The
cerebellum (Latin for
little brain) is a region of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
that plays an important role in the integration of
sensory perceptionIn philosophy, psychology, and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. It is a task far more complex than was imagined in the 1950s and 1960s, when it was predicted that building perceiving machines would take about a decade,...
, coordination and motor control. In order to coordinate motor control, there are many
neural pathwayA neural pathway, or neural tract, connects one part of the nervous system with another and usually consists of bundles of elongated, myelin-insulated neurons, known collectively as white matter...
s linking the cerebellum with the cerebral
motor cortexMotor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into six main parts:...
(which sends information to the
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s causing them to move) and the
spinocerebellar tractThe spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position ....
(which provides
proprioceptiveProprioception is the sense of the relative position of neighbouring parts of the body...
feedback on the position of the body in space). The cerebellum integrates these pathways using the constant feedback to fine-tune motor activity.
Because of this 'updating' function of the cerebellum,
lesionA lesion is any abnormal tissue found on or in an organism, usually damaged by disease or trauma. Lesion is derived from the Latin word laesio which means injury.-Causes of lesions:...
s within it are not so debilitating as to cause
paralysisParalysis is the complete loss of muscle function for one or more muscle groups. Paralysis can cause loss of feeling or loss of mobility in the affected area.-Causes:Paralysis is most often caused by damage to the nervous system, especially the spinal cord...
, but rather present as
feedbackFeedback describes the situation when output from an event or phenomenon in the past will influence the same event/phenomenon in the present or future....
deficits resulting in disorders in fine movement,
equilibriumEquilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still.-Normal balance functioning:...
,
postureHuman position refers to a position of a human body.There are several synonyms that refer to the human position, often used interchangeably, but having specific flavors.*position is a general term for a configuration of the human body...
, and
motor learningMotor learning is the process of improving the motor skills, the smoothness and accuracy of movements. It is obviously necessary for complicated movements such as speaking, playing the piano, climbing trees and eating bananas but it is also important for calibrating simple movements like reflexes,...
. Initial observations by
physiologistsPhysiology is the science of the functioning of living systems. It is a subcategory of biology...
during the
18th centuryThe 18th century lasted from 1701 to 1800 in the Gregorian calendar, in accordance with the Anno Domini/Common Era numbering system.However, Western historians sometimes specifically define the 18th century otherwise for the purposes of their work...
indicated that patients with cerebellar damage show problems with
motor coordinationMotor coordination is a term used to refer to the coordination of movements, usually between different subsequent parts of the same movement or movements of several limbs or even several actors.Motor coordination arise from a complex coordination between:...
and movement. Research into cerebellar function during the early to mid
19th centuryThe 19th century was a period in history marked by the collapse of the Spanish, Portuguese, Chinese, Ottoman, Holy Roman and Mughal empires...
was done via lesion and ablation studies in animals. Research physiologists noted that such lesions led to animals with strange movements, awkward gait, and
muscular weaknessMuscle weakness, also known as muscle fatigue, is a direct term for the inability to exert force with one's muscles to the degree that would be expected given the individual's general physical fitness. A test of strength is often used during a diagnosis of a muscular disorder before the etiology...
. These observations and studies led to the conclusion that the cerebellum was a motor control structure. However, modern research shows that the cerebellum has a broader role in a number of key
cognitive functionsIn some forms of psychological testing, particularly those related to the Myers-Briggs Type Indicator, the cognitive functions are defined as different ways of perceiving and judging the world...
, including
attentionAttention is the cognitive process of selectively concentrating on one aspect of the environment while ignoring other things. Attention has also been referred to as the allocation of processing resources...
and the processing of language, music, and other sensory temporal stimuli.
General features
The cerebellum is located in the inferior posterior portion of the head (the
hindbrainThe rhombencephalon is a developmental categorization of portions of the central nervous system in vertebrates.The rhombencephalon can be subdivided in a variable number of transversal swellings called rhombomeres...
), directly
dorsalIn anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
to the
ponsThe pons is a structure located on the brain stem. It is cranial to the medulla oblongata, caudal to the midbrain, and ventral to the cerebellum...
, and inferior to the
occipital lobeThe occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
(Figs. 1 and 3). Because of its large number of tiny
granule cellIn neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum, layer 3, inner most layer, of cerebral cortex ), the dentate gyrus of the hippocampus, and in the olfactory bulb.While anatomically...
s, the cerebellum contains more than 50% of all
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s in the brain, but it only takes up 10% of total brain volume. The cerebellum receives nearly 200 million input fibers; in contrast, the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
is composed of a mere one million fibers.
The cerebellum is divided into two large
hemisphereA sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
s, much like the
cerebrumThe cerebrum or telencephalon, together with the diencephalon, constitute the forebrain, along with the front lobe. It is the most anterior or, especially in humans, most superior region of the vertebrate central nervous system. "Telencephalon" refers to the embryonic structure, from which the...
