sarcomere
Summary
A sarcomere is the basic contractile unit of skeletal and cardiac muscle, extending from one Z-line to the next. It contains organized thick (myosin) and thin (actin) filaments that slide past each other to produce muscle contraction, per the sliding filament theory.
Detail
The sarcomere consists of the A-band (dark, contains myosin thick filaments and overlapping actin), I-band (light, contains only actin thin filaments, bisected by the Z-line), H-zone (central region of A-band with only myosin), and M-line (center of sarcomere where myosin filaments are anchored). Titin provides elastic recoil and structural stability, connecting Z-line to M-line, while nebulin regulates thin filament length. During contraction (via calcium-induced tropomyosin/troponin shift exposing actin-binding sites), the I-band and H-zone shorten while the A-band remains constant length—key for understanding muscle physiology questions. Cardiac sarcomeres differ from skeletal in having intercalated discs (gap junctions, desmosomes) enabling functional syncytium behavior and are relevant in understanding hypertrophic cardiomyopathy, caused by mutations in sarcomere proteins (beta-myosin heavy chain, myosin-binding protein C, troponin T)—an important board topic given its association with sudden cardiac death in young athletes. Sarcomere length-tension relationships also explain Frank-Starling mechanism in cardiac physiology and are tested via muscle physiology graphs showing force generation as a function of sarcomere length (optimal overlap of actin-myosin maximizes cross-bridge formation).
Sources
- Guyton and Hall Textbook of Medical Physiology
- First Aid for the USMLE Step 1
- Kaplan USMLE Physiology Lecture Notes
- Robbins Basic Pathology (for HCM sarcomere mutations)
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.