Myofibril

A myofibril is a threadlike contractile structure inside a muscle cell, built from repeating sarcomeres containing actin and myosin filaments.

Sarcomere: The repeating contractile unit of striated muscle, bounded by Z discs and containing organized actin and myosin filaments. Repeating sarcomeres are the structural units that make up a myofibril.
Skeletal muscle fiber: A long, multinucleated skeletal muscle cell containing many contractile myofibrils. Myofibrils are packed within each skeletal muscle fiber.
Muscle contraction: The process by which muscle generates tension, usually through interactions between actin and myosin. Myofibrils are the contractile machinery that produces muscle force.
Myofilament: A protein filament, such as actin or myosin, that forms part of a sarcomere. Myofilaments are components of sarcomeres, whereas myofibrils are chains of sarcomeres.
Sliding filament theory: The model explaining muscle contraction as actin filaments sliding past myosin filaments without either filament substantially shortening. It explains how sarcomeres shorten while their constituent filaments retain their lengths.
Thick filament: A sarcomere filament assembled mainly from myosin molecules. Thick filaments provide the motor proteins that interact with actin.
Muscle hypertrophy: An increase in muscle size, commonly involving growth of existing muscle fibers and their contractile components. Training-related growth can increase myofibrillar protein and contractile capacity.
Smooth muscle: A type of involuntary muscle whose cells lack the regular sarcomere pattern of striated muscle. Smooth muscle contracts without the organized myofibrils characteristic of striated muscle.
Myosin: A family of motor proteins that bind actin and use ATP to generate movement and force. Myosin heads pull on actin within each sarcomere.
Thin filament: A sarcomere filament made primarily of actin, with regulatory proteins tropomyosin and troponin. Thin filaments organize actin and its regulators within each sarcomere.

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