, and contains ten smaller lobules. The
cytoarchitectureCytoarchitectonics is the study of the cellular composition of the body's tissues under the microscope...
(
cellularThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
organization) of the cerebellum is highly uniform, with connections organized into a rough,
three-dimensionalIn mathematics and physics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify each point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
array of perpendicular
circuitIn neuroscience, a neural network describes a population of physically interconnected neurons or a group of disparate neurons whose inputs or signalling targets define a recognizable circuit. Communication between neurons often involves an electrochemical process...
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.
Development and evolution
Phylogenetic
The circuits in the cerebellar cortex look similar across all
classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
es of
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s, including fish, reptiles, birds, and mammals (e.g., Fig. 2). There is also an analogous brain structure in
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
s with well developed brains such as
octopusThe octopus is a cephalopod of the order Octopoda. The octopus inhabits many diverse regions of the ocean, especially coral reefs. The term may also be used to refer only to those creatures in the genus Octopus...
es. This has been taken as evidence that the cerebellum performs functions important to all animal
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
with a brain.
Embryonic
During the early stages of
embryonic developmentEmbryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum which, after fertilization, is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth and cellular differentiation, leading to...
, the brain starts to form in three distinct segments: the
prosencephalonIn the anatomy of the brain of vertebrates, the prosencephalon is the rostral-most portion of the brain. The prosencephalon, the mesencephalon , and rhombencephalon are the three primary portions of the brain during early development of the central nervous system...
,
mesencephalonIn biological anatomy, the mesencephalon comprises the tectum , tegmentum, the ventricular mesocoelia , and the cerebral peduncles, as well as several nuclei and fasciculi...
, and
rhombencephalonThe rhombencephalon is a developmental categorization of portions of the central nervous system in vertebrates.The rhombencephalon can be subdivided in a variable number of transversal swellings called rhombomeres...
. 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
rhombomereIn the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the...
s. The cerebellum arises from two rhombomeres located in the
alar plateThe alar plate is a neural structure in the embryonic nervous system, part of the dorsal side of neural tube, that involves the communication of general somatic and general visceral sensory impulses. It later becomes a sensory region and part of the spinal cord.-External links:* Category:Embryology...
of the
neural tubeIn the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
, 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).
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 ventricleThe fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the fourth ventricle...
. This area produces
Purkinje cellFor the cells of the electrical conduction system of the heart, see Purkinje fibersPurkinje cells, or Purkinje neurons , are a class of GABAergic neurons located in the cerebellar cortex...
s and deep cerebellar
nuclearIn neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...
neurons. These cells are the primary output neurons of the cerebellar cortex and cerebellum. The second germinal zone (cellular birthplace) is known as the Rhombic lip, neurons then move by embryonic week 27 to the external granular layer. This layer of cells—found on the exterior the cerebellum—produces the granule neurons. 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 matterWhite matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
may be a third germinal zone in the cerebellum; however, its function as a germinal zone is controversial.
Aging
The human cerebellum changes with
ageAgeing or aging is the accumulation of changes in an organism or object over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline...
.
These changes may be different from those of other parts of the brain, e.g., the
gene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA...
pattern in the human cerebellum shows less age-related alteration than in the human
cerebral cortexThe cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. In preserved brains, it has a grey color, hence the name "grey matter"...
.
A
stereologicalStereology was originally defined as "the spatial interpretation of sections". It is an interdisciplinary field that is largely concerned with the three-dimensional interpretation of planar sections of materials or tissues...
study has found that human cerebellar
white matterWhite matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
is reduced by 26% with age (over the age range 19–84).
The researchers of the study could detect no global loss of
PurkinjeFor the cells of the electrical conduction system of the heart, see Purkinje fibersPurkinje cells, or Purkinje neurons , are a class of GABAergic neurons located in the cerebellar cortex...
or
granule cellIn neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum, layer 3, inner most layer, of cerebral cortex ), the dentate gyrus of the hippocampus, and in the olfactory bulb.While anatomically...
s, however in the anterior lobe there is a significant loss of these cell types as well as a 30% volume loss.
With
magnetic resonance imagingMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
a moderate volumetric reduction with age in vermis and the cerebellar hemisphere has been observed.
An autoradiography study of the human cerebellum found an increasing
bindingMolecular binding is an attractive interaction between two molecules which results in a stable association in which the molecules are in close proximity to each other...
of H-3-
ketanserinKetanserin is drug with affinity for multiple GPCR receptors. Initially it was believed to be a highly selective antagonist for serotonin 5-HT2A receptors, however this not true. Ketanserin only has weak selectivity for 5-HT2A receptors over 5-HT2C receptors...
with age.
(ketanserin binds primarily to the
5-HT2A neuroreceptorThe mammalian 5-HT
2A receptor is a subtype of the 5-HT
2 receptor which belongs to the serotonin receptor family and is a G protein coupled receptor...
