Muscle's smallest motor unit is a striped molecular machine
A sarcomere is the repeating contractile segment between two Z-lines in striated muscle. Thick myosin and thin actin filaments slide to shorten it; smooth muscle skips this lattice. Van Leeuwenhoek first noted the stripes that sarcomeres paint across skeletal and cardiac fibers.
The name comes from Greek words for flesh and part. Skeletal muscle is built from long cells called fibres, formed in the embryo; each fibre is packed with myofibrils, and each myofibril is a chain of sarcomeres, the repeating units that make skeletal and heart muscle look striped under a microscope. Van Leeuwenhoek was the first to describe those stripes. Smooth muscle, by contrast, does not organise its myofibrils this way.
Each unit runs from one Z-line to the next; the letter comes from the German for between, and the line anchors the thin actin filaments. Nearest the Z-line sits the pale I-band, where thin filaments lack any thick-filament overlap. Next comes the dark A-band, which spans a whole thick myosin filament, about 1.85 micrometres long in mammalian skeletal muscle. In its middle is the brighter H-zone, where no actin reaches, split by the M-line, whose name means middle. Two giant proteins act as rulers: titin, the largest highly elastic protein known, stretches from Z-line to M-band, while nebulin runs along the thin filaments.
Contraction is a sliding act. A myosin head splits ATP, cocks into a high-energy shape, grabs actin and pulls, so the I-bands and H-zone narrow while the A-band keeps its length and the Z-lines draw together. At rest, tropomyosin blocks the binding sites on actin. When a motor neuron releases acetylcholine, an electrical signal races along the T-tubules, calcium channels open, and ryanodine receptors dump calcium from the sarcoplasmic reticulum, a step called calcium-induced calcium release. Calcium binds troponin C, which shifts tropomyosin aside.
Relaxation follows when pumps return calcium to storage. Sarcomere geometry also governs strength: overlap between the filaments produces the length-tension curve, so force drops if the muscle is overstretched, leaving fewer cross-bridges, or squashed until actin filaments collide. Longer sarcomeres can form more cross-bridges and exert more force.
Source: Sarcomere
