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The trilobed structure of the brain, lying posterior to the pons and medulla oblongata and inferior to the occipital lobes of the cerebral hemispheres, that is responsible for the regulation and coordination of complex voluntary muscular movement as well as the maintenance of posture and balance.
[Medieval Latin, from Latin, diminutive of cerebrum, brain.]
cerebellar cer'e·bel'lar (-bĕl'ər) adj.Cerebellum
Definition
The cerebellum is a cauliflower-shaped brain structure located just above the brainstem, beneath the occipital lobes at the base of the skull.
Description
The word cerebellum comes from the Latin word for "little brain." The cerebellum has traditionally been recognized as the unit of motor control that regulates muscle tone and coordination of movement. There is an increasing number of reports that support the idea that the cerebellum also contributes to non-motor functions such as cognition (thought processes) and affective state (emotion).
The cerebellum comprises approximately 10% of the brain's volume and contains at least half of the brain's neurons. The cerebellum is made up of two hemispheres (halves) covered by a thin layer of gray matter known as the cortex. Beneath the cortex is a central core of white matter. Embedded in the white matter are several areas of gray matter known as the deep cerebellar nuclei (the fastigial nucleus, the globise-emboliform nucleus, and the dentate nucleus). The cerebellum is connected to the brainstem via three bundles of fibers called peduncles (the superior, middle, and inferior).
Anatomy
The cerebellum is a complex structure. At the basic level, it is divided into three distinct regions: the vermis, the paravermis (also called the intermediate zone), and the cerebellar hemispheres. Fissures, deep folds in the cortex that extend across the cerebellum, further subdivide these regions into 10 lobules, designated lobules I–X. Two of these fissures in particular, the posterolateral fissure and the primary fissure, separate the cerebellum into three lobes that have different functions: the flocculonodular lobe, or the vestibulocerebellum (lobule X); the anterior lobe (lobules I–V); and the posterior lobe (lobules VI–IX).
The cerebellum plays an important role in sending and receiving messages (nerve signals) necessary for the production of muscle movements and coordination. There are both afferent (input) and efferent (output) pathways. The major input pathways (also called tracts) include:
The major output pathways include the following:
There is a network of fibers (cells) within the cerebellum that monitors information to and from the brain and the spinal cord. This network of neural circuits links the input pathways to the output pathways. The Purkinje fibers and the deep nuclei play key roles in this communication process. The Purkinje fibers regulate the deep nuclei, which have axons that send messages out to other parts of the central nervous system.
Function
The flocculonodular lobe helps to maintain equilibrium (balance) and to control eye movements. The anterior lobe parts of the posterior lobe (the vermis and paravermis) form the spinocerebellum, a region that plays a role in control of proximal muscles, posture, and locomotion such as walking. The cerebellar hemispheres (part of the posterior lobe) are collectively known as the cerebrocerebellum (or the pontocerebellum); they receive signals from the cerebral cortex and aid in initiation, coordination, and timing of movements. The cerebrocerebellum is also thought to play a role in cognition and affective state.
The cerebellum has been reported to play a role in psychiatric conditions such as
Disorders
There are a variety of disorders that involve or affect the cerebellum. The cerebellum can be damaged by factors including:
Typical symptoms of cerebellar disorders include
Resources
BOOKS
Manto, Mario U., and Massimo Pandolfo, eds. The Cerebellum and its Disorders. Cambridge, England: Cambridge University Press, 2001.
De Zeeuw, C. I., P. Strata, and J. Voogd, eds. The Cerebellum: From Structure to Control. St Louis, MO: Elsevier Science, 1997.
PERIODICALS
Daum, I., B. E. Snitz, and H. Ackermann. "Neuropsychological Deficits in Cerebellar Syndromes." International Review of Psychiatry 13 (2001): 268–275.
Desmond, J. E. "Cerebellar Involvement in Cognitive Function: Evidence from Neuroimaging." International Review of Psychiatry 13 (2001): 283–294.
Leroi, I., E. O'Hearn, and R. Margolis. "Psychiatric Syndromes in Cerebellar Degeneration." International Review of Psychiatry 13 (2001): 323–329.
