Muscle contraction physiology
From a nerve signal to a muscle twitch, contraction is a chain of electrical and chemical events powered by ATP.
The neuromuscular junction
A motor neuron meets a muscle fibre at the neuromuscular junction. When a nerve impulse arrives, the neuron releases acetylcholine, which binds receptors on the sarcolemma, triggering an electrical impulse (action potential) in the muscle fibre.
Excitation-contraction coupling
The muscle action potential travels along the sarcolemma and down T-tubules, triggering the sarcoplasmic reticulum to release calcium ions. Calcium binds troponin, which shifts tropomyosin away from the binding sites on actin, so myosin can attach. When stimulation stops, calcium is pumped back and the muscle relaxes.
The role of ATP
ATP powers contraction: its energy cocks the myosin head, and a new ATP is needed for myosin to detach from actin. ATP also powers the pumps that return calcium. After death, ATP runs out, myosin cannot detach, and muscles lock: rigor mortis.
Energy sources
Muscles have only a few seconds' worth of stored ATP. Creatine phosphate quickly regenerates ATP for about 10-15 seconds of all-out effort. Anaerobic glycolysis then provides fast energy, producing lactate. For sustained activity, aerobic respiration in mitochondria supplies most ATP from glucose and fat.
Motor units and grading force
A motor unit is one motor neuron and all the fibres it controls. Fine movements, like eye muscles, use small motor units; powerful muscles like the thigh use large ones. The nervous system increases force by recruiting more motor units and by stimulating them more rapidly, which merges twitches into a sustained contraction (tetanus).
Types of contraction
In isotonic contractions the muscle changes length: concentric (shortening, as when lifting) or eccentric (lengthening under load, as when lowering). In isometric contractions tension rises but length stays the same, as when pushing against a wall or holding a plank.
Fibre types
Slow-twitch (type I) fibres are rich in mitochondria and myoglobin, resist fatigue, and suit endurance; marathon runners have many. Fast-twitch (type II) fibres contract quickly and powerfully but tire fast; sprinters have more. Training can shift fibre properties, but genetics sets much of the mix.
Worked example
A sprinter runs 100 metres in about 10 seconds. Stored ATP and creatine phosphate supply most of that burst, with anaerobic glycolysis contributing. A marathon runner relies on aerobic respiration in slow-twitch fibres, burning glucose and fat for hours.
Key takeaways
Acetylcholine at the neuromuscular junction triggers a muscle action potential. Calcium released from the sarcoplasmic reticulum binds troponin, exposing actin for myosin. ATP powers cross-bridge cycling and calcium pumping; its absence causes rigor mortis. Force is graded by motor unit recruitment and frequency. Contractions can be concentric, eccentric, or isometric.
Flashcard
Where nerve meets muscle
Check your understanding
Why do muscles stiffen in rigor mortis?