10-13).
In summary, the vestibulocerebellum influences motor neurons through the vestibulospinal tract, the medial longitudinal fasciculus, and reticulospinal fibers. It is concerned with adjustment of muscle tone in response to vestibular stimuli. It coordinates the actions of muscles that maintain equilibrium and participates in other motor responses, including those of the eyes, to vestibular stimulation (see Chapter 22). The posterior vermis also contributes to the cerebellar control of eye movements.
The following four afferent systems project to the spinocerebellar cortex.
FIGURE 10-14 Connections of the spinocerebellum. Afferents to the cerebellum are blue, cerebellar efferents are red, and other neurons are black.
and 9-2), which project to the cerebellum. These two precerebellar reticular nuclei also receive afferent fibers from primary motor and sensory areas of the cerebral cortex. Another precerebellar reticular nucleus that projects to the vermis and medial parts of the hemispheres is the reticulotegmental nucleus in the pons (see Fig. 9-1). This nucleus receives afferents from the cerebral cortex and from the vestibular nuclei (see Fig. 10-13).
Collateral branches of the axons from all the various afferent sources terminate in the interposed nuclei, which also receive a small contingent of fibers from the red nucleus.
Each half of the body is represented in the ipsilateral cerebellar cortex; if afferent fibers have crossed the midline from cells of origin at lower levels, they cross again in the white matter of the cerebellum. In monkeys and probably also in humans, the half-body is represented in two areas. One is upside down, in and alongside the vermis of the anterior lobe. The other is the right way up, in the medial part of the hemisphere on the inferior surface of the posterior lobe. The two “head areas” are in the vermis and adjacent cortex of the posterior lobe, and they are separated by an area that receives auditory and visual input from the tectum, both directly and by way of a tecto-ponto-cerebellar circuit. Somatotopic representation in the spinocerebellum is less clearly defined than in some areas of the cerebral cortex; there is overlap of different inputs, so that trains of impulses from various sources may reach the same Purkinje cell.
The spinocerebellar cortex projects to the fastigial nucleus (from the vermis) and to the interposed (globose and emboliform) nuclei (from the paravermal zones of the hemispheres). Synergy of muscle action and control of muscle tone are effected in part through fastigiobulbar connections, as described for the vestibulocerebellum. Axons from the interposed nuclei traverse the superior cerebellar peduncle and terminate in the central group of reticular nuclei. Thus, the spinocerebellum may influence motor neurons through reticulospinal fibers and a similar projection to motor nuclei of cranial nerves. Alpha and gamma motor neurons are involved in cerebellar control of muscle action, and the influence of the spinocerebellum on the skeletal musculature is ipsilateral.
Some axons from the interposed nuclei traverse the superior cerebellar peduncle and end in the red nucleus, which, in turn, projects to the inferior olivary nucleus. Others pass through or around the red nucleus and continue to the ventral lateral nucleus of the thalamus, which projects to the primary motor area of the cerebral cortex.
In summary, the spinocerebellum receives information from proprioceptive and exteroceptive sensory endings and, indirectly, from the cerebral cortex. Visual and auditory input to areas of the spino- and pontocerebellar cortex also takes place. These data are processed in the circuitry of the cerebellar cortex, which modifies and refines the discharge of signals from the central nuclei. Motor neurons are influenced mainly through relays in the vestibular nuclei, the reticular formation, and the primary motor area of the cerebral cortex. The end result is control of muscle tone and synergy of collaborating muscles, as appropriate at any moment for the adjustment of posture and in many types of movement, including those of locomotion.
Pontocerebellar fibers constitute the whole of the middle cerebellar peduncle. They originate in the pontine nuclei (nuclei pontis) of the opposite side. Pontocerebellar axons have branches that synapse with neurons in the dentate nucleus, and they are distributed throughout the cortex of the cerebellar hemispheres and the superior vermis of the posterior lobe. The corticopontine tracts originate in widespread areas of the contralateral cerebral cortex (especially that of the frontal and parietal lobes but also temporal and occipital) and end in the pontine nuclei. Through the corticopontine and pontocerebellar projections, the cortex of a cerebellar hemisphere receives information
concerning volitional movements that are anticipated or in progress. Some of the pontine nuclei receive afferents from the superior colliculus and relay data used by the cerebellum in the control of visually guided movements.
In addition to pontine afferents, the superior vermis of the posterior lobe, similar to the spinocerebellar cortex, receives tectocerebellar fibers from the superior and inferior colliculi. There are also olivary afferents, the axons of cells in the contralateral inferior olivary nucleus.
FIGURE 10-15 Connections of the pontocerebellum. Afferents to the cerebellum are blue, cerebellar efferents are red, and other neurons are black.
Purkinje cell axons from the pontocerebellar cortex terminate in the dentate nucleus, the efferent fibers of which compose most of the superior cerebellar peduncle. After traversing the decussation of the peduncles, some dentatothalamic fibers give off branches to the red nucleus, but the majority passes through or around the red
nucleus and end in the ventral lateral nucleus of the thalamus. In turn, this thalamic nucleus projects to the primary motor area of cerebral cortex in the frontal lobe. Through these connections, the pontocerebellum can modify activity in corticospinal, corticoreticular, and reticulospinal pathways (Fig. 10-15).