O'Hearn, E., and M. E. Molliver. "Organizational Principles and Microcircuitry of the Cerebellum." International Review of Psychiatry 13 (2001): 232–246.
Rapoport, M. "The Cerebellum in Psychiatric Disorders." >International Review of Psychiatry 13 (2001): 295–301.
Schmahmann, J. D. "The Cerebrocerebellar System: Anatomic Substrates of the Cerebellar Contribution to Cognition and Emotion." International Review of Psychiatry 13 (2001): 247–260.
Shill, H. A., and M. Hallett. "Cerebellar Diseases." International Review of Psychiatry 13 (2001): 261–267.
WEBSITES
"BrainInfo Web Site." Cerebellum Information Page. Neuroscience Division, Regional Primate Research Center, University of Washington, 2000. (May 22, 2004.) http://braininfo.rprc.washington.edu.
The Cerebellum Database Site. (May 22, 2004). http://www.cerebellum.org/8home/.
The National Institute of Neurological Disorders and Stroke (NINDS). Cerebellar Degeneration Information Page. PO Box 5801 Bethesda, MD, 2003. (May 22, 2004). http://www.ninds.nih.gov/health_and_medical/disorders/cerebellar_degeneration.htm.
The National Institute of Neurological Disorders and Stroke (NINDS). Cerebellar Hypoplasia Information Page. PO Box 5801 Bethesda, MD, 2003. (May 22, 2004). http://www.ninds.nih.gov/health_and_medical/disorders/cerebellar_hypoplasia.htm.
ORGANIZATIONS
National Institute of Mental Health. 6001 Executive Boulevard, Room 8184, MSC 9663, Bethesda, MD 20892-9663. (301) 443-4513 or (866) 615-6464; TTY: (301) 443-8431; Fax: (301) 443-4279. nimhinfo@nih.gov. http://www.nimh.nih.gov/.
National Institute of Neurological Disorders and Stroke (NINDS), NIH Neurological Institute. P.O. Box 5801, Bethesda, MD 20824. (301) 496-5751 or (800) 352-9424; TTY: (301) 468-5981. http://www.ninds.nih.gov/.
Cerebellum (‘little brain’): an intricately corrugated ball of nervous tissue that lies under the rear end of the cerebral hemispheres and is attached to the brain stem by huge bundles of nerve fibres, the cerebellar peduncles, which carry information to and from other parts of the brain.
The cerebellum makes up more than one-tenth of the volume of the human brain. The basic circuitry of nerves within it is essentially similar in all vertebrates and during evolution it has changed much less in size, relative to the body, than have the cerebral hemispheres. These facts suggest that it has some essential, basic function in all vertebrates. Although its exact mechanisms remain unclear, its fundamental role is in the control of movement. This was clearly recognized by the seventeenth-century physician Thomas Willis in his book Cerebri Anatome (1664) and the idea can be traced back to the observations and interpretations of Galen (c.130-210 ad).
The cerebellum comprises an outer, thin layer of grey matter — the cerebellar cortex — covering a core of white matter, within which lie three lumps of grey matter on each side of the midline (the deep cerebellar nuclei). Closest to the midline is the fastigial nucleus and furthest from it is the dentate nucleus, with the interpositus nucleus between.
The surface area of the cortex is greatly augmented by folds that run across from side to side — deep ones that divide the surface into ten lobules, and numerous shallower ones cutting each lobule into folia. If the cortex were flattened out, it would be a ribbon much longer than it is wide.
The cortex is divided up functionally into longitudinal (i.e. fore-and-aft) strips or zones, each interconnected with a particular deep nucleus. The vermis, running down the middle, connects with the right and left fastigial nuclei. This is flanked on each side by a paravermal cortical zone related to nucleus interpositus; and most lateral is the pair of large cerebellar hemispheres, linked to the dentate nuclei. Since the 1960s studies in animals have shown that each cortical zone comprises many narrower micro-zones, each relating to a particular ‘private’ portion of the corresponding deep nucleus.
The fine structure of the cortex and the circuits that link it with the deep nuclei vary little from place to place, which suggests that all parts of the cerebellum perform a similar basic ‘computation’ or operation. If different parts of the cerebellum have different functional roles, this must be due to differences in their input and output connections rather than their internal wiring.
