Sliding Filament Theory & Muscle Contraction - Vocabulary Flashcards

A set of vocabulary flashcards covering key concepts, structures, and processes from the lecture notes on the sliding filament theory and muscle energy systems.

Sarcomere

The repeating contractile unit of a myofibril, bounded by Z-lines and containing actin (thin) and myosin (thick) filaments.

Myofibril

Long, rod-like organelles inside muscle fibers composed of many sarcomeres in series; responsible for contraction.

Myofilament

Protein filaments (actin and myosin) that form the contractile elements of the sarcomere.

Actin (Thin Filament)

Thin filament that interacts with myosin during contraction; sites on actin bind myosin heads.

Myosin (Thick Filament)

Thick filament with protruding heads that form cross-bridges with actin during contraction.

Cross-Bridge

Attachment of a energized myosin head to an actin filament during the contraction cycle.

Power Stroke

Pivoting of the myosin head that pulls the actin filament toward the center of the sarcomere.

Cocked Head (Pre-stroke Position)

Energetically primed myosin head after ATP hydrolysis, ready to bind to actin.

Troponin

Regulatory protein that binds calcium and, with tropomyosin, exposes actin’s myosin-binding sites.

Tropomyosin

Regulatory protein that blocks actin’s myosin-binding sites when the muscle is relaxed.

Calcium (Ca2+)

Ion released from the sarcoplasmic reticulum that binds to troponin to enable contraction.

Troponin-Tropomyosin Complex

Regulatory complex that controls access to actin’s myosin-binding sites in response to Ca2+.

Sarcoplasmic Reticulum (SR)

Organelle surrounding myofibrils that stores, releases, and reabsorbs Ca2+ during contraction.

Transverse Tubules (T-tubules)

Invaginations of the sarcolemma that transmit action potentials into the muscle fiber to trigger Ca2+ release from SR.

Sarcolemma

Plasma membrane of a muscle fiber.

Neuromuscular Junction

Synapse where a motor neuron releases acetylcholine to stimulate a muscle fiber.

Acetylcholine (ACh)

Neurotransmitter released at the neuromuscular junction to initiate muscle excitation.

Acetylcholinesterase

Enzyme that breaks down acetylcholine to terminate the signal and end contraction.

Excitation-Contraction Coupling (EC Coupling)

Sequence by which an action potential triggers Ca2+ release, leading to muscle contraction.

Cross-Bridge Cycle

Series of events: cross-bridge formation, power stroke, detachment with ATP, and re-cocking of the myosin head.

Z-Line (Z-disc)

Boundary of a sarcomere; anchors actin filaments and marks the start/end of a sarcomere.

I-Band

Light region of the sarcomere containing thin filaments; shortens during contraction.

A-Band

Dark region containing thick filaments; length remains constant during contraction and includes overlap with thin filaments.

H-Zone

Central part of the A-band containing only thick filaments; narrows during contraction.

M-Line

Center line of the sarcomere that holds thick filaments together.

Sliding Filament Theory

Muscle contraction occurs as actin slides past myosin, shortening the sarcomere without shortening the filaments themselves.

Phase 1: Neuromuscular Junction

Event where nerve impulse triggers ACh release and motor neuron initiates muscle excitation.

Phase 2: EC Coupling & Cross-Bridge Cycle

Calcium release, troponin-tropomyosin shift, cross-bridge formation, power stroke, detachment, and head re-cocking.

Phase 3: Relaxation

Acetylcholine is degraded, Ca2+ is reabsorbed, active sites are blocked, and the muscle relaxes.

ATP (Adenosine Triphosphate)

Primary energy source for muscle contraction; powers cross-bridge cycling and detachment.

Creatine Phosphate (CP)

Immediate, small-energy source that donates phosphate to ADP to form ATP for rapid bursts; lasts ~15 seconds.

Glycolysis

Anaerobic breakdown of glucose in the cytoplasm to pyruvate, yielding net 2 ATP; can occur with or without oxygen.

Pyruvate

End product of glycolysis; can enter mitochondria to form acetyl-CoA or be reduced to lactate under anaerobic conditions.

Acetyl-CoA

Product of pyruvate oxidation that enters the Krebs cycle for aerobic respiration.

Krebs Cycle (Citric Acid Cycle)

Mitochondrial cycle producing NADH, FADH2, and a small amount of ATP per glucose.

NADH & FADH2

Electron carriers produced by glycolysis and the Krebs cycle that fuel the Electron Transport Chain.

Electron Transport Chain (ETC)

Series of protein complexes in the mitochondrial membrane that produces the bulk of ATP via oxidative phosphorylation.

Aerobic Respiration

Cellular respiration with oxygen; yields the most ATP and occurs in mitochondria through glycolysis, Krebs cycle, and ETC.

Anaerobic Respiration / Lactic Acid Fermentation

Glycolysis without oxygen; produces a small amount of ATP and lactate that can be reused later by the liver.

Lactic Acid

End product of anaerobic glycolysis; accumulates during intense exercise and is processed by the liver.

Cori Cycle

Process by which lactate produced in muscles is converted to glucose in the liver for reuse.