Muscle Contraction
Learn the sliding filament theory and how nerve signals trigger skeletal muscle to generate force.
The sliding filament theory
According to the sliding filament theory, muscle contraction happens when thin actin filaments slide past thick myosin filaments within the sarcomere, shortening the muscle fiber without either filament changing length.
Actin and myosin filaments sliding past each other
This diagram of a sarcomere shows the actin and myosin filaments whose sliding motion shortens each contractile unit of a muscle fiber.
Nerve impulse
A nerve impulse arrives at the neuromuscular junction, triggering the release of acetylcholine.
Calcium release
Calcium ions flood the muscle fiber and bind to troponin, exposing binding sites on actin.
Cross-bridge cycling
Myosin heads attach to actin and pull the thin filaments toward the center of the sarcomere.
ATP and release
ATP binds to myosin, releasing it from actin so the cycle can repeat or the muscle can relax.
From nerve signal to movement
Muscle contraction begins with a nerve signal and ends with filaments sliding past each other, a chain of events that repeats thousands of times to sustain a single sustained contraction.
The filaments themselves don't shorten
It is a common misconception that the actin and myosin filaments shrink during contraction. In fact, they stay the same length throughout — it is the overlap between them that increases.
Contraction shortens the sarcomere, not the filaments.
The actin and myosin filaments themselves stay the same length during contraction — it is the amount of overlap between them that increases, shortening each sarcomere and, in turn, the whole muscle fiber.