Damage to part or even all of the human cerebellum, on its own, does not lead to clear impairment of intellect, emotion, or vegetative functions (such as the control of the heart and breathing). But there is abundant evidence that the control of movements is markedly disordered. Typically, patients with cerebellar damage are unsteady on their feet, and their hands shake as they try to point or lift objects (‘intention tremor’) ; their eyes swing uncontrollably from side to side (nystagmus) ; and even their speech can be jerky (‘scanning speech’). These three typical signs, described by the great nineteenth-century French neurologist Charcot, are known as ‘Charcot's Triad’.
The movements most affected vary somewhat depending on the location of the damage. No type of movement is completely lost, but movements ranging in complexity from simple reflex actions to walking, speech, and highly skilled manipulations may all be defective in rate, range, force, and timing. Extremely rarely, individuals are born with little or no cerebellum, and although some of its functions may be taken over by other parts of the brain, movements are permanently clumsy and poorly co-ordinated, suggesting that the learning of motor skills is impaired.
The 600 000 nerve cells in the deep nuclei send messages out of the cerebellum along their fibres (or axons), which run through the peduncles to a number of nuclei in the brain stem and thalamus. These in turn are connected to the spinal cord and to regions of the cerebral cortex concerned with the control of movement.
Studies of the activity of nerve cells in animals have been the main source of knowledge of how the cerebellum works. Even when movements are not being made, neurons of the deep nuclei are continuously active, producing impulses at rates of 30-50 per second. This continuous background firing arises because the huge number of excitatory nerve fibres that enter the cerebellum, carrying information to the cortex, send side branches into the deep nuclei. In addition, they receive the axons of the 15 million Purkinje cells, the largest cells in the cortex. These are all inhibitory, using gamma-amino-butyric acid (GABA) as their transmitter. So, the variation of firing of cells in the deep nuclei, which constitutes the output of the cerebellum and hence modulates movement, is dependent on the relationship between incoming activity and the resulting firing of Purkinje cells.
The incoming nerve fibres, which ultimately control the firing of Purkinje cells, are of two types. The first are the axons of cells in a nucleus in the medulla of the brain stem that glories in the name inferior olive, and which receives signals, indirectly, from parts of the cerebral cortex concerned with movement. Each of these axons wraps itself around the huge bush of processes (dendrites) of just one Purkinje cell (hence their name, ‘climbing fibres’), ending in around 2000 synapses. As a result, even a single impulse in a climbing fibre will make its Purkinje cell fire an impulse.
The other class of incoming axons are called ‘mossy fibres’. They are the fibres of several different kinds of nuclei in the brain stem and spinal cord. Some 40 million of them arise from cells in a region of the pons called the pontine nuclei. Some mossy fibres carry signals from the eyes, inner ears, skin, muscles, and tendons, providing information about the state and posture of the body. Because movements inevitably generate sensory stimulation, these messages must include ‘feedback’ information regarding current patterns of movement. Other mossy fibres (the majority) carry signals originating in various areas of the cerebral cortex, probably including copies of the current ‘commands-to-move’ emanating from motor areas of the cortex. They inform the cerebellum about movement intentions even before any motion has begun, enabling it to modify movements before errors have started to occur. This essentially ‘predictive’ revision is thought to reduce the extent to which the control of movement depends on feedback from sensory receptors about actual, achieved movement. This is very useful because feedback obviously cannot begin until movement has started, and the delay in a control system causes oscillations and other errors, as engineers well know.
The mossy fibres do not contact Purkinje cells directly: they end mainly on the 50 billion tiny granule cells in the cortex, whose long parallel fibres each form synaptic connections on many Purkinje cells. About 95% of the impulses produced by Purkinje cells result from the stream of signals from granule cells.