)
The same research team found no significant correlation with age in their homogenate binding study.
Somewhat in line with the autoradiography study a
positron emission tomographyPositron emission tomography is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
study with the
altanserinAltanserin is a compound that binds to the 5-HT
2A receptor .It is a yellowish solid.Labeled with the isotope fluorine-18 it is used as a radioligand in positron emission tomography studies of the brain, i.e., studies of the serotonin-2A neuroreceptors.Besides human neuroimaging studies...
5-HT
2A receptor
radioligandA radioligand is a radioactive biochemical substance that is used for diagnosis or for research-oriented study of the receptor systems of the body....
found a
positive correlationIn statistics, correlation indicates the strength and direction of a relationship between two random variables. The commonest use refers to a linear relationship. In general statistical usage, correlation or co-relation refers to the departure of two random variables from independence...
between age and cerebellar nonspecific binding.
Anatomy
The cerebellum contains similar
gray"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's collection Night Shift...
and white matter divisions as the cerebrum. Embedded within the white matter—which is known as the
arbor vitaeThe arbor vitae is the cerebellar white matter, so called for its branched, tree-like appearance. It brings sensory and motor information to and from the cerebellum....
(Tree of Life) in the cerebellum due to its branched,
treeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
like appearance—are four
deep cerebellar nucleiThe cerebellum has four deep cerebellar nuclei embedded in the white matter in its center.-Inputs:These nuclei receive inhibitory inputs from Purkinje cells in the cerebellar cortex and excitatory inputs from mossy fiber and climbing fiber pathways. Most output fibers of the cerebellum originate...
. 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.
OxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
ated
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
is supplied by three
arterialArteries are blood vessels that carry blood away from the heart. All arteries, with the exception of the pulmonary and umbilical arteries, carry oxygenated blood.The circulatory system is extremely important for sustaining life...
branches off the
basilarIn human anatomy, the basilar artery is one of the arteries that supplies the brain with oxygen-rich blood.The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with...
and
vertebral arteriesThe vertebral arteries are branches of the subclavian arteries.The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with blood supplied to the anterior part of the...
.
Divisions
The cerebellum can be divided according to three different criteria: gross anatomical, phyologenetical, and functional.
Gross anatomical divisions
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 lobeThe posterior lobe of cerebellum is the portion of the cerebellum caudal to the primary fissure.It is sometimes equated to the "neocerebellum"....
(dorsal to the "primary fissure"). The latter two can be further divided in a midline
cerebellar vermisPart of the structure of animal brains, the cerebellar vermis is a narrow, wormlike structure between the hemispheres of the cerebellum.-Function:It is the site of termination of the spinocerebellar pathways that carry subconscious proprioception....
and lateral
cerebellar hemispheres.
|
>
|
Phylogenetic and functional divisions
The cerebellum can also be divided in three parts based on both
phylogeneticIn biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
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 The flocculonodular lobe is a lobe of the cerebellum consisting of the nodule and the flocculus.It is closely associated with the vestibulocerebellum.-External links:* via the Neuroscience Information Framework...
(and immediately adjacent vermis) |
vestibularThe vestibular system, which contributes to our balance and our sense of spatial orientation, is the sensory system that provides the dominant input about movement and equilibrioception. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear,...
input from both the semicircular canals and from the vestibular nuclei-Subnuclei:There are 4 subnuclei; they are situated at the floor of the fourth ventricle.* The medial vestibular nucleus , corresponding to the lower part of the area acustica in the rhomboid fossa; the caudal end of this nucleus is sometimes termed the descending or spinal vestibular nucleus.* The...
, and sends fibres back to the medial and lateral vestibular nuclei. It also receives visualThe visual system is the part of the central nervous system which enables organisms to see.It interprets the information from visible light to build a representation of the world surrounding the body...
input from the superior colliculi and from the visual cortexThe term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17...
(the latter via the pontine nucleiThe pontine nuclei are a part of the pons which store the memory of intention during motor activity. Corticopontine fibres carry information from the primary motor cortex to the ipsilateral pontine nucleus in the ventral pons, and the pontocerebellar projection then carries that information to the...
, forming a cortico-ponto-cerebellar pathway). Lesions of the vestibulocerebellum cause disturbances of balance and gaitGait is the pattern of movement of the limbs of terrestrial animals, including humans, during locomotion. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency...
.
>-
| Spinocerebellum (Paleocerebellum) |
Vermis and intermediate parts of the hemispheres ("paravermis") |
spinal cord The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord together make up the central nervous system. It is around 45 cm long in men and around 43 cm long in women. The length of the spinal cord is much shorter than...
(including the spinocerebellar tractThe spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position ....
) as well as from the trigeminal nerveThe trigeminal nerve is responsible for sensation in the face. Sensory information from the face and body is processed by parallel pathways in the central nervous system....
, as well as from visual and auditory- Ear :- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.The sound waves...
