Skeletal Muscle Fibers – Key Vocabulary

32 vocabulary flashcards summarizing critical terms and definitions from the lecture on skeletal muscle fibers, covering structure, energy sources, contraction mechanics, and fiber types.

Skeletal Muscle Fiber

A single skeletal muscle cell; long, multinucleated, and capable of contraction.

Sarcomere

The basic contractile unit in striated muscle, extending from one Z disc to the next.

Z Disc (Z Line)

Protein boundary of a sarcomere to which thin filaments anchor.

Thick Filament

Filament composed primarily of myosin molecules with protruding heads that form cross-bridges.

Thin Filament

Filament mainly made of actin (F-actin) strands, along with troponin and tropomyosin.

F-Actin

Filamentous polymer of actin subunits that forms the backbone of the thin filament.

H Zone

Central region of a sarcomere that contains only thick filaments; narrows during contraction.

Cross-Bridge

Temporary linkage between a myosin head and an actin binding site that generates force.

Length-Tension Relationship

Relationship between sarcomere length and the force a muscle can generate; optimal when all possible cross-bridges can form.

Overstretched Muscle Fiber

State in which sarcomere length is too long, leaving many myosin heads unable to bind actin, reducing force.

ATP (Adenosine Triphosphate)

Immediate energy source required for muscle contraction and relaxation.

Residual ATP

Small store of ATP already present in muscle fibers, used up within about one second of activity.

Substrate-Level Phosphorylation

Direct transfer of a phosphate group to ADP (often from creatine phosphate) to form ATP.

Creatine Phosphate

High-energy compound that donates a phosphate to ADP, rapidly regenerating ATP in muscle.

Glycolysis

Anaerobic breakdown of glucose to pyruvate (or lactate), producing a small, quick supply of ATP.

Lactic Acid Fermentation

Conversion of pyruvate to lactate under anaerobic conditions; can contribute to muscle fatigue.

Oxidative Phosphorylation

Aerobic ATP production in mitochondria involving electron transport chain, chemiosmosis, and ATP synthase; produces large ATP yield.

Twitch

A single contraction-relaxation cycle of a muscle fiber.

Latent Period

Brief delay (≈2 ms) between a muscle action potential and the onset of contraction.

Summation

Increased muscle tension produced by multiple, rapid action potentials that keep Ca²⁺ levels elevated.

Incomplete (Unfused) Tetanus

Sustained but wavering muscle contraction when stimuli are frequent but allow partial relaxation.

Complete (Fused) Tetanus

Maximal, smooth, sustained contraction produced by very high-frequency stimulation with no relaxation.

Motor Unit

One somatic motor neuron and all the skeletal muscle fibers it innervates.

Recruitment

Activation of additional motor units to increase overall muscle force.

Slow-Twitch Fiber (Type I)

Muscle fiber adapted for endurance; slow contraction, rich in mitochondria, myoglobin, and capillaries; relies on oxidative metabolism.

Fast-Twitch Fiber (Type II)

Muscle fiber adapted for rapid, powerful movements; fast contraction, larger diameter, primarily glycolytic metabolism.

Myosin ATPase

Enzymatic site on the myosin head that hydrolyzes ATP, powering the cross-bridge cycle.

Calcium ATPase (Ca²⁺ Pump)

ATP-driven pump in the sarcoplasmic reticulum membrane that returns Ca²⁺ to storage, enabling relaxation.

DHP Receptor

Voltage-sensitive receptor in the T-tubule that triggers opening of ryanodine receptors on the SR.

Ryanodine Receptor

Calcium-release channel in the sarcoplasmic reticulum that supplies Ca²⁺ for contraction.

Myoglobin

Oxygen-binding pigment in muscle fibers, especially abundant in slow-twitch fibers.