contraction

Meaning: In skeletal muscle, a motor neuron's acetylcholine depolarizes the sarcolemma, the signal travels down T-tubules, and the sarcoplasmic reticulum releases Ca²⁺. Ca²⁺ binds troponin, which shifts tropomyosin off the myosin-binding sites on actin. Myosin heads form cross-bridges, release ADP and Pi in the power stroke, and pull actin toward the M line. A new ATP binds myosin and detaches it, and hydrolysis re-cocks the head. Relaxation occurs when Ca²⁺ is pumped back into the sarcoplasmic reticulum.

On the MCAT: Expect a sarcomere diagram asking which regions change. During contraction the sarcomere, I band and H zone shorten, while the A band (thick filament length) stays constant. Other questions ask what happens without Ca²⁺ (no contraction) or without ATP (heads stay bound, as in rigor mortis). You may also compare muscle types: smooth muscle has no troponin and uses Ca²⁺ with calmodulin and myosin light-chain kinase. Cardiac muscle is striated like skeletal muscle. Repeated stimulation can cause summation and tetanus.

Common confusion: Students think the filaments themselves shorten. They slide past each other, and only their overlap changes. Another error is thinking ATP is needed only for the power stroke. ATP binding is what releases myosin from actin. Also, contraction means active force generation and does not always shorten the muscle, as in an isometric contraction.