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, Pontocerebellum) |
Lateral parts of the hemisphere The cerebellum consists of three parts, a median and two lateral, which are continuous with each other, and are substantially the same in structure. The median portion is constricted, and is called the vermis, from its annulated appearance which it owes to the transverse ridges and furrows upon it;...
s |
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 The parietal lobe is a lobe in the brain. It is positioned above the occipital lobe and behind the frontal lobe....
) via the pontine nuclei (forming cortico-ponto-cerebellar pathways), and sends fibres mainly to the ventrolateral thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
(in turn connected to motor areas of the premotor cortexThe premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for the...
and primary motor area of the cerebral cortex) and to the red nucleusThe red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
(in turn connected to the inferior olivary nucleusThe inferior olivary nucleus is the largest nucleus situated in the olivary body, part of the medulla oblongata.-Function:It is closely associated with the cerebellum, meaning that it is involved in control and coordination of movements, and likely also sensory processing and cognitive tasks.There...
, which links back to the cerebellar hemispheres). The neocerebellum is involved in planning movement that is about to occur 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.
Deep nuclei
The deep nuclei of the cerebellum act as the main centers of communication, and the four different nuclei of the cerebellum (dentate, interpositus, fastigial, and vestibular) receive and send information to specific parts of the brain. In addition, these nuclei receive both inhibitory and excitatory signals from other parts of the brain which in turn affect the nucleus's outgoing signals.
Cortical layers
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.
MossyMossy fibers are one of the major inputs to cerebellum. There are many sources of this pathway, the largest of which is the cerebral cortex, which sends input to the cerebellum via the pontocerebellar pathway. Other contributors include the vestibular nerve and nuclei, the spinal cord, the...
and
climbing fiberClimbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata.
[ Image of Parallel fiber]...
s 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 synapse. This synapse regulates the extent to which mossy and climbing fibers activate the deep nuclei, and thus control the ultimate effect of the cerebellum on motor function. The synaptic strength of almost every synapse in the cerebellar cortex has been shown to undergo
synaptic plasticityIn neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength. There are several underlying mechanisms that cooperate to achieve synaptic plasticity, including changes in the quantity of neurotransmitters released into a synapse and...
. This allows the circuitry of the cerebellar cortex to continuously adjust and fine-tune the output of the cerebellum, forming the basis of some types of motor learning and coordination. Each layer in the cerebellar cortex contains the various cell types that comprise this circuitry.
Granular layer
The innermost layer contains the cell bodies of three types of cells: the numerous and tiny
granule cellIn neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum, layer 3, inner most layer, of cerebral cortex ), the dentate gyrus of the hippocampus, and in the olfactory bulb.While anatomically...
s, a bit larger unipolar brush cells and the much larger
Golgi cellIn neuroscience, Golgi cells are inhibitory interneurons found within the granular layer of the cerebellum. These cells synapse onto the soma of granule cells and unipolar brush cells. They receive excitatory input from mossy fibres, also synapsing on granule cells, and parallel fibers, which are...
s. 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 fiberParallel fibers arise from granule cells in the cerebellar cortex. They form synapses onto the dendrites of Purkinje cells ....
s—up into the superficial molecular layer, where they form hundreds of thousands of synapses with Purkinje cell
dendriteDendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s. The human cerebellum contains on the order of 60 to 80 billion granule cells, making this single
cell typeA cell type is a distinct morphological or functional form of cell. When a cell switches state from one cell type to another, it undergoes cellular differentiation. A complete list of distinct cell types in the adult human body may include about 210 distinct types....
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.
Purkinje layer
The middle layer contains only one type of cell body—that of the large
Purkinje cellFor the cells of the electrical conduction system of the heart, see Purkinje fibersPurkinje cells, or Purkinje neurons , are a class of GABAergic neurons located in the cerebellar cortex...
. 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
orthogonallyIn mathematics, two vectors are orthogonal if they are perpendicular, i.e., they form a right angle. The word comes from the Greek , meaning "straight", and , meaning "angle".- Definitions :...
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,
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
invariant) spiking of the Purkinje cell.
Purkinje cells also receive input from the
inferior olivary nucleusThe inferior olivary nucleus is the largest nucleus situated in the olivary body, part of the medulla oblongata.-Function:It is closely associated with the cerebellum, meaning that it is involved in control and coordination of movements, and likely also sensory processing and cognitive tasks.There...
via
climbing fiberClimbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata.
[ Image of Parallel fiber]...
s. 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 potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
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.
Molecular layer
This outermost layer of the cerebellar cortex contains two types of inhibitory
interneuronAn interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s: the
stellateIn neuroscience, stellate cells are neurons and astrocytes with several dendrites radiating from the cell body giving them a star shaped appearance...
and
basket cellBasket cells are inhibitory GABAergic interneurons found in several brain regions: the molecular layer of the cerebellum, the hippocampus, and the cortex. In the cerebellum, they synapse on the cell bodies of Purkinje cells, and are multipolar and stellate, with freely branching dendrites, which...
s. It also contains the dendritic arbors of Purkinje neurons and parallel fiber tracts from the granule cells. Both stellate and basket cells form
GABAergicγ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone...
synapses onto Purkinje cell dendrites.