But how does it all work? Although we know more about the micro-anatomy of the cerebellum than of any other area of the brain, there is still intense debate about exactly what it does. One of the complicating factors is that the strength of each synapse between any parallel fibre and the large number of Purkinje cells that it contacts can be changed, in ways that are invisible even under the microscope. Technically elegant experiments, involving recording from Purkinje cells in slices of cerebellum, maintained alive in vitro, show that when the cell is activated by its climbing fibre, the synapses of any parallel fibres that are simultaneously active are decreased in effectiveness, and that this ‘long-term depression’ lasts a very long time. This implies that the climbing fibre can, in effect, ‘teach’ the Purkinje cell to alter its response to any recurrence of the particular pattern of mossy fibre input it was experiencing (representing a particular sensory and motor state of the body) when the climbing fibre was activated. This line of thinking is not universally accepted but has prompted attempts to identify the circumstances (behavioural contexts) in which the climbing fibres increase their activity. At present, the slim available evidence suggests that this occurs when a mismatch develops between the commands-to-move issued to the muscles by the central nervous system and the movements that actually ensue. Climbing fibres may, therefore, function (at least in part) as error-detectors in movement control.
This hypothesis also implies that the cerebellar cortex is the repository of many learned responses or ‘motor memories’ that help to ensure the prompt and accurate execution of skilled movements. Whenever these memories prove to be inadequate, either because a novel movement command is required or because they are fading, control errors will be made and the teaching effect of the climbing fibres will automatically come into play, gradually reducing the errors and improving the skill.
— David M. Armstrong
See nervous system. See also brain; cerebral cortex; memory; movement, control of.
A major division of the brain, behind the cerebrum and above the pons and fourth ventricle, consisting of a median lobe, two lateral lobes, and major connections through pairs of peduncles to the cerebrum, pons, and medulla oblongata. The cerebellum is intimately connected with the auditory vestibular apparatus and the proprioceptive system of the body and hence is involved in maintenance of body equilibrium, orientation in space, and muscular coordination and tonus.
For more information on cerebellum, visit Britannica.com.
Part of the brain behind the medulla and the pons. It has numerous connections with other parts of the brain and plays a crucial part in many aspects of locomotion. It is involved in the control of posture and muscle tone, and it helps to produce smooth, coordinated, locomotory movements. The cerebellum continuously and subconsciously integrates information from the primary motor cortex, other motor areas of the brain, and sensory receptors, especially proprioceptors.
The part of the brain that helps control muscle coordination.
The part of the metencephalon situated on the back of the brainstem, to which it is attached by three cerebellar peduncles on each side; it consists of a median lobe (vermis) and two lateral lobes (the hemispheres). Structures in the cerebellum include cingulum, cerebellar cortex, culmen, pyramid of cerebellum, uvula and vermis. See also brain.
| Brain: Cerebellum | ||
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| Figure 1a: A human brain, with the cerebellum in purple. | ||
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| Figure 1b: MRI image showing a mid-sagittal view of the human brain, with the cerebellum in purple. | ||
| Part of | Brain | |
| Artery | SCA, AICA, PICA | |
| Vein | superior, inferior | |
The cerebellum (Latin: "little brain") is a region of the brain that plays an important role in the integration of sensory perception and motor output. Many neural pathways link the cerebellum with the motor cortex—which sends information to the muscles causing them to move—and the spinocerebellar tract—which provides feedback on the position of the body in space (proprioception). The cerebellum integrates these pathways, using the constant feedback on body position to fine-tune motor movements.
Because of this 'updating' function of the cerebellum, lesions within it are not so
debilitating as to cause paralysis, but rather present as feedback deficits resulting in disorders in fine movement, equilibrium,
The cerebellum is located in the inferior posterior portion of the head (the hindbrain), directly dorsal to the pons, and inferior to the occipital lobe (Figs. 1 and 3). Because of its large number of tiny granule cells, the cerebellum contains more than 50% of all neurons in the brain, but it only takes up 10% of total brain volume.[2] The cerebellum receives nearly 200 million input fibers; in contrast, the optic nerve is composed of a mere one million fibers.
