Twitching is caused by a minor muscle contraction or uncontrollable twitching of a muscle group served by a single motor nerve fiber.
From the kentucky.com
Small ones have single, thin nerve fibers that innervate several muscle fibers, whereas the lone, thicker nerve fiber in a larger motor unit may innervate several thousand of them.
From the sciencedaily.com
In around 1% of infections, poliovirus spreads along certain nerve fiber pathways, preferentially replicating in and destroying motor neurons within the spinal cord, brain stem, or motor cortex.