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cerebellum

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.

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Cerebellum

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 IX. 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 IV); and the posterior lobe (lobules VIIX).

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:

  • dorsal spinocerebellar pathway
  • ventral spinocerebellar pathway
  • corticopontocerebellar pathway
  • cerebo-olivocerebellar pathway
  • cerebroreticulocerebellar pathway
  • cuneocerebellar pathway
  • vestibulocerebellar pathway

The major output pathways include the following:

  • globose-emboliform-rubral pathway
  • fastigial reticular pathway
  • dentatothalamic pathway
  • fastigial vestibular pathway

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 schizophrenia , autism , mood disorders, dementia , and attention deficit hyperactivity disorder (ADHD). Currently, the relationship between the cerebellum and psychiatric illness remains unclear. It is hoped that further research will reveal insights into the cerebellar contribution to these conditions.

Disorders

There are a variety of disorders that involve or affect the cerebellum. The cerebellum can be damaged by factors including:

  • toxic insults such as alcohol abuse
  • paraneoplastic disorders; conditions in which autoantibodies produced by tumors in other parts of the body attack neurons in the cerebellum
  • structural lesions such as strokes, multiple sclerosis , or tumors
  • inherited cerebellar degeneration such as in Friedreich ataxia or one of the spinocerebellar ataxias
  • congenital anomalies such as cerebellar hypoplasia (underdevelopment or incomplete development of the cerebellum) found in Dandy-Walker syndrome , or displacement of parts of the cerebellum such as in Arnold-Chiari malformation

Typical symptoms of cerebellar disorders include hypotonia (poor muscle tone), movement decomposition (muscular movement that is fragmented rather than smooth), dysmetria (impaired ability to control the distance, power, and speed of an act), gait disturbances (abnormal pattern of walking), abnormal eye movement, and dysarthria (problems with speaking).

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): 268275.

Desmond, J. E. "Cerebellar Involvement in Cognitive Function: Evidence from Neuroimaging." International Review of Psychiatry 13 (2001): 283294.

Leroi, I., E. O'Hearn, and R. Margolis. "Psychiatric Syndromes in Cerebellar Degeneration." International Review of Psychiatry 13 (2001): 323329.

O'Hearn, E., and M. E. Molliver. "Organizational Principles and Microcircuitry of the Cerebellum." International Review of Psychiatry 13 (2001): 232246.

Rapoport, M. "The Cerebellum in Psychiatric Disorders." >International Review of Psychiatry 13 (2001): 295301.

Schmahmann, J. D. "The Cerebrocerebellar System: Anatomic Substrates of the Cerebellar Contribution to Cognition and Emotion." International Review of Psychiatry 13 (2001): 247260.

Shill, H. A., and M. Hallett. "Cerebellar Diseases." International Review of Psychiatry 13 (2001): 261267.

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/>.

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cerebellum

cerebellum (sĕr´əbĕl´əm), portion of the brain that coordinates movements of voluntary (skeletal) muscles. It contains about half of the brain's neurons, but these particular nerve cells are so small that the cerebellum accounts for only 10% of the brain's total weight. The cerebellum operates automatically, without intruding into consciousness; motor impulses from the cerebrum are organized and modulated before being transmitted to muscle. As the muscle tissue responds, its sensory nerve cells return information to the cerebellum. Thus, throughout periods of muscular activity, the cerebellum adjusts speed, force, and other factors involved in movement. The overall effect is a smooth, balanced muscular activity. If the cerebellum is injured, an activity like walking becomes spasmodic: the muscles involved contract too much or too little and operate out of sequence. Maintaining muscle tone is also a function of the cerebellum. Filling most of the skull behind the brain stem and below the cerebrum, the human cerebellum approximates an orange in size and consists of two hemispherical lobes. The grooved surface of the cerebellum is gray matter, composed chiefly of nerve cells. The interior, dense with nerve fibers, makes up the white matter. Five different nerve cell types make up the cerebellum: stellate, basket, Purkinje, Golgi, and granule cells. The Purkinje cells are the only ones to send axons out of the cerebellum. Three main nerve tracts link the cerebellum with other brain areas. Injury to the cerebellum usually results in disruption of eye movements, balance, or muscle tone.

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cerebellum

cerebellum One of the three major parts of the brain. The cerebellum forms from a dorsal outgrowh from the metencephalon region of the brain stem and is involved in co-ordination and regulation of motor activities, such as balance or escape movements. The grey matter of the cerebellum receives impulses from various other co-ordination centres as well as from tendon stretch receptors and acoustic areas of the myelencephalon.

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cerebellum

cerebellum (se-ri-bel-ŭm) n. the largest part of the hindbrain, bulging back behind the pons and the medulla oblongata and overhung by the occipital lobes of the cerebrum. The cerebellum is essential for the maintenance of muscle tone, balance, and the synchronization of activity in groups of muscles under voluntary control, converting muscular contractions into smooth coordinated movement.
cerebellar adj.

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cerebellum

cerebellum The part of the vertebrate brain concerned with the coordination and regulation of muscle activity and the maintenance of muscle tone and balance. In mammals it consists of two connected hemispheres, composed of a core of white matter and a much-folded outer cortex of grey matter containing numerous Purkyne cells, and it is situated above the medulla oblongata and partly beneath the cerebrum.

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cerebellum

cer·e·bel·lum / ˌserəˈbeləm/ • n. (pl. -bel·lums or -bel·la / -ˈbelə/ ) Anat. the part of the brain at the back of the skull in vertebrates. Its function is to coordinate and regulate muscular activity. DERIVATIVES: cer·e·bel·lar adj.

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cerebellum

cerebellum Part of the brain located at the base of the cerebrum. It is involved in maintaining muscle tone, balance, and finely coordinated movement. It is divided into hemispheres, and each controls a side of the body.

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cerebellum

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