Peduncles
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
>-
| SuperiorThe superior cerebellar peduncles , two in number, emerge from the upper and medial part of the white substance of the hemispheres and are placed under cover of the upper part of the cerebellum....
|
dentate nucleus The Dentate nucleus is located within the deep white matter of each cerebellar hemisphere. It is the largest of the four deep cerebellar nuclei, the others being the fastigial nucleus and the interposed nucleus . It is responsible for the planning, initiation and control of volitional movements...
which in turn project to various midbrain structures including the red nucleusThe red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
, the ventral lateral/ventral anterior nucleus of the thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
, and the medullaThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
. The dentatorubrothalamocortical (dentate nucleus > red nucleusThe red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
> thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
> premotor cortexThe premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for the...
) and cerebellothalamocortical (cerebellum > thalamus > premotor cortex) pathways are two major pathways that pass through this peduncle and are important in motor planning.
>-
| MiddleThe middle cerebellar peduncles are composed entirely of centripetal fibers, which arise from the cells of the nuclei pontis of the opposite side and end in the cerebellar cortex; the fibers are arranged in three fasciculi, superior, inferior, and deep.* The superior fasciculus, the most...
|
pontine nuclei The pontine nuclei are a part of the pons which store the memory of intention during motor activity. Corticopontine fibres carry information from the primary motor cortex to the ipsilateral pontine nucleus in the ventral pons, and the pontocerebellar projection then carries that information to the...
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.
>-
| InferiorThe upper part of the posterior district of the medulla oblongata is occupied by the inferior peduncle, a thick rope-like strand situated between the lower part of the fourth ventricle and the roots of the glossopharyngeal and vagus nerves....
|
This carries many types of input and output fibers that are mainly concerned with integrating proprioceptive Proprioception is the sense of the relative position of neighbouring parts of the body...
sensory input with motor vestibular functionsThe vestibular system, which contributes to our balance and our sense of spatial orientation, is the sensory system that provides the dominant input about movement and equilibrioception. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear,...
such as balance and posture maintenance. Proprioceptive information from the body is carried to the cerebellum via the dorsal spinocerebellar tractThe spinocerebellar tract is a set of axonal fibers originating in the spinal cord and terminating in the ipsilateral cerebellum. This tract conveys information to the cerebellum about limb and joint position ....
. This tract passes through the inferior cerebellar peduncle and synapses within the paleocerebellum. Vestibular information projects onto the archicerebellum.
The climbing fiberClimbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata. [ Image of Parallel fiber]...
s of the inferior oliveThe inferior olivary nucleus is the largest nucleus situated in the olivary body, part of the medulla oblongata.-Function:It is closely associated with the cerebellum, meaning that it is involved in control and coordination of movements, and likely also sensory processing and cognitive tasks.There...
run through the inferior cerebellar peduncle.
This peduncle also carries information directly from the Purkinje cells out to the vestibular nuclei-Subnuclei:There are 4 subnuclei; they are situated at the floor of the fourth ventricle.* The medial vestibular nucleus , corresponding to the lower part of the area acustica in the rhomboid fossa; the caudal end of this nucleus is sometimes termed the descending or spinal vestibular nucleus.* The...
in the dorsal brainstem located at the junction between the ponsThe pons is a structure located on the brain stem. It is cranial to the medulla oblongata, caudal to the midbrain, and ventral to the cerebellum...
and medullaThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the 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 cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord together make up the central nervous system. It is around 45 cm long in men and around 43 cm long in women. The length of the spinal cord is much shorter than...
. 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.
Relationship with cerebral cortex
The
local field potentialA local field potential is a particular class of electrophysiological signals, which is related to the sum of all dendritic synaptic activity within a volume of tissue.-Background:...
s of the neocortex and cerebellum oscillate coherently at (6–40 Hz) in awake behaving animals. These appear to be under the control of output from the cerebral cortex. This output would be mediated by a pathway from layer 5/6 neurons in the neocortex through that project either to the pons or the inferior olive. If through the pon this would go to mossy fibers that synapse with granule and Golgi neurons with the granule cells then targeting Purkinje neurons via their excitatory parallel fibers. If the inferior olive it would go via excitatory climbing fiber inputs to Purkinje neurons. These return this output back to the cerebral cortex through the ventrolateral thalamus completing the loop.
Blood supply
Three arteries supply blood to the cerebellum (Fig. 7): the
superior cerebellar arteryThe superior cerebellar artery arises near the termination of the basilar artery.It passes lateralward, immediately below the oculomotor nerve, which separates it from the posterior cerebral artery, winds around the cerebral peduncle, close to the trochlear nerve, and, arriving at the upper...