The cerebellum is divided into two large hemispheres, much like the cerebrum, and contains ten smaller lobules. The cytoarchitecture (cellular organization) of the cerebellum is highly uniform, with connections organized into a rough, three-dimensional array of perpendicular circuit elements. This organizational uniformity makes the nerve circuitry relatively easy to study. To envision this "perpendicular array," one might imagine a tree-lined street with wires running straight through the branches of one tree to the next.[clarify]
During the early stages of embryonic development, the brain starts to form in three distinct segments: the prosencephalon, mesencephalon, and rhombencephalon. The rhombencephalon is the most caudal (toward the tail) segment of the embryonic brain; it is from this segment that the cerebellum develops. Along the embryonic rhombencephalic segment develop eight swellings, called rhombomeres. The cerebellum arises from two rhombomeres located in the alar plate of the neural tube, a structure that eventually forms the brain and spinal cord. The specific rhombomeres from which the cerebellum forms are rhombomere 1 (Rh.1) caudally (near the tail) and the "isthmus" rostrally (near the front).[3]
Two primary regions are thought to give rise to the neurons that make up the cerebellum. The first region is the ventricular zone in the roof of the fourth ventricle. This area produces Purkinje cells and deep cerebellar nuclear neurons. These cells are the primary output neurons of the cerebellar cortex and cerebellum. The second germinal zone (cellular birthplace) is known as the external granular layer. This layer of cells—found on the exterior the cerebellum—produces the granule neurons. Once born, the granule neurons migrate from this exterior layer to form an inner layer known as the internal granule layer. The external granular layer ceases to exist in the mature cerebellum, leaving only granule cells in the internal granule layer. The cerebellar white matter may be a third germinal zone in the cerebellum; however, its function as a germinal zone is controversial.
The cerebellum is of archipalliar phylogenetic origin. The pallium is a term for gray matter that forms the cortex. The archipallium is the one of the most evolutionarily primitive brain regions. The circuits in the cerebellar cortex look similar across all classes of vertebrates, including fish, reptiles, birds, and mammals (e.g., Fig. 2). This has been taken as evidence that the cerebellum performs functions important to all vertebrate species.
The cerebellum contains similar gray and white matter divisions as the cerebrum. Embedded within the white matter—which is known as the arbor vitae (Tree of Life) in the cerebellum due to its branched, treelike appearance—are four deep cerebellar nuclei. Three gross phylogenetic segments are largely grouped by general function. The three cortical layers contain various cellular types that often create various feedback and feedforward loops. Oxygenated blood is supplied by three arterial branches off the basilar and vertebral arteries.
The cerebellum can be divided according to three different criteria: gross anatomical, phyologenetical, and functional.
On gross inspection, three lobes can be distinguished in the cerebellum: the flocculonodular lobe, the anterior lobe (rostral to the "primary fissure"), and the posterior lobe (dorsal to the "primary fissure"). The latter two can be further divided in a midline cerebellar vermis and lateral cerebellar hemispheres.
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The cerebellum can also be divided in three parts based on both phylogenetic criteria (the evolutionary age of each part) and on functional criteria (the incoming and outgoing connections each part has and the role played in normal cerebellar function). From the phylogenetically oldest to the newest, the three parts are:
| Functional denomination (phylogenetic denomination) | Anatomical parts | Role |
| Vestibulocerebellum (Archicerebellum) | Flocculonodular lobe (and immediately adjacent vermis) | The vestibulocerebellum regulates balance and eye movements. It receives vestibular input from both the semicircular canals and from the vestibular nuclei, and sends fibres back to the medial and lateral vestibular nuclei. It also receives visual input from the superior colliculi and from the visual cortex (the latter via the pontine nuclei, forming a cortico-ponto-cerebellar pathway). Lesions of the vestibulocerebellum cause disturbances of balance and gait. |
| Spinocerebellum (Paleocerebellum) | Vermis and intermediate parts of the hemispheres ("paravermis") | The spinocerebellum regulates body and limb movements. It receives proprioception input from the dorsal columns of the spinal cord (including the spinocerebellar tract) as well as from the trigeminal nerve, as well as from visual and auditory systems. It sends fibres to deep cerebellar nuclei which in turn project to both the cerebral cortex and the brain stem, thus providing modulation of descending motor systems. The spinocerebellum contains sensory maps as it receives data on the position of various body parts in space: in particular, the vermis receives fibres from the trunk and proximal portions of limbs, while the intermediate parts of the hemispheres receive fibres from the distal portions of limbs. The spinocerebellum is able to elaborate proprioceptive input in order to anticipate the future position of a body part during the course of a movement, in a "feed forward" manner. |
| Cerebrocerebellum (Neocerebellum) | Lateral parts of the hemispheres | The neocerebellum is involved in planning movement and evaluating sensory information for action. It receives input exclusively from the cerebral cortex (especially the parietal lobe) via the pontine nuclei (forming cortico-ponto-cerebellar pathways), and sends fibres mainly to the ventrolateral thalamus (in turn connected to motor areas of the premotor cortex and primary motor area of the cerebral cortex) and to the red nucleus (in turn connected to the inferior olivary nucleus, which links back to the cerebellar hemispheres). The neocerebellum is involved in planning movement that is about to occur[4] and has purely cognitive functions as well. |
Much of what is understood about the functions of the cerebellum stems from careful documentation of the effects of focal lesions in human patients who have suffered from injury or disease or through animal lesion research.
