Home > Cerebellum
1 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 modulating, rather than initiating movements. 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 proprioceptive input to the cerebellar cortex.
- Relay: Information is relayed out of the cerebellum via the deep cerebellar nuclei (a bit like the cerebellum's equivalent of the thalamus).
- Outputs: To the thalamus and back to cortex.
2 Anatomy
The circuits in the cerebellar cortex looks similar in all animals, from fish to mice to humans. This has been taken as evidence that it performs a common function.
- The cerebellar cortex has three layers and contains neurons known as granule cells which constitute about 90% of the neurons in the brain.
- The Purkinje cells are the output cells which send inhibitory projections to the deep cerebellar nuclei.
- Each Purkinje cell receives synapses from around 1 million parallel fibres (the axons the granule cells of the cerebellar cortex).
- When 'unrolled' the human cerebellum is about 3 metres long.
- The cerebellum handles a lot of information. It gets 20 million input coming in from the pontine nuclei. In comparison the optic nerve has around 1 million fibres.
3 Function
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, are demonstrating that the timing and amplitude of learned movements are encoded by the cerebellum.
4 Disfunctions
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.
4.1 Lesions of the cerebellum
Patients with cerebellar lesions generally exhibit deficits during movement execution. For example, they show `intention tremor' which occurs during execution of a 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 a target. Another common sign of cerebellar damage in 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 effect arm movements or speech. The cerebellum represents information ipsilaterally so damage to it effects the ipsilateral side of the body.
5 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, some of both of these claims is true. Studies of motor learning in the vestibulo-ocular reflex and eyeblink conditioning are demonstrating that timing and amplitude of learned movements are encoded by the cerebellum. A recent review article (2004) 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 synapsenerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical ones. For the technology festival, see Synapse Festival. Synapses are specialized junctions through which cells of the nervous system signs.
