A part of the brain which occupies the lower back part of the cranial cavity within the skull, consisting of two hemispheres united in the midline by the vermis. It is connected to the brain stem by three pairs of structures called peduncles, and forms part of the wall of the fourth ventricle. Its principal function is the control of posture, repetitive movements, and the geometric accuracy of voluntary movements; it also appears to have an important role in learning. Damage to the cerebellum in both animals and humans results in cerebellar ataxia (inco-ordination of movement).
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The cerebellum (Latin: "little brain") is a region of the brain that plays an important role in the integration of sensory perception and motor output. Many neural pathways link the
cerebellum with the motor cortex—which sends information to the muscles causing them to move—and the spinocerebellar tract—which provides feedback on the position of the body in space
(proprioception). The cerebellum integrates these pathways, using the constant feedback on body position to fine-tune motor movements.
Because of this 'updating' function of the cerebellum, lesions within it are not so debilitating as to cause paralysis, but rather present as feedback deficits resulting in disorders in fine movement, equilibrium, posture, and motor learning. 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. Because of its large number of tiny granule cells, the cerebellum contains nearly 50% of all neurons in the brain, although it constitutes only 10% of total brain volume. The cerebellum receives nearly 200 million input fibers;
The cerebellum is divided into two large hemispheres, much like the cerebrum, and contains ten smaller lobules.
Development and evolution
During the early stages of embryonic development, the brain starts to form in three distinct segments: the prosencephalon, mesencephalon, and rhombencephalon. 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).
The neural tube is organized so that the alar plate typically gives rise to structures involved in sensory functions; Given its alar plate origins, the cerebellum would be expected to be devoted primarily to sensory functions. Despite its embryological origin, one of the many ironies of the cerebellum is that it functions primarily to modulate motor function.
Two primary regions are thought to give rise to the neurons that make up the cerebellum. This area produces Purkinje cells and deep cerebellar nuclear neurons. These cells are the primary output neurons of the cerebellar cortex and cerebellum. 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;
The cerebellum is of archipalliar phylogenetic origin. This has been taken as evidence that the cerebellum performs functions important to all vertebrate species.
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.
Divisions
There are three phylogenetic divisions within the cerebellum: the flocculonodular, anterior, and posterior lobes (Fig. These divisions are divided from the front to the back of the cerebellum;
The cerebellum can also be divided by function rather than evolutionary age.
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.
Cerebellar structure and function from a phylogenetic perspective
Archicerebellum
The archicerebellum is associated with the flocculonodular lobe and is mainly involved in balance (vestibular system) and eye movement functions. It receives input from the inferior and medial vestibular nuclei and sends fibers back to the vestibular nuclei, creating a feedback loop that allows for the constant maintenance of balance.
Paleocerebellum
The paleocerebellum controls proprioception related to muscle tone (constant, partial muscle contraction that is important for the maintenance of posture).
Neocerebellum
The neocerebellum receives input from the pontocerebellar tract and projects to the deep cerebellar nuclei.
The functional organization of the cerebellum
Vermis
The vermis receives its inputs mainly from the spinocerebellar tracts from the trunk of the body. The vermis sends projections to the fastigial nucleus of the cerebellum, which then sends output to the vestibular nuclei.
Intermediate zone
The intermediate zone (or paravermis) receives input from the corticopontocerebellar fibers that originate from the motor cortex.
Lateral zone
The lateral zone receives input from the parietal cortex via pontocerebellar mossy fibers regarding the location of the body in the world.
Deep nuclei
The four deep cerebellar nuclei are in the center of the cerebellum, embedded in the white matter. These nuclei receive inhibitory (GABAergic) inputs from Purkinje cells in the cerebellar cortex and excitatory (glutamatergic) inputs from mossy fiber pathways. Most output fibers of the cerebellum originate from these nuclei. One exception is that fibers from the flocculonodular lobe synapse directly on vestibular nuclei without first passing through the deep cerebellar nuclei. The vestibular nuclei in the brainstem are analogous structures to the deep nuclei, since they receive both mossy fiber and Purkinje cell inputs.
From lateral to medial, the four deep cerebellar nuclei are the dentate, emboliform, globose, and fastigial.
