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Cerebellum

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The cerebellum is a brain region important for a number of motor and cognitive functions, including learning (particularly learning of unconscious motor tasks such as riding a bike), time perception, and precise movement. Studies of simple forms of motor learning, such as adaptation of the vestibulo-ocular reflex and eyeblink conditioning, demonstrate that the timing and amplitude of learned movements are encoded by the cerebellum.

Contents

General features

  • Location: It is found at the bottom rear of the head (the hindbrain), directly above the brainstem.
  • Role: The cerebellum is involved in computing movements, directing attention, measuring time, and many other motor and cognitive functions. It is involved in guiding movements based on sensory feedback (particularly vision). Movement commands themselves are sent from the primary motor cortex to motorneurons in the spinal cord.
  • Inputs: Receive inputs from much of the cerebral cortex but mainly from visual cortex and somatosensory cortex. These inputs go to the cerebellar cortex via the pontine nuclei. Another major and important input is from the inferior olive which relays error signals to the cerebellar cortex.
  • Outputs: To motorneurons, thalamus and cortex.
  • Although the cerebellum looks small, it contains nearly 50% of all the neurons in the brain.
  • The cerebellum handles a lot of information. It gets nearly 200 million inputs. In contrast, the optic nerve has a mere 1 million fibres.

The circuits in the cerebellar cortex look similar in all animals, from fish to mice to humans. This has been taken as evidence that it performs a common function.

Anatomy

  • The cerebellum is divided up into two parts: the cerebellar cortex and the deep cerebellar nuclei.
  • The cerebellum is comprised of three lobes: the anterior lobe, the posterior lobe, and the flocculonodular lobe. The primary fissure runs between the anterior and posterior lobes. The posterolateral fissure runs between the posterior and flocculonodular lobes.
  • The midline of cerebellum is called the vermus zone or vermis. The region just lateral to the vermis is called the intermediate zone. The most lateral region is called, logically enough, the lateral zone.

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Cerebellar cortex layers

  • The cerebellar cortex has three layers: The outermost layer, called the molecular layer; an intermediate layer called the Purkinje cell layer, and the innermost layer, called the granular layer.
  • The molecular layer is made up of two types of inhibitory interneurons: the stellate cells and the basket cells. It also contains the dendritic arbor of Purkinje cells and parallel fiber tracts.
  • The Purkinje cells are the output cells which send inhibitory projections to the deep cerebellar nuclei and vestibular nuclei. Each Purkinje cell receives synapses from around 100,000 parallel fibres (the axons of the granule cells of the cerebellar cortex), and only one climbing fiber (axon from inferior olivary neurons).
  • The granular layer contains granule cells which receive inputs from outside the cerebellum (via mossy fibers) and sent their outputs to other cells in the cerebellum (via parallel fibers).

Dysfunctions

Patients with cerebellar dysfunction have problems with walking and balance, and accurate hand and arm movements. Recent brain imaging studies (using fMRI) show that the cerebellum is important for language processing and selective attention. The cerebellum is thought to be deficient in neuropsychiatric disorders such as dyslexia and autism. It is also important in development of certain ataxias, including a form of cerebral palsy.

Lesions of the cerebellum

Patients with cerebellar lesions generally exhibit deficits during movement execution. For example, they show `intention tremor' - a tremor occurring only during movement rather than at rest (as seen in Parkinson's). Patients may also show dysmetria, which is an overestimation or underestimation of force. This results in over-shoot or under-shoot when reaching for a target. Another common sign of cerebellar damage is an inability to perform rapid alternating movements. There is some degree of organisation in the cerebellum: for example, alcohol abuse leads to degeneration of the anterior cerebellum which leads to a wide staggering gait but does not affect arm movements or speech. The anterior and medial aspects of the cerebellum represent information ipsilaterally so damage to this region on one side affects the movement on the same side of the body. The posterior and lateral aspects of the cerebellum represent information bilaterally and damage to this region has been shown to impair sensory-motor adaption, while leaving motor contol unaffected.

Computational theories

Two main theories address the function of the cerebellum. 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 other claims that the cerebellum operates as a learning machine, encoding information like a computer. This was first proposed by Marr and Albus in the early 1970s. Like many controversies in biology, there is evidence supporting parts of both hypotheses. Studies of motor learning in the vestibulo-ocular reflex and eyeblink conditioning demonstrate that timing and amplitude of learned movements are encoded by the cerebellum. A recent review article (2004) (http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.neuro.27.070203.144238) explores how different mechanisms of the cerebellum may contribute to learning and other behaviors. Many synaptic plasticity mechanisms have been found throughout the cerebellum. The Marr-Albus model mostly attributes motor learning to a single plasticity mechanism, long-term depression of parallel fiber synapses.

Development and evolution

The embryonic cerebellum develops from the superior dorsal aspect of the rhombencephalon. In the mature mammallian brain, the cerebellum comprises a distinct structure at the back of the brain. The cerebellum is of archipalliar phylogenetic origin, shared as a prototypical brain structure by animals from the most elementary to the most advanced.

The cerebellum arises from the alar plate of the neural tube. In general, sensory structures are produced from the alar plate suggesting that the cerebellum maybe in fact sensory in nature. In reality, it performs a combination of sensory and motor functions.

There are two primary regions that are thought to give rise to neurons that make up the cerebellum. The first region is the ventricular zone (the roof of the four ventricle). This area produces Purkinje cells and deep cerebellar nuclear neurons. These cells are the primary output neurons of the cerebellar cortex and cerebellum (respectively). The second germinal zone is known as the external granular layer or EGL. This layer of cells, found on the exterior the cerebellum, produces the cells known as granule neurons. Once born, the granule neurons migrate from this exterior layer to form the inner layer known as the internal granule cell layer (IGL). Once the cerebellum has reached maturity, the EGL ceases to exist, leaving only granule cells in the IGL. The cerebellar white matter is perhaps a third germinal zone in the cerebellum, however this germinal zone is somewhat controversial.

See also

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