The four deep cerebellar nuclei are in the center of the cerebellum, embedded in the white matter. These nuclei receive inhibitory (GABAergic) inputs from Purkinje cells in the cerebellar cortex and excitatory (glutamatergic) inputs from mossy fiber pathways. Most output fibers of the cerebellum originate from these nuclei. One exception is that fibers from the flocculonodular lobe synapse directly on vestibular nuclei without first passing through the deep cerebellar nuclei. The vestibular nuclei in the brainstem are analogous structures to the deep nuclei, since they receive both mossy fiber and Purkinje cell inputs.
From lateral to medial, the four deep cerebellar nuclei are the dentate, emboliform, globose, and fastigial. An easy mnemonic device to remember these names and positions relative to their position from the midline is the phrase "Don't Eat Greasy Food", where each letter indicates the lateral to medial location in the cerebellar white matter. Some animals do not have distinct emboliform and globose nuclei, instead having a single, fused nucleus interpositus (interposed nucleus). In animals with distinct emboliform and globose nuclei, the term interposed nucleus is often used to refer collectively to these two nuclei.
In general, each pair of deep nuclei is associated with a corresponding region of cerebellar surface anatomy. The dentate nuclei are deep within the lateral hemispheres, the interposed nuclei are located in the paravermal (intermediate) zone, and the fastigial nuclei are in the vermis. These structural relationships are generally maintained in the neuronal connections between the nuclei and associated cerebellar cortex, with the dentate nucleus receiving most of its connections from the lateral hemispheres, the interposed nuclei receiving inputs mostly from the paravermis, and the fastigial nucleus receiving primarily afferents from the vermis.
There are three layers to the cerebellar cortex; from outer to inner layer, these are the molecular, Purkinje, and granular
layers. The function of the cerebellar cortex is essentially to modulate information flowing through the deep nuclei. The
microcircuitry of the cerebellum is schematized in Figure 5. Mossy and climbing fibers carry sensorimotor information into the deep nuclei, which in turn pass it on to various
premotor areas, thus regulating the gain and timing of motor actions. Mossy and climbing fibers
also feed this information into the cerebellar cortex, which performs various computations, resulting in the regulation of
Purkinje cell firing. Purkinje neurons feed back into the deep nuclei via a potent inhibitory
The innermost layer contains the cell bodies of two types of cells: the numerous and tiny granule cells, and the larger Golgi cells. Mossy fibers enter the granular layer from their main point of origin, the pontine nuclei. These fibers form excitatory synapses with the granule cells and the cells of the deep cerebellar nuclei. The granule cells send their T-shaped axons—known as parallel fibers—up into the superficial molecular layer, where they form hundreds of thousands of synapses with Purkinje cell dendrites. The human cerebellum contains on the order of 60 to 80 billion granule cells, making this single cell type by far the most numerous neuron in the brain (roughly 70% of all neurons in the brain and spinal cord, combined). Golgi cells provide inhibitory feedback to granule cells, forming a synapse with them and projecting an axon into the molecular layer.
