MYONEURAL JUNCTION DISORDERS
Hypotonia due to myoneural involvement has different characteristics depending on the site of the myoneural junction involved (animation below). Myoneural junction disorders may involve presynaptic or postsynaptic areas. The presynaptic area is involved in infantile botulism, hypermagnesemia, and some of the congenital myasthenic syndromes. Aminoglycosides combines presynaptic and postsynaptic blocks. The postsynaptic area is involved in transient myasthenia gravis and most congenital myasthenia gravis syndromes.
Animation 137.1.— The advancing depolarization (blue ragged objects) arrives to the synaptic button. The depolarization of the membrane of the synaptic button. leads to opening of the calcium channels. Calcium enters the synaptic button. The calcium leads to migration towards the region of the synaptic vesicles abutting with the myoneural junction and to the opening of the synaptic vesicles into the myoneural junction. Acetylcholine travels through the myoneural junction and excites the acetylcholine dependent sodium channels in the muscle at the myoneural junction. The entry of sodium produces depolarization of the muscle leading to a muscle contraction.