Syndromes myasthéniques : Toxiques et troubles électrolytiques

Pour rappel, schéma de la jonction neuro-musculaire :

Jonction neuro-musculaire - physiologie

Troubles électrolytiques : calcium et magnésium

Les hypoMg et hypoCa se caractérisent par une hyper-excitabilité de la jonction neuro-musculaire alors que les hyperMg et hyperCa se caractérise par une dépression neuro-musculaire. L'intoxication au Mg se recontre surtout chez l'insuffisant rénal terminal. Les effets de ces troubles électrolytiques sont multiples... les modes d'action prépondérant étant :

• Calcium :
  • Cardiomyocytes : module la durée de repolarisation via des canaux Ca voltage dépendant
  • Muscles lisses : module la contractilité via la concentration intra-cellulaire
  • Système nerveux et muscle squelettique : module le seuil d'ouverture de canaux Na voltage dépendant
• Magnésium :
  • Cardiomyocytes, muscles lisses et jonction neuro-musculaire : inhibiteur compétitif du Ca
  • Système nerveux central : antagoniste des récepteurs NMDA,...

Autres toxiques

• Venins de serpents (cobras, mambas, serpents de mer,... en Belgique : vipère commune) : neurotoxines se fixant sur les R(Acch) → syndromes myasthéniques aigus parfois très sévères.
• Arthropodes (Amérique, Australie) et aux organo-phosphorés
• Iatrogènes : D-pénicillamine,...
• Oranophosphorés et carbamates (pesticides et gaz de combat)
• Curares
• Toxine botulinique (cf Syndromes myasthéniques : Botulisme)

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