skeletal muscle

Meaning: Skeletal muscle is striated, voluntary muscle attached to bones by tendons. Each fiber is a long, multinucleated cell filled with myofibrils, which are chains of sarcomeres (Z line to Z line). Acetylcholine from a motor neuron triggers an action potential that spreads down the T-tubules, and the sarcoplasmic reticulum releases Ca²⁺. Ca²⁺ binds troponin, tropomyosin moves off the myosin-binding sites on actin, and myosin heads cycle through attachment, power stroke, detachment, and recocking using ATP.

On the MCAT: Expect a table comparing skeletal, smooth, and cardiac muscle (striated or not, voluntary or not, number of nuclei, how Ca²⁺ regulates contraction). Sarcomere figures ask which region changes during contraction: the I band and H zone shrink, and the A band stays the same. Fast-twitch fibers rely on glycolysis, have few mitochondria, and fatigue quickly. Slow-twitch fibers have many mitochondria and abundant myoglobin and use oxidative metabolism. Rapid repeated stimulation causes summation and tetanus, and force is graded by recruiting more motor units.

Common confusion: The filaments do not shorten. They slide past each other, so the A band keeps its length. ATP binding, not the power stroke, releases myosin from actin. That is why muscles stiffen after death (rigor mortis) when ATP runs out. Skeletal muscle uses troponin to sense Ca²⁺, while smooth muscle uses calmodulin.