A 52-year-old woman with a 10-year history of myasthenia gravis (MG) is on long-term neostigmine therapy. She presents to the neurology clinic for routine follow-up. Her neurologist explains that she has developed a tolerance to the drug and is considering switching to a longer-acting agent. The neurologist also mentions the risk of a "cholinergic crisis" if the dose is increased too much. Which of the following best explains the mechanism of cholinergic crisis in myasthenia gravis patients on excessive cholinesterase inhibitors?