Benign fasciculations can lead to a cramp, but are unassociated with muscle weakness. They go away. The bad twitches, such as the one I saw in Mrs. Nagle’s hand, result from the dying of nerve cells in the spinal cord. They occur when a group of muscle fibers loses the connection to its controlling nerve cell—the dying cell—and an adjacent surviving cell then takes over control of those fibers. This new, larger, motor unit cannot easily be sustained by the single nerve cell, and becomes very unstable, producing the spontaneous contractions in the remolded motor unit.
