What is the muscle response to a single, brief threshold stimulus called?

• A. Muscle twitch
• B. Tetanus
• C. Treppe
• D. Fatigue

Answer: (A)

A muscle twitch is the basic contractile response to a single stimulus. When a brief threshold stimulus is applied, one action potential travels along the motor neuron to the muscle fiber, releasing calcium and triggering cross-bridge cycling for a short moment. The fiber then relaxes as calcium is re-sequestered, producing a brief, singular contraction and relaxation. If stimuli are delivered rapidly, contractions can sum and fuse into a sustained tetanus; if the muscle is warmed and multiple identical stimuli are applied in early trials, a staircase rise in force called treppe can occur; fatigue occurs when stimulation continues and the muscle’s energy sources become depleted.