The middle layer contains only one type of cell body—that of the large Purkinje cell. Purkinje cells are the primary integrative neurons of the cerebellar cortex and provide its sole output. Purkinje cell dendrites are large arbors with hundreds of spiny branches reaching up into the molecular layer (Fig. 6). These dendritic arbors are flat—nearly all of them lie in planes—with neighboring Purkinje arbors in parallel planes. Each parallel fiber from the granule cells runs orthogonally through these arbors, like a wire passing through many layers. Purkinje neurons are GABAergic—meaning they have inhibitory synapses—with the neurons of the deep cerebellar and vestibular nuclei in the brainstem. Each Purkinje cell receives excitatory input from 100,000 to 200,000 parallel fibers. Parallel fibers are said to be responsible for the simple (all or nothing, amplitude invariant) spiking of the Purkinje cell.
Purkinje cells also receive input from the inferior olivary nucleus via climbing fibers. A good mnemonic for this interaction is the phrase "climb the other olive tree", given that climbing fibers originate from the contralateral inferior olive. In striking contrast to the 100,000-plus inputs from parallel fibers, each Purkinje cell receives input from exactly one climbing fiber; but this single fiber "climbs" the dendrites of the Purkinje cell, winding around them and making a large number of synapses as it goes. The net input is so strong that a single action potential from a climbing fiber is capable of producing a "complex spike" in the Purkinje cell: a burst of several spikes in a row, with diminishing amplitude, followed by a pause during which simple spikes are suppressed.
This outermost layer of the cerebellar cortex contains two types of inhibitory interneurons: the stellate and basket cells. It also contains the dendritic arbors of Purkinje neurons and parallel fiber tracts from the granule cells. Both stellate and basket cells form GABAergic synapses onto Purkinje cell dendrites.
Similarly, the cerebellum follows the trend of "threes", with three major input and output peduncles (fiber bundles). These are the superior (brachium conjunctivum), middle (brachium pontis), and inferior (restiform body) cerebellar peduncles.
| Peduncle | Description |
| Superior | While there are some afferent fibers from the anterior spinocerebellar tract that are conveyed to the anterior cerebellar lobe via this peduncle, most of the fibers are efferents. Thus, the superior cerebellar peduncle is the major output pathway of the cerebellum. Most of the efferent fibers originate within the dentate nucleus which in turn project to various midbrain structures including the red nucleus, the ventral lateral/ventral anterior nucleus of the thalamus, and the medulla. The dentatorubrothalamocortical (dentate nucleus > red nucleus > thalamus > premotor cortex) and cerebellothalamocortical (cerebellum > thalamus > premotor cortex) pathways are two major pathways that pass through this peduncle and are important in motor planning. |
| Middle | This is composed entirely of afferent fibers originating within the pontine nuclei as part of the massive corticopontocerebellar tract (cerebral cortex > pons > cerebellum). These fibers descend from the sensory and motor areas of the cerebral neocortex and make the middle cerebellar peduncle the largest of the three cerebellar peduncles. |
| Inferior | This carries many types of input and output fibers that are mainly concerned with integrating proprioceptive sensory input with motor vestibular functions
such as balance and posture maintenance. Proprioceptive information from the body is carried to the cerebellum via the dorsal
spinocerebellar tract. This tract passes through the inferior cerebellar peduncle
and synapses within the paleocerebellum. Vestibular information projects onto the archicerebellum. The climbing fibers of the inferior olive run through the inferior cerebellar peduncle. This peduncle also carries information directly from the Purkinje cells out to the vestibular nuclei in the dorsal brainstem located at the junction between the pons and medulla. |
There are three sources of input to the cerebellum, in two categories consisting of mossy and climbing fibers, respectively. Mossy fibers can originate from the pontine nuclei, which are clusters of neurons located in the pons that carry information from the contralateral cerebral cortex. They may also arise within the spinocerebellar tract whose origin is located in the ipsilateral spinal cord. Most of the output from the cerebellum initially synapses onto the deep cerebellar nuclei before exiting via the three peduncles. The most notable exception is the direct inhibition of the vestibular nuclei by Purkinje cells.
Three arteries supply blood to the cerebellum (Fig. 7): the superior cerebellar artery (SCA), anterior inferior cerebellar artery (AICA), and posterior inferior cerebellar artery (PICA).
The SCA branches off the lateral portion of the basilar artery, just inferior to its bifurcation into the posterior cerebral artery. Here it wraps posteriorly around the pons (to which it also supplies blood) before reaching the cerebellum. The SCA supplies blood to most of the cerebellar cortex, the cerebellar nuclei, and the middle and superior cerebellar peduncles.