Pathological conditions are broadly classified into those that affect the vermis and flocculonodular lobe (the vestibulocerebellum and spinocerebellum) and those that affect the hemispheres (pontocerebellum).
The midline portions of the cerebellum may be the site of a tumor, typically, a malignant “medulloblastoma” that occurs in childhood. In adults, a similar syndrome may be seen in chronic alcoholism, which causes degeneration of the vermis. The patient has an unsteady, staggering ataxic gait, walks on a wide base, and sways from side to side. Cerebellar nystagmus is usually in the horizontal plane and is most pronounced when the eyes are looking to one side. It is attributed to interruption of connections of the vermis with the ocular motor nuclei by way of the vestibular nuclei and the reticular formation. At first, the signs are limited to a disturbance of equilibrium; additional cerebellar signs appear when a tumor invades other parts of the cerebellum.
With respect to the cerebellar hemispheres, signs of dysfunction accompany lesions that interrupt afferent pathways, cause destruction of the cortex and white matter, or involve the central nuclei or the efferent pathways in the superior cerebellar peduncle. The motor disorder is more severe and more enduring when a lesion involves the central nuclei or the superior cerebellar peduncle. When the lesion is unilateral, the signs of motor dysfunction are on the same side of the body.
The following signs, in varying degrees of severity, are those of a neocerebellar syndrome:
The deficits noted are superimposed on volitional movements that are otherwise intact.
A cerebellar cognitive affective syndrome can result from damage to the posterior but not the anterior lobe of the cerebellum. In addition to the motor changes of a neocerebellar syndrome, there are effects more usually attributable to destructive lesions in the cerebral cortex. These include uninhibited behavior and impairment of planning, reasoning, and verbal fluency, which are functions of the anterior part of the frontal lobe. Testing also reveals blunting of affect, poor visuospatial organization and memory, loss of the vocal cadence that normally puts feeling and expression into speech, and failure to connect words in a grammatically correct way. These disorders are otherwise seen in patients with lesions in various parts of the temporal and parietal lobes (see Chapters 15 and 18).
The output of the dentate nucleus, similar to that of the other cerebellar nuclei, fluctuates according to the excitatory input from extracerebellar sources and the refinement of discharge by the inhibitory action of Purkinje cells. Mainly through its influence on the cerebral motor cortex, the pontocerebellum ensures
a smooth and orderly sequence of muscle contractions and the intended precision in the force, direction, and extent of volitional movements. These functions are particularly important for the upper limbs. A cerebellar hemisphere influences the musculature of the same side of the body because of the compensating decussations of the superior cerebellar peduncles and of the descending motor pathways.
ADDITIONAL CEREBELLAR CONNECTIONS AND FUNCTIONS
The climbing fibers from the inferior olivary complex are believed to carry instructions relating to movements that have not yet been performed. The patterns or programs concerned are stored in the cerebellum, probably as structural or functional modifications of synapses. It has been suggested that activity of the climbing fibers excites the Purkinje cell dendrites but also lowers their sensitivity to excitatory input from the much more numerous parallel fibers. Protracted but reversible changes in synaptic efficiency constitute a proposed mechanism of memory. The execution and coordination of learned movements are mediated by the mossy fiber afferents, of which those from the pontine nuclei are the most numerous in primates. When a monkey makes an intended movement, the neurons in the dentate nucleus (which receives its excitatory afferents from the pontine nuclei) are active several milliseconds before those in the primary motor area (which receives signals from the cerebellum by way of the dentatothalamo-cortical projection).
The movements coordinated by the pontocerebellum are usually guided by input from the special senses, especially vision. The vermis receives visual and auditory input by way of tectocerebellar and tecto-ponto-cerebellar projections. Stimuli perceived by the eyes and ears can also influence the cerebellum through corticopontine fibers that originate in visual and auditory areas of the cerebral cortex.
Results of animal experiments have shown that the cerebellum also has a role in visceral functions. Under certain conditions, electrical stimulation of the spinocerebellar cortex produces respiratory, cardiovascular, pupillary, and urinary bladder responses. These responses are sympathetic in nature when the anterior lobe is stimulated and parasympathetic when the tonsils (see Fig. 10-3) of the posterior lobe are stimulated. The postulated pathway includes the interposed nuclei, reticular formation, and hypothalamus.
NONMOTOR FUNCTIONS OF THE CEREBELLUM
The human cerebellar hemispheres are large, and they receive afferents (by way of the pontine nuclei) from all the lobes of the cerebral cortex. This anatomy suggests involvement of the cerebellum in more activities of the brain than just the coordination of movements. Functional imaging techniques such as positron emission tomography (PET) and functional nuclear magnetic resonance imaging (fMRI) (see Chapter 4) reveal increased activity in the cerebellum in a variety of sensory and cognitive tasks in addition to the expected activation seen in specific areas of the cerebral cortex. For example, there is a four times greater increase of oxygen use in the cerbellar cortex, dentate nucleus, and red nucleus in response to passive touching of the skin (with no movement) than in response to moving the skin across a stationery surface. Heightened cerebellar activity is also seen in association with recognition of words and faces. These are cognitive functions of the parietal and temporal lobes.
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