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Unifying hypothesis for the motoneuronal code in neurological disorders

par Manto, Mario ;Sauvage, Caroline;Roark, Rick
Référence Bioscience Hypotheses, 1, 2, page (93-99)
Publication Publié, 2008

Article révisé par les pairs

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Résumé :

The elucidation of the motoneuronal code in disorders characterized by prominent motor deficits, such as stroke, Parkinson disease (PD), chronic cerebellar degeneration (CCD) and amyotrophic lateral sclerosis (ALS), would have major impacts for early diagnosis, understanding of disease progression, monitoring the effects of conventional or emerging therapies, and rehabilitation. We propose that the fundamental rules of motoneuronal coding are violated in these neurological diseases. Unraveling of the motoneuronal signatures will lead to a new classification of movement disorders. We suggest that: (1) paresia due to a stroke is associated with a reduction in the mean motor unit firing rates (MUFR), abnormal commonality between pairs of motor units (MU), and an abnormal MU/force relationship; (2) in PD, MU do not discharge at the appropriate MUFR at given recruitment thresholds due to underactivity of the motor cortex. Treatment restores the linear relationship between recruitment threshold and MUFR; (3) in CCD, the hierarchical organization of MU is impaired, the commonality between pairs of MU is deregulated, the derecruitment of MU is abnormal, and EMG discharges show periods of electrical silences in MU discharges (ESIMUD); (4) in the early stages of ALS, a premature recruitment of high-threshold MU at low force levels occurs. The hierarchical organization is lost. Fluctuations in timing intervals between successive discharges of given MU are increased. Impaired MU contractile properties resulting from upper motor neuron disease are associated with decreased maximal M wave following lower motor neuron disease. The decline in mechanical force output following fatigue is associated with frequency shifts in the myoelectric signal and reduced muscle fiber conduction velocity. We suggest that patterns of MU discharges are distinct in acute diseases and in chronic neurological conditions, as a result of plasticity, compensatory patterns and reorganization of motor cortex output involving changes in functional connectivity in the central nervous system (CNS). © 2008 Elsevier Ltd. All rights reserved.