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Focal dystonia is a neurological condition that affects a muscle or group of muscles in a specific part of the body causing involuntary muscular contractions and abnormal postures. For example, in focal hand dystonia, the fingers either curl into the palm or extend outward without control. In musicians, the condition is referred to as Musician's Focal Dystonia, or simply musician's dystonia. In sports, it is commonly referred to as (the) yips. ==Causes== The cause of dystonia is not precisely understood. Misfiring of neurons in the sensorimotor cortex, a thin layer of neural tissue covering the brain, is thought to cause contractions. The source of this misfiring may be a result of impaired inhibitory mechanisms during muscle contraction. When the brain tells a given muscle to contract, it simultaneously silences muscles that would oppose the intended movement. In dystonia, it appears that the ability of the brain to inhibit the surrounding muscles is impaired, leading to loss of selectivity. The sensorimotor cortex is organized as discrete "maps" of the human body. Under normal conditions, each body part (such as individual fingers) occupies a distinct area on these cortical maps. In dystonia, these maps lose their distinct borders and overlap occurs. Exploration of this initially involved over-training particular finger movements in non-human primates, which resulted in the development of focal hand dystonia. Examination of the primary somatosensory cortex in the trained animals showed grossly distorted representations of the maps pertaining to the fingers when compared to the untrained animals. Additionally, these maps in the dystonic animals had lost the distinct borders that were noted in the untrained animals. Imaging studies in humans with focal dystonia have confirmed this finding. Also, synchronous afferent stimulation of peripheral muscles induces organizational changes in motor representations, characterized both by an increase in map size of stimulated muscles and a reduction in map separation, as assessed using transcranial magnetic stimulation.〔Schabrun SM, & Ridding MC (2007). “The influence of correlated afferent input on motor cortical representations in humans”. ''Experimental Brain Research'', 183(1): 41—49, doi: 10.1007/s00221-007-1019-8〕 The cross-connectivity between areas that are normally segregated in the sensory cortex may prevent normal sensorimotor feedback and so contribute to the observed co-contraction of antagonist muscle groups, and inappropriately timed and sequenced movements that underlie the symptoms of focal dystonia. It is hypothesized that a deficit in inhibition caused by a genetically mediated loss of inhibitory interneurons may be the underlying cause of the deficits observed in dystonia. While usually painless, in some instances the sustained contraction and abnormal posturing in dystonia may cause pain. Focal dystonia most typically affects those who rely on fine motor skills (musicians, writers, surgeons, etc.). It is thought that the excessive motor training in these individuals may contribute to the development of dystonia as their cortical maps become enlarged and begin to overlap. Focal dystonia is generally "task specific," meaning that it is only problematic during certain activities. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「focal dystonia」の詳細全文を読む スポンサード リンク
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