A 36-year-old man who had been healthy all his life now has progressive, symmetric muscular weakness. A year ago, he noted weakness in the area of the head and neck, which caused difficulty with speech, eye movements, and swallowing. In the past year, the weakness in the upper and lower extremities has increased, and he can no longer stand, walk, or feed himself. His mental function remains intact. Which of the following cells is most likely being destroyed in this man?