(SCA),
anterior inferior cerebellar arteryThe anterior inferior cerebellar artery passes backward to be distributed to the anterior part of the under surface of the cerebellum, anastomosing with the posterior inferior cerebellar branch of the vertebral...
(AICA), and
posterior inferior cerebellar arteryThe posterior inferior cerebellar artery , the largest branch of the vertebral, is one of the three main arterial blood supplies for the cerebellum.-Course:...
(PICA).
The SCA branches off the lateral portion of the basilar artery, just inferior to its bifurcation into the
posterior cerebral arteryThe posterior cerebral artery is one of a pair of blood vessels that supplies oxygenated blood to the posterior aspect of the brain in human anatomy]...
. 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 angleThe cerebellopontine angle is a space filled with spinal fluid. It has the brain stem as its medial boundary, the cerebellum as its roof and posterior boundary, and the posterior surface of the temporal bone as its lateral boundary. The floor of the cerebellopontine angle is formed by the lower...
before reaching the cerebellum. This artery supplies blood to the anterior portion of the inferior cerebellum, and to the
facialThe facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and taste to the anterior two-thirds of the tongue...
(CN VII) and
vestibulocochlear nerveThe vestibulocochlear nerve is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium information from the inner ear to the brain....
s (CN VIII). Obstruction of the AICA can cause
paresisParesis is a condition typified by partial loss of movement, or impaired movement. When used without qualifiers, it usually refers to the limbs, but it also can be used to describe the muscles of the eyes and also the stomach...
,
paralysisParalysis is the complete loss of muscle function for one or more muscle groups. Paralysis can cause loss of feeling or loss of mobility in the affected area.-Causes:Paralysis is most often caused by damage to the nervous system, especially the spinal cord...
, and loss of sensation in the face; it can also cause
hearing impairmentA hearing impairment or deafness is a full or partial decrease in the ability to detect or understand sounds. Caused by a wide range of biological and environmental factors, loss of hearing can happen to any organism that perceives sound...
. Moreover, it could cause an infarct of the cerebellopontine angle. This could lead to hyperacusia (dysfunction of the
stapedius muscleThe stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes....
, innervated by CN VII) and
vertigoVertigo is a specific type of dizziness, a major symptom of a balance disorder. It is the sensation of spinning or swaying while the body is actually stationary with respect to the surroundings.The effects of vertigo may be slight...
(wrong interpretation from the vestibular semi-circular canal's
endolymphEndolymph is the fluid contained in the membranous labyrinth of the inner ear.It is also called Scarpa's fluid, after Antonio Scarpa.-Composition:...
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 ambiguusThe nucleus ambiguus is a region of histologically disparate cells located just dorsal to the inferior olivary nucleus in the lateral portion of the upper medulla...
, the
vagusThe vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem...
motor nucleus, the spinal
trigeminalThe trigeminal nerve is responsible for sensation in the face. Sensory information from the face and body is processed by parallel pathways in the central nervous system....
nucleus, the
solitary nucleusThe solitary tract and nucleus are structures in the brainstem that carry and receive visceral sensation and taste from the facial , glossopharyngeal and vagus cranial nerves.-Anatomy:...
, and the
vestibulocochlearThe vestibulocochlear nerve is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium information from the inner ear to the brain....
nuclei.
Function
Functionally, the
climbing fiberClimbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata.
[ Image of Parallel fiber]...
and the
mossy fiberMossy fiber may refer to two different bundles of axons in the brain:* Mossy fiber * Mossy fiber...
-granule cell-parallel fiber pathways are the two main types of afferents to the cerebellum as a whole and to the Purkinje cells in particular. These afferent systems differ dramatically in their connectivity. The Purkinje cell and its climbing fiber afferent have a one-to-one relationship and the overall projection is organized to produce synchronous activation of specific groupings of Purkinje cells in a rostrocaudal orientation. The relationship between the Purkinje cell and the mossy fiber-parallel fiber system can be characterized as many-to-many, with the directionality being mediolateral orientation within the molecular layer, i.e. at right angles to the Purkinje cell dendrites, which are isoplanar .
The climbing fiber system
Originates from the contralateral inferior olive. There are different views concerning the role of the climbing fibre system. According to a very influential idea first proposed by Marr and
AlbusJames Sacra Albus is an American engineer and Senior NIST Fellow, Founder and former Chief of the Intelligent Systems Division of the Manufacturing Engineering Laboratory at the National Institute of Standards and Technology .- Biography :...
the climbing fibers cause synaptic changes in the cerebellar cortex which underlie motor learning. Evidence from many labs and using different learning paradigms has confirmed this. An alternative view is that, as a result of the electrical coupling between inferior olivary neurons, their dynamic decoupling via return inhibition from the cerebellar nuclei and the topography of the olivocerebellar projection, this system generates synchronous (on a millisecond
time scaleA time scale specifies divisions of time and possibly an epoch that marks the time when the time scale began .*A time standard is a specification of either the rate at which time passes, or points in time, or both....
) complex spike activation of Purkinje cells, in rostrocaudally oriented bands. These activity bands are about 250 μm wide in the mediolateral direction but can be several millimeters long in the rostrocaudal direction and extend down the walls of the cerebellar folia and across several lobules. The moment–to–moment synchrony distribution of motor control is dynamically modulated by the inferior olive with the major role of the olivary afferents being to determine the pattern of "effective" electronic coupling between olivary neurons and thereby the distribution of synchronous complex spike activity across the cerebellar cortex. Changes in synchrony patterns are associated with movements made by animals performing a motor task. The olivocerebellar system can be considered an electrically malleable substrate from which unique motor synergies can be sculpted.
The mossy fiber-parallel fiber system
In contrast to the punctate nature of cerebellar activation by the olivocerebellar system, the mossy fiber-parallel fiber system provides a continuous and very delicate regulation of the excitability of the cerebellar nuclei, brought about by the tonic activation of simple spikes in Purkinje cells, which ultimately generates the fine control of movement known as motor coordination. The fact that the mossy fibers inform the cerebellar cortex of both ascending and descending messages to and from the motor centers in the spinal cord and brainstem gives us an idea of the ultimate role of the mossy fiber system: it informs the cortex of the place and rate of movement of limbs and puts the motor intentions generated by the brain into the context of the status of the body at the time the movement is to be executed. Moreover, through its effects on the inhibitory GABAergic cerebellar nuclear cells, which project back to the inferior olive, it helps shape the pattern of coupling among olivary cells and hence the synchrony distribution in the upcoming olivocerebellar discharge.
The cerebellar nuclei
The Purkinje cells are the only output of the cerebellar cortex and are inhibitory in nature. Their axons contact the cerebellar and Deiters vestibular nucleus as their only target. The activity of the cerebellar nuclei is regulated in three ways: (1) by excitatory input from collaterals of the cerebellar afferent systems, (2) by inhibitory inputs from Purkinje cells activated over the mossy fiber pathways, and (3) by inputs from Purkinje cells activated by the climbing fiber system.
Overall cerebellar function
The output of the cerebellum (the cerebellar nuclei axons) proceeds to generate the background activity that serves to set the overall tone and posture that gives the motor cortex the ability to execute movements on the basis of intention (the strategy of movement). In this context the cerebellum provides the tactics of the multiple muscle activation required to support such definite movements. While the motor brain determines where to move (executive imperative), the cerebellum implements its proper timing and modulates the force given to every motor command, as the coordination of movement is a non-continuous function.
Cerebellar learning
Several investigators have felt it unlikely that the cerebellum could serve the functions of coordination and fine-tuning of movement unless it had mechanisms for learning. It was proposed by Marr and Albus (see above) that the cerebellar Purkinje cells could learn to change their responses to particular parallel fibre inputs if these were repeatedly paired with simultaneuous inputs from the climbing fibres. In a pioneering study by Gilbert and Thach from 1977, Purkinje cell recordings from monkeys learning a reaching task seemed to be consistent with this suggestion. The idea of the cerebellum as a site of motor learning has since been pursued by several research groups working with different learning paradigms, such as the vestibulo-ocular reflex and eyeblink conditioning and also with synaptic mechanisms both in vivo and in vitro.
Eyeblink conditioning
In the
eyeblink conditioningEyeblink conditioning is a form of classical conditioning that has been used extensively to study neural structures and mechanisms that underlie learning and memory. The procedure is relatively simple and usually consists of pairing an auditory or visual stimulus with an eyeblink-eliciting...
paradigm, a neutral
conditioned stimulus such as a tone or a light is repeatedly paired with an
unconditioned stimulus, such as an air puff, that elicits a blink response. After such repeated presentations of the CS and US, the CS will eventually elicit a blink before the US, a
conditioned response or CR. It was discovered by McCormick and Thompson in 1984 that lesions to the cerebellum abolished classically conditioned blink responses. The localization of the learning site was further narrowed down to the anterior interpositus nucleus and the hemispheral lobule VI in lesion studies. It was also shown that this area received convergent mossy and climbing fibre input as required by the Marr-Albus hypothesis. Physiological studies later confirmed this and demonstrated that a number of small cortical areas, most prominently in the C3 zone in HVI, controlled the eyelids . There was considerable disagreement among researchers about the nature of the cerebellar involvement, but it is now generally accepted that the critical learning mechanisms are located in the cerebellum . There has remained a disagreement concerning the relative roles of the cerebellar cortex and the deep nuclei, however. It is clear both from lesion studies that the cortex is involved in the learning, but there are also studies suggesting that the deep nuclei have a role. Crucial evidence for the role of the cortex has recently come from recordings of Purkinje cell behaviour during conditioning. Paired CS-US presentations cause the acquisition of a pause in simple spike firing called a Purkinje cell CR . Because of the inhibitory action of Purkinje cells on the deep nuclei, this would be translated into an excitatory output signal the eyelid. Because acquisition of conditioned Purkinje cell responses also occurred when the conditioned and unconditioned stimuli consisted of direct mossy and climbing fibre stimulation, this provides striking confirmation of the original Marr-Albus proposal.
Motor coordination
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. The second, the "
Tensor Network TheoryTensor network theory is a theory of brain function by Llinas and Pellionisz which provides a mathematical model of transformation of sensory space-time coordinates into motor coordinates by cerebellar neuronal networks....
", provides a mathematical model of transformation of sensory (covariant) space-time coordinates into motor (contravariant) coordinates by cerebellar neuronal networks. Like many controversies in the
physical sciencePhysical science is an encompassing term for the branches of natural science and science that study non-living systems, in contrast to the biological sciences...
s, there is evidence supporting each of the hypotheses. Studies of motor learning in the
vestibulo-ocular reflexThe vestibulo-ocular reflex or oculovestibular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head...
and
eyeblink conditioningEyeblink conditioning is a form of classical conditioning that has been used extensively to study neural structures and mechanisms that underlie learning and memory. The procedure is relatively simple and usually consists of pairing an auditory or visual stimulus with an eyeblink-eliciting...
demonstrate that the timing and
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of learned movements are encoded by the cerebellum.
Many
synaptic plasticityIn neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength. There are several underlying mechanisms that cooperate to achieve synaptic plasticity, including changes in the quantity of neurotransmitters released into a synapse and...
mechanisms have been found throughout the cerebellum. The Marr-Albus model mostly attributes motor learning to a single plasticity mechanism: the
long-term depressionLong-term depression , in neurophysiology, is the weakening of a neuronal synapse that lasts from hours to days. It results from either strong synaptic stimulation or persistent weak synaptic stimulation . Long-term potentiation is the opposing process...
of parallel fiber synapses. The Tensor Network Theory of sensorimotor transformations by the cerebellum has also been experimentally supported.
Ataxia is a complex of motor symptoms, generally involving a lack of coordination, that is often found in disease processes affecting the cerebellum. To identify cerebellar problems, the
neurological examinationA neurological examination is the assessment of sensory neuron and motor responses, especially reflexes, to determine whether the nervous system is impaired...
includes assessment of gait (a broad-based gait being indicative of ataxia), finger-pointing tests and assessment of posture. Structural abnormalities of the cerebellum (hemorrhage, infarction, neoplasm, degeneration) may be identified on cross-sectional imaging.
Magnetic resonance imagingMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
is the modality of choice, as
computed tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
is insufficiently sensitive for detecting structural abnormalities of the cerebellum.
With the advent of more sophisticated
neuroimagingNeuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
techniques such as
positron emission tomographyPositron emission tomography is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET)
and fMRI, 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.
Cerebellar modeling
There have been many attempts to
modelA mathematical model uses mathematical language to describe a system. Mathematical models are used not only in the natural sciences and engineering disciplines but also in the social sciences ; physicists, engineers, computer scientists, and economists use...
the cerebellar function.
The insights provided by the models have also led to extrapolations in the domains of
artificial intelligenceArtificial intelligence is the intelligence of machines and the branch of computer science which aims to create it. Textbooks define the field as "the study and design of intelligent agents,"...
methodologies, especially
neural networksNeural Networks is the official journal of the three oldest societies dedicated to research in neural networks: International Neural Network Society, European Neural Network Society and Japanese Neural Network Society, published by Elsevier. A subsription to the journal is part of the membership...
. Some of the notable achievements have been
Cerebellatron,
Cerebellar Model Associative Memory or
CMAC networks,
SpikeFORCE for robotic movement control, and the "Tensor Network Theory".
External links
- A worldwide list of
- Cerebellum and multiple sclerosis at mult-sclerosis.org
- Basal ganglia and cerebellum at Washington University in St. Louis
Washington University in St. Louis is a nonsectarian, private research university located in suburban St. Louis, Missouri. Founded in 1853 and named for George Washington, the university has students and faculty from all fifty U.S. states and more than one hundred and twenty five nations...
- Cerebellum - Cell Centered Database
- "The Treasure at the Bottom of the Brain” at newhorizons.org
- Cerebellum images at About.com
About.com is an online source for original information and advice. It is written in English, and is aimed primarily at North Americans. It is owned by The New York Times Company....
- Histological section of primate cerebellum at University of Birmingham
The University of Birmingham is a British 'Redbrick' university located in the city of Birmingham, England...
- Cerebellum and Motor Control, Neurophysiology Lecture Site, Ronald E. Young, Ph.D., University of West Indies, Mona.
- Comparative Neuroscience at Wikiversity
- NIF Search - Cerebellum via the Neuroscience Information Framework
The Neuroscience Information Framework is a repertory of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources.- Description :...