The AICA branches off the lateral portion of the basilar artery, just superior to the junction of the vertebral arteries. From its origin, it branches along the inferior portion of the pons at the cerebellopontine angle before reaching the cerebellum. This artery supplies blood to the anterior portion of the inferior cerebellum, and to the facial (CN VII) and vestibulocochlear nerves (CN VIII).
Obstruction of the AICA can cause paresis, paralysis, and loss of sensation in the face; it can also cause hearing impairment. Moreover, it could cause an infarct of the cerebellopontine angle. This could lead to hyperacusia (dysfunction of the stapedius muscle, innervated by CN VII) and vertigo (wrong interpretation from the vestibular semi-circular canal's endolymph acceleration caused by alteration of CN VIII).
The PICA branches off the lateral portion of the vertebral arteries just inferior to their junction with the basilar artery. Before reaching the inferior surface of the cerebellum, the PICA sends branches into the medulla, supplying blood to several cranial nerve nuclei. In the cerebellum, the PICA supplies blood to the posterior inferior portion of the cerebellum, the inferior cerebellar peduncle, the nucleus ambiguus, the vagus motor nucleus, the spinal trigeminal nucleus, the solitary nucleus, and the vestibulocochlear nuclei.
Ataxia is a complex of symptoms, generally involving a lack of coordination, that is often found in disease processes affecting the cerebellum. To identify cerebellar problems, the neurological examination includes assessment of gait (a broad-based gait being indicative of ataxia), finger-pointing tests and assessment of posture.[1] Structural abnormalities of the cerebellum (hemorrhage, infarction, neoplasm, degeneration) may be identified on cross-sectional imaging. Magnetic resonance imaging is the modality of choice, as computed tomography is insufficiently sensitive for detecting structural abnormalities of the cerebellum.[5]
Two main theories address the function of the cerebellum, both dealing with motor coordination. One claims that the cerebellum functions as a regulator of the "timing of movements". This has emerged from studies of patients whose timed movements are disrupted.[6]
The second, "Tensor Network Theory" provides a mathematical model of transformation of sensory (covariant) space-time coordinates into motor (contravariant) coordinates by cerebellar neuronal networks.[7][8]
Like many controversies in the physical sciences, there is evidence supporting each of the above hypotheses. Studies of motor learning in the vestibulo-ocular reflex and eyeblink conditioning demonstrate that the timing and amplitude of learned movements are encoded by the cerebellum.[9] Many synaptic plasticity mechanisms have been found throughout the cerebellum. The Marr-Albus model mostly attributes motor learning to a single plasticity mechanism: the long-term depression of parallel fiber synapses. The Tensor Network Theory of sensorimotor transformations by the cerebellum has also been experimentally supported.[10][11]
With the advent of more sophisticated neuroimaging techniques such as positron emission tomography (PET),[12] and fMRI,[13] numerous diverse functions are now at least partially attributed to the cerebellum. What was once thought to be primarily a motor/sensory integration region is now proving to be involved in many diverse cognitive functions. Paradoxically, despite the importance of this region and the heterogeneous role it plays in motor and sensory functions, people who have lost their entire cerebellum through disease, injury, or surgery can live reasonably normal lives.
As mentioned in the preceding section, there have been many attempts to model the cerebellar function.[14] The insights provided by the models have also led to extrapolations in the domains of artificial intelligence methodologies, especially neural networks. Some of the notable achievements have been Cerebellatron ,[15] Cerebellar Model Associative Memory or CMAC networks, and SpikeFORCE for robotic movement control,[16] and "Tensor Network Theory".[17]
 Lobes |
 Diencephalon |
 Scheme showing the connections of the several parts of the brain. |
 Upper surface of the cerebellum. |
 Under surface of the cerebellum. |
 Sagittal section of the cerebellum, near the junction of the vermis with the hemisphere. |
 Dissection showing the projection fibers of the cerebellum. |
 Scheme of roof of fourth ventricle. The arrow is in the foramen of Majendie. |
 Dissection showing the course of the cerebrospinal fibers. |
 Diagram showing the positions of the three principal subarachnoid cisternæ. |
