Skeletal Muscle Physiology Flashcards

Comprehensive vocabulary flashcards covering the structure, regulatory proteins, neuromuscular transmission, excitation-contraction coupling, mechanics, and metabolic fiber types of skeletal muscle.

Sarcolemma

The plasma membrane of a skeletal muscle cell.

Sarcoplasm

The cytoplasm of a skeletal muscle cell.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum in skeletal muscle cells that sequesters and stores $Ca^{2+}$ during muscle relaxation.

Actin

The primary contractile protein of thin filaments, consisting of globular G-actin monomers that polymerize into two intertwined helical strands of F-actin.

Myosin

The contractile protein of thick filaments, featuring a tail of two heavy chains and heads containing light chains, an actin-binding site, and an ATP-hydrolyzing site (myosin ATPase).

Tropomyosin

A threadlike regulatory protein located in the groove of thin filaments that covers myosin-binding sites on actin when muscle is relaxed.

Troponin

A regulatory protein complex attached to tropomyosin with three subunits: Inhibitory (TnI), Tropomyosin-binding (TnT), and $Ca^{2+}$-binding (TnC).

Sarcomere

The basic functional contractile unit of a myofibril, demarcated from one Z disk to the adjacent Z disk.

A Band

The dark region of a sarcomere corresponding to the entire length of the thick filaments; its width does not shorten during muscle contraction.

I Band

The light region of a sarcomere comprising thin filaments not overlapped by thick filaments; centered on the Z disk and shortens during contraction.

H Zone

The central portion of the A band containing only thick filaments at rest, which shortens during muscle contraction.

Motor Unit

A single \null\text{\beta} or \null\text{\alpha} motor neuron and all the individual skeletal muscle fibers it innervates.

Motor Neuron Pool

The collection of all motor neurons that innervate the muscle fibers of a single whole muscle.

Motor End Plate

The specialized, highly folded region of the muscle sarcolemma that forms the postsynaptic component of the neuromuscular junction.

Nicotinic Acetylcholine Receptor ($N_1$ or $N_M$)

A ligand-gated, non-selective cation channel on the motor end plate that opens upon binding acetylcholine, allowing $Na^+$ influx and $K^+$ efflux.

End Plate Potential (EPP)

A large local depolarization generated at the motor end plate by acetylcholine-gated channel opening, which reaches threshold to trigger a muscle action potential.

Safety Factor

The extent to which an end plate potential exceeds the threshold potential required to fire a muscle membrane action potential.

Transverse Tubules (T Tubules)

Invaginations of the muscle sarcolemma that conduct action potentials into the interior of the muscle fiber near the sarcomeres.

Triad

A structural arrangement in skeletal muscle consisting of one T tubule flanked by two adjacent terminal cisternae of the sarcoplasmic reticulum.

Dihydropyridine Receptor (DHPR)

A modified voltage-gated $Ca^{2+}$ channel in the T-tubule membrane that acts as a voltage sensor physically linked to ryanodine receptors.

Ryanodine Receptor (RYR)

A $Ca^{2+}$ release channel located in the sarcoplasmic reticulum membrane that opens in response to conformational changes in DHPR.

SERCA (SR $Ca^{2+}$-ATPase)

A primary active transport pump that actively returns $Ca^{2+}$ from the sarcoplasm back into the sarcoplasmic reticulum to enable relaxation.

Calsequestrin

A $Ca^{2+}$-binding protein inside the sarcoplasmic reticulum lumen that sequesters $Ca^{2+}$ to lower the free $Ca^{2+}$ concentration gradient against SERCA.

Cross-Bridge Cycle

The cyclic four-step process of myosin binding to actin, flexing (power stroke), ATP-mediated detachment, and ATP hydrolysis to recock the head group.

Rigor Complex

The continuous, persistent attachment between actin and myosin heads that occurs in the absence of fresh ATP, causing rigor mortis.

Preload

The degree of stretch or muscle fiber length before contraction begins, which determines the baseline actin-myosin overlap and active force capacity.

Afterload

The force or resistance opposing muscle contraction, which decreases shortening velocity as its value increases.

Isometric Contraction

A contraction in which muscle length remains constant because the afterload equals or exceeds the maximum contractile force generated by the muscle.

Tetanus

A sustained, maximal muscle contraction without relaxation produced by high-frequency action potentials keeping cytosolic $Ca^{2+}$ elevated.

Myoglobin

An oxygen-binding protein located in muscle fibers that contains a single heme group and acts as an intracellular $O_2$ storage reservoir.

Slow-Oxidative Fibers (Type 1)

Slow-twitch muscle fibers characterized by low myosin-ATPase activity, high oxidative capacity, high myoglobin content, and high resistance to fatigue.

Fast-Oxidative-Glycolytic Fibers (Type 2A)

Fast-twitch muscle fibers possessing high myosin-ATPase activity, high oxidative capacity, intermediate glycolytic capacity, and moderate fatigue resistance.

Fast-Glycolytic Fibers (Type 2X)

Fast-twitch muscle fibers characterized by high myosin-ATPase activity, high glycolytic capacity, low myoglobin content, and rapid rate of fatigue.