The
cerebellum (
Latin for
little brain) is a region
of the
brain that plays an important role in
the integration of
sensory perception,
coordination and
motor control. In order
to coordinate motor control, there are many
neural pathways linking the cerebellum with
the
cerebral motor
cortex (which sends information to the
muscles causing them to move) and the
spinocerebellar tract (which provides
proprioceptive 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,
lesions within it are not so debilitating as to cause
paralysis, but rather present as
feedback deficits resulting in disorders in fine
movement,
equilibrium,
posture, and
motor
learning. Initial observations by
physiologists during the
18th century indicated that patients with
cerebellar damage show problems with
motor coordination and movement. Research
into cerebellar function during the early to mid
19th century 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 weakness. 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 functions, including
attention 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
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. 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
hemisphere, 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.
Development and evolution
Phylogenetic
The circuits in the cerebellar cortex look similar across all
class of
vertebrates, including fish, reptiles, birds, and
mammals (e.g., Fig. 2). There is also an analogous brain structure
in
cephalopods with well developed brains
such as
octopuses. This has been taken as
evidence that the cerebellum performs functions important to all
animal
species with a brain.
Embryonic
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).
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 Rhombic lip, neurons then
move by human 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 matter may be a third germinal zone in
the cerebellum; however, its function as a germinal zone is
controversial.
Aging
The human cerebellum changes with
age.These
changes may be different from those of other parts of the brain,
e.g., the
gene expression pattern in
the human cerebellum shows less age-related alteration than in the
human
cerebral cortex.
A
stereological study has found that
human cerebellar
white matter is
reduced by 26% with age (over the age range 19–84).The researchers
of the study could detect no global loss of
Purkinje or
granule
cells, however in the
anterior
lobe there is a significant loss of these cell types as well as
a 30% volume loss.With
magnetic resonance imaging 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
binding of H-3-
ketanserin with age.(ketanserin binds primarily
to the
5-HT2A
neuroreceptor)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 tomography
study with the
altanserin
5-HT
2A receptor
radioligandfound a
positive correlation between age and cerebellar
nonspecific binding.
Anatomy
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.
Divisions
The cerebellum can be divided according to three different
criteria: gross anatomical, phylogenetical, 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
lobe (dorsal to the "primary fissure"). The latter two
can be further divided in a midline
cerebellar vermis and lateral
cerebellar hemispheres.
|
||
Phylogenetic and functional divisions
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, Pontocerebellum) || Lateral parts of the hemisphere || 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 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.
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
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 plasticity. 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
cells, a bit larger
unipolar
brush cells and the much 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.
Purkinje 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.
Molecular layer
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.
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
|-
| 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.
Relationship with cerebral cortex
The
local field potentials 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
Figure 7: The three major arteries of the cerebellum: the SCA,
AICA, and PICA
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.
Variations among vertebrates
There is considerable variation in the size and shape of the
cerebellum in different vertebrate species. It is generally largest
in
cartilaginous and
bony fish,
birds, and
mammals, but somewhat smaller in
reptiles. The large paired and convoluted lobes
found in humans are typical of mammals, but the cerebellum is
generally a single median lobe in other groups, and is either
smooth or only slightly grooved. In mammals, the neocerebellum is
the major part of the cerebellum by mass, but in other vertebrates,
it is typically the spinocerebellum.
In
amphibians,
lampreys, and
hagfish the
cerebellum is little developed; in the latter two groups it is
barely distinguishable from the brain-stem. Although the
spinocerebellum is present in these groups, the primary structures
are small paired nuclei corresponding to the
vestibulocerebellum.
Function
Functionally, the
climbing fiber and
the
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
Albus 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 scale)
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 conditioning
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 Theory", 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
sciences, there is evidence supporting each of the 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.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.
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 examination 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
imaging is the modality of choice, as
computed tomography is insufficiently
sensitive for detecting structural abnormalities of the
cerebellum.
With the advent of more sophisticated
neuroimaging techniques such as
positron emission tomography
(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
model the cerebellar function.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,
Cerebellar Model
Associative Memory or
CMAC networks,
SpikeFORCE for robotic movement control, and the "Tensor
Network Theory".
Additional images
Image:CT of brain of Mikael Häggström S3 I8.JPG|Computed tomography of head, with cerebellum visible at lower part
Image:Illu cerebrum lobes.jpg|Lobes
Image:Illu diencephalon .jpg|Diencephalon
Image:Gray677.png|Scheme showing the connections of the several parts of the brain.
Image:Gray702.png|Upper surface of the cerebellum
Image:Gray703.png|Under surface of the cerebellum
Image:Gray704.png|Sagittal section of the cerebellum, near the junction of the vermis with the hemisphere
Image:Gray705.png|Dissection showing the projection fibers of the cerebellum
Image:Gray708.svg|Scheme of roof of fourth ventricle. The arrow is in the foramen of Majendie.
Image:Gray745.png|Dissection showing the course of the cerebrospinal fibers
Image:Gray768.png|Diagram showing the positions of the three principal subarachnoid cisternæ
Image:Human cerebellum anterior view description.JPG|Human cerebellum anterior view
Image:Human brain midsagittal view description.JPG|Human brain midsagittal view
References
Further reading
- Ito M. Cerebellum and Neural Control. New York: Raven
Press; 1984. ISBN 0-89004-106-7
- Kandel ER, Schwartz JH, Jessell
TM. Principles of
Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN
0-8385-7701-6
- Llinás, R, Sotelo C. The Cerebellum Revisited. New
York: Springer; 1992. ISBN 0-387-97693-0
- Parent A, Carpenter MB. Carpenter's Human
Neuroanatomy. 9th ed. Philadelphia: Williams and Wilkins;
1995. ISBN 0-683-06752-4
External links
Top topics
Wikis
Quiz
Map
Search
