Dysmetria
| Dysmetria | |
|---|---|
| Classification and external resources | |
| ICD-10 | R27 |
| ICD-9-CM | 781.3 |
| DiseasesDB | 2218 |
| MeSH | D002524 |
Dysmetria (English: wrong length) refers to a lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye. It is a type of ataxia. It is sometimes described as an inability to judge distance or scale.
Hypermetria and hypometria refer, respectively, to overshooting and undershooting the intended position.
The cerebellum is the area of the brain that contributes to coordination and motor processes and is anatomically inferior to the cerebrum.Sensorimotor integration is the brain’s way of integrating the information received from the sensory (or proprioceptive) neurons from the body, including any visual information. To be more specific, information needed to perform a motor task comes from retinal information pertaining to the eyes’ position and has to be translated into spatial information. Sensorimotor integration is crucial for performing any motor task and takes place in the post parietal cortex. After the visual information has been translated into spatial information, the cerebellum must use this information to perform the motor task. If there is damage to any pathways that connect the pathways, dysmetria may result.
Motor dysmetria is the customary term used when a person refers to dysmetria. Dysmetria of the extremities caused by hemispheric syndromes is manifested in multiple ways: dysrhythmic tapping of hands and feet and dysdiadochokinesis, which is the impairment of alternating movements. Damage to the cerebellum makes a person slow to orient their extremities in space.
Motor control as a learning process
Recent research has also shed light upon a specific process that if interrupted, may be the cause of ataxia and dysmetria. According to sources cited in this article, motor control is a learning process that occurs in the synapses of Purkinje dendrites. There have been varying theories as to the makeup of the cerebellum, which controls this process. Some predicted that the cerebellum was an array of adjustable pattern generators (APGs), each of which generate a “burst command” with varying intensity and duration. Other models, which apply mostly in robotic applications, propose that the cerebellum acquires an “inverse model of the motor apparatus". More recent research in electrophysiology has shown modular structures in the spinal cord known as “motor primitives". Based on the APG model, modules of APG are the features that control motor learning. The entire process is a positive feedback loop. Inhibitory input is transmitted and received from various components of the cortex, including the cerebellar nucleus, a motor cortical cell and Purkinje cells. Purkinje cells send the inhibitive information by obtaining learning information from parallel fibers of granule cells. This model of APGs is useful in that it effectively describes the motor learning process.
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