In general, each pair of deep nuclei is associated with a corresponding region of cerebellar surface anatomy. The dentate nuclei are deep within the lateral hemispheres, the interposed nuclei are located in the paravermal (intermediate) zone, and the fastigial nuclei are in the vermis. These structural relationships are generally maintained in the neuronal connections between the nuclei and associated cerebellar cortex, with the dentate nucleus receiving most of its connections from the lateral hemispheres, the interposed nuclei receiving inputs mostly from the paravermis, and the fastigial nucleus receiving primarily afferents from the vermis.
Cortical layers
There are three layers to the cerebellar cortex; The function of the cerebellar cortex is essentially to modulate information flowing through the deep nuclei. Mossy and climbing fibers also feed this information into the cerebellar cortex, which performs various computations, resulting in the regulation of Purkinje cell firing. 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. 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 two types of cells: the numerous and tiny granule cells, and the larger Golgi cells. These fibers form excitatory synapses with the granule cells and the cells of the deep cerebellar nuclei. 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).
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 neurons are GABAergic—meaning they have inhibitory synapses—with the neurons of the deep cerebellar and vestibular nuclei in the brainstem.
Purkinje cells also receive input from the inferior olivary nucleus via climbing fibers.
Molecular layer
This outermost layer of the cerebellar cortex contains two types of inhibitory interneurons: the stellate and basket cells.
Peduncles
Similarly, the cerebellum follows the trend of "threes", with three major input and output peduncles (fiber bundles). There are three sources of input to the cerebellum, in two categories consisting of mossy and climbing fibers, respectively. Most of the output from the cerebellum initially synapses onto the deep cerebellar nuclei before exiting via the three peduncles.
Superior cerebellar peduncle
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. premotor cortex) and cerebellothalamocortical (cerebellum >
Middle cerebellar peduncle
This is composed entirely of afferent fibers originating within the pontine nuclei as part of the massive corticopontocerebellar (cerebral cortex > 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 cerebellar peduncle
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.
Blood supply
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).
Superior cerebellar artery
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.
Anterior inferior cerebellar artery
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).
Posterior inferior cerebellar artery
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.
Dysfunction
Patients with cerebellar dysfunction experience problems in walking, balance, and accurate hand and arm movement. Recent brain imaging studies using functional magnetic resonance imaging (fMRI) show that the cerebellum is important for language processing and selective attention. Neuropsychiatric disorders such as dyslexia, schizophrenia and autism appear to be associated with a deficiency in the cerebellum, which may also play a role in the development of certain ataxias, including a form of cerebral palsy. It is believed that opsoclonus myoclonus syndrome is caused by an autoimmune attack on the cerebellum among other brain regions.
Lesions of the cerebellum
Patients with cerebellar lesions (injuries) typically exhibit deficits during movement execution.
The anterior and medial aspects of the cerebellum represent information ipsilaterally; The posterior and lateral aspects of the cerebellum represent information bilaterally; Such localized lesions cause a wide variety of symptoms related to their location in the cerebellum.
A lesion to the paleocerebellum causes severe disturbance in muscle tone and bodily posture, resulting in weakness to the side of the body opposite the lesion.
Alcohol abuse is also a common cause of cerebellar lesions. Alcohol abuse can lead to thiamine deficiency, which in the cerebellum will cause degeneration of the anterior lobe.
Ischemia and thrombosis
An obstruction of the posterior inferior cerebellar artery (known as 'PICA syndrome') can cause a wide range of characteristic effects. PICA syndrome manifests as a loss of sensation in the contralateral limbs due to damage of the inferior cerebellar peduncle as well as dizziness and nausea due to loss of blood to the nucleus ambiguus and vestibulocochlear nuclei.
Theories about cerebellar function
Three main theories address the function of the cerebellum. One claims that the cerebellum functions as a regulator of the "timing of movements". The second theory claims that the cerebellum operates as a learning machine, encoding information as does a computer. 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. The Tensor Network Theory of sensorimotor transformations by the cerebellum has also been experimentally supported.
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. Paradoxically, despite the importance of this region and the heterogeneous role it plays in motor and sensory functions, people who have lost their entire cerebellum through disease, injury, or surgery can live reasonably normal lives.
Cerebellar modeling
As mentioned in the preceding section, there have been many attempts to model the cerebellar function.
Additional images
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