Muscle Tissue – Vocabulary Review

These vocabulary flashcards cover key structures, proteins, physiological processes, and clinical terms introduced in the lecture on skeletal, cardiac, and smooth muscle tissues.

Skeletal Muscle

Striated, voluntary muscle attached to bones (or skin for facial muscles); capable of powerful contractions.

Cardiac Muscle

Striated, involuntary muscle found only in the heart; connected by intercalated discs.

Smooth Muscle

Non-striated, involuntary muscle in walls of hollow organs and vessels; spindle-shaped cells.

Striations

Alternating dark A bands and light I bands seen in skeletal and cardiac muscle fibers.

Voluntary Muscle

Muscle whose contractions are consciously controlled; skeletal muscle.

Involuntary Muscle

Muscle that contracts without conscious control; cardiac and smooth muscle.

Muscle Fiber

Individual skeletal muscle cell; long, cylindrical, multinucleated.

Sarcolemma

Plasma membrane of a muscle fiber.

Myofibril

Rod-like contractile element inside a muscle fiber; composed of repeating sarcomeres.

Sarcomere

Smallest contractile unit; region between two Z discs in a myofibril.

Thick Filament

Filament made of myosin molecules; spans entire A band.

Thin Filament

Filament made chiefly of actin plus tropomyosin and troponin; spans entire I band and part of A band.

Myosin

Motor protein forming thick filaments; has heads that bind actin and ATP.

Actin

Protein forming thin filaments; contains binding sites for myosin heads.

Tropomyosin

Regulatory protein that blocks actin’s active sites in resting muscle.

Troponin

Regulatory protein that binds calcium and moves tropomyosin off actin’s active sites.

Z Disc

Protein boundary of a sarcomere; anchors thin filaments.

H Zone

Central lighter region in A band where thick and thin filaments do not overlap.

M Line

Protein line in center of H zone that holds thick filaments in place.

Sarcoplasmic Reticulum (SR)

Smooth ER in muscle fibers; stores and releases calcium ions.

T Tubule

Sarcolemma invagination that conducts action potentials into fiber interior.

Triad

T tubule flanked by two terminal cisterns of SR; site of excitation-contraction coupling.

Sliding Filament Theory

Model in which myosin heads pull actin filaments toward the M line, shortening sarcomeres.

Excitation-Contraction Coupling

Sequence linking muscle fiber action potential to cross-bridge cycling and contraction.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron axon terminal and a skeletal muscle fiber.

Acetylcholine (ACh)

Neurotransmitter released at NMJ to trigger muscle action potential.

Synaptic Cleft

Fluid-filled gap between axon terminal and muscle fiber at NMJ.

