I have heard that acetylcholine can be excitatory or inhibitory, but I am confused as to which it is when it comes to muscles. On the one hand, I believe that muscle atonia during REM sleep is associated with an abundance of acetylcholine in the brain stem, suggesting it has an inhibitory response on the motor neurons. On the other hand I know that the illness myasthenia gravis is caused by antibodies blocking acetylcholine receptors, which suggests to me that acetylcholine is excitatory. Could it be that with myasthenia gravis, the effect occurs at the neuromuscular junction where acetylcholine is excitatory, but it has an inhibitory effect on the motor neurons in the brain stem?
Thank you in advance.