Skeletal muscle contracts only when a motor nerve tells it to. The signal crosses a junction onto the fiber and makes proteins inside it pull against each other.
Neuromuscular junction
A motor neuron releases acetylcholine onto the muscle fiber. Acetylcholine binds receptors on the fiber membrane and starts an action potential that spreads along the fiber.
Calcium release
The action potential runs down T tubules into the fiber and makes the sarcoplasmic reticulum release calcium. Calcium uncovers the binding sites on the thin actin filaments.
Sliding filaments
Myosin heads grab actin, pull, let go and grab again, using ATP for each cycle. The filaments slide past each other, so the sarcomere shortens while the filaments keep their length.
Relaxation
When the nerve signal stops, acetylcholinesterase breaks down acetylcholine and calcium is pumped back into storage. Relaxation also needs ATP, which is why muscles stiffen after death (rigor mortis).
At the bedside
Neuromuscular blockers used for intubation block the acetylcholine receptor, and myasthenia gravis attacks the same receptor. Weakness that worsens with repeated effort points to a problem at the junction.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).