- Skeletal muscle contraction is controlled voluntarily through nervous signals from the brain. When a nerve impulse reaches the motor end plate, it causes acetylcholine to be released, which stimulates a muscle impulse. - The muscle impulse travels through the muscle fiber and causes calcium ions to be released from the sarcoplasmic reticulum. Calcium ions bind to troponin and allow muscle contraction by exposing actin binding sites for myosin. - Repeated stimuli can cause muscle fibers to contract without fully relaxing through summation, resulting in a sustained tetanic contraction. Forceful exercise causes muscle hypertrophy through recruitment of fast twitch fibers, while disuse leads to muscle atrophy.

1. Controlling Muscle Contraction & Muscle Response Chapter 8: Muscular System Unit 2: Support and Movement

4. Neuromuscular Junction

19. Series of Twitches

20. Summation