The Muscular System
The muscular system domain tests how skeletal, cardiac and smooth muscle produce force, how sarcomeres shorten, and how nerves and muscles communicate at the neuromuscular junction. Questions often require you to follow excitation-contraction coupling or distinguish muscle actions and fiber types.
Core rules and concepts
- Skeletal muscle is striated, multinucleated and voluntarily controlled. Cardiac muscle is striated, branched and has intercalated discs. Smooth muscle is nonstriated, spindle-shaped and involuntary.
- A sarcomere runs from one Z disc to the next. The A band contains thick filaments; the I band contains thin filaments; the H zone is thick filaments only. Sliding filaments shorten the sarcomere without shortening the filaments themselves.
- An action potential travels along the sarcolemma and T tubules, triggering calcium release from the sarcoplasmic reticulum. Calcium binds troponin, moving tropomyosin and exposing actin-binding sites on myosin.
- ATP is needed for myosin cross-bridge detachment and for calcium reuptake. Rigor mortis occurs when ATP falls and cross-bridges cannot detach.
- A motor unit is one motor neuron and the muscle fibers it innervates. Recruitment and firing frequency increase force. Slow oxidative fibers resist fatigue; fast glycolytic fibers produce rapid, powerful contractions.
- Isotonic contraction changes muscle length and moves a load; isometric contraction generates tension without changing length.
Worked example
A drug blocks acetylcholine receptors at the neuromuscular junction. Acetylcholine cannot bind, the sarcolemma does not depolarize, calcium is not released, and cross-bridges do not form. The result is flaccid paralysis rather than a contracture. If acetylcholinesterase were inhibited instead, acetylcholine would persist and cause prolonged stimulation, showing that the site of interference determines the effect.
Traps that cost marks
Do not say the A band shortens during contraction; the sarcomere shortens as filaments slide. Do not confuse troponin with tropomyosin: troponin binds calcium and moves tropomyosin. Do not call all muscle contraction “isotonic.” Remember that smooth muscle can maintain tone with little ATP use and can be activated by stretch, hormones or autonomic nerves.
Worked example
During a biceps curl, the biceps shortens while lifting the weight, so the contraction is isotonic. Holding the weight steady at the top is isometric.