Muscle Physiology: Structure, Excitation-Contraction Coupling, and Energetics

Vocabulary flashcards covering the structural proteins, excitation-contraction coupling mechanisms, electrical summation, metabolic pathways, and fiber types of skeletal muscle.

Myosin

The protein that exclusively forms the thick filament, consisting of a tail, a flexible hinge region that alters its angle to generate force, and a head that binds to actin.

Actin

The primary structural component of the thin filament, arranged like interconnected balls forming a rope-like structure that contains specific binding sites for myosin heads.

Tropomyosin

A string-like protein wrapped around actin filaments that covers and physically blocks myosin-binding sites under resting baseline conditions.

Troponin complex

A multi-protein complex located intermittently along tropomyosin that binds calcium ions, altering its conformation to pull tropomyosin away from actin binding sites.

Power stroke

The mechanical movement in muscle contraction where, following the release of a phosphate group from the myosin head, the hinge angle pivots to pull the actin filament and generate tension.

Sarcoplasmic reticulum

The specialized endoplasmic reticulum of muscle cells that serves as a massive internal compartment dedicated to storing and releasing calcium ions.

Sarcoplasm

The specialized cytoplasm of a muscle cell from which calcium ions are actively pumped into internal storage or out of the cell under baseline conditions.

Acetylcholine

The primary neurotransmitter released by motor neurons at neuromuscular junctions that binds to ionotropic receptors to trigger sodium influx and muscle membrane depolarization.

T-tubuli

Tubular invaginations of the muscle cell membrane that propagate action potentials deep into large muscle fibers to trigger mechanical opening of calcium channels on the sarcoplasmic reticulum.

Twitch

A single, isolated contraction of a muscle fiber that typically lasts approximately $20\,\text{ms}$, initiated by an action potential lasting $2\text{--}3\,\text{ms}$.

Summed twitch

The additive buildup of mechanical force observed when successive action potentials stimulate a muscle fiber before it has fully returned to baseline relaxation.

Tetanus

A sustained, maximal plateau of muscle force generation produced by high-frequency, repeated action potentials that continuously stimulate the muscle without intermediate relaxation.

Creatine phosphate

A vertebrate phosphagen molecule that buffers cellular energy by holding a high-energy phosphate group transferred from excess ATP during rest, supporting approximately $5\text{--}10\,\text{s}$ of contraction.

Creatine kinase

The enzyme that catalyzes the transfer of a phosphate group from creatine phosphate back to ADP to rapidly regenerate ATP during muscle activity.

Anaerobic metabolism

A rapid but inefficient metabolic pathway utilized when oxygen is low, yielding $2\,\text{ATP}$ and $2\,\text{lactate}$ molecules per molecule of glucose.

Aerobic metabolism

A highly efficient, sustainable pathway utilizing oxygen and the citric acid cycle to generate $30\,\text{ATP}$ per glucose molecule without producing lactic acid.

Slow oxidative fibers

Skeletal muscle fibers characterized by slow contraction speed, high fatigue resistance, small fiber diameter, abundant capillaries, and high myoglobin content for sustained activities like posture.

Fast glycolytic fibers

Muscle fibers adapted for rapid force generation through anaerobic pathways that fatigue quickly, characterized by large diameters, fewer capillaries, and low myoglobin content (e.g., eye movement muscles).

Fast oxidative fibers

An intermediate muscle fiber type that combines rapid twitch speed with oxidative metabolism, providing greater fatigue resistance than fast glycolytic fibers.

Myoglobin

An iron-containing, dark red, oxygen-binding protein located within muscle fibers that stores oxygen to facilitate aerobic metabolism.