Muscle structure
A skeletal muscle is made of bundles (fascicles) of long cells called muscle fibres. Each fibre contains many myofibrils, chains of repeating units named sarcomeres, the smallest contractile units. A sarcomere is bounded by Z discs and contains two kinds of protein filament: thin filaments (mostly actin, with the regulatory proteins troponin and tropomyosin) and thick filaments (myosin). Dark A bands match the length of the thick filaments, while lighter I bands contain only thin filaments.
Sliding filament theory
Muscle shortens because filaments slide past each other, not because they themselves shorten. Myosin heads form cross-bridges with actin, pivot, and pull the thin filaments toward the sarcomere centre. The A band stays the same length, while the I band and the H zone narrow.
From nerve to contraction
- A motor neuron releases acetylcholine at the neuromuscular junction.
- An action potential spreads over the sarcolemma and down the T tubules.
- The sarcoplasmic reticulum releases calcium ions into the cell.
- Calcium binds troponin, which moves tropomyosin and uncovers binding sites on actin.
- Myosin heads attach, perform the power stroke, and the sarcomere shortens.
- A fresh ATP binds myosin, which lets go of actin. The head is then re-energised and can attach again further along.
- When nerve signals stop, calcium is pumped back into the sarcoplasmic reticulum, the sites are covered, and the muscle relaxes.
Energy and organisation
Stored ATP lasts only seconds. Creatine phosphate quickly regenerates ATP, glycolysis provides more without needing oxygen, and aerobic respiration in mitochondria supplies the most ATP for longer activity. A motor unit is one motor neuron plus all the fibres it controls. When muscles move a joint, the prime mover (agonist) produces the movement, the antagonist opposes it, and synergists assist.
Worked example: why rigor mortis happens
After death, cells stop making ATP. Calcium leaks out of the sarcoplasmic reticulum, so myosin heads attach to actin. Normally a new ATP would release them; with no ATP available, the cross-bridges stay locked and the muscles stiffen. Later the proteins break down and the stiffness fades. Compare this with a living contraction: attachment needs calcium, but detachment needs ATP. Both are essential, and removing either one stops normal movement.