Sarcomere

A sarcomere is the repeating contractile unit of striated muscle, extending from one Z disc to the next. Its organized protein filaments generate force as they slide past one another.

Sliding filament theory: The model that explains muscle contraction as thin and thick filaments sliding past one another. It describes how sarcomeres shorten without the filaments themselves becoming shorter.
Z disc: A protein-rich boundary that anchors thin filaments and marks the ends of a sarcomere. Adjacent Z discs set the sarcomere’s repeating length.
Length–tension relationship: The relationship between a muscle’s length and the force it can produce. Sarcomere length determines filament overlap and therefore the number of possible cross-bridges.
Smooth muscle: Involuntary muscle tissue whose contractile filaments are not arranged into sarcomeres. Its contraction uses actin and myosin without the repeating banded architecture seen here.
Cross-bridge cycle: The cyclic binding, force-generating, and release actions of myosin heads on actin. Myosin cross-bridges produce the force that draws sarcomere filaments past each other.
M line: The central region of a sarcomere where proteins align and stabilize thick filaments. It anchors and organizes thick filaments at the sarcomere’s midpoint.
Muscle hypertrophy: An increase in muscle-fiber size, usually through growth of contractile and supporting components. Training-related growth changes the amount and organization of sarcomeric material.
Cardiac muscle: Striated, involuntary muscle tissue that forms the walls of the heart. It contains sarcomeres like skeletal muscle, but contracts through specialized cardiac excitation and control.
Actin: A protein that forms thin filaments and participates in force production in muscle cells. Thin filaments anchored at Z discs provide the tracks myosin pulls.
A band: The sarcomere region spanning the full length of thick filaments, including overlap with thin filaments. Its width remains nearly constant as filaments slide during contraction.

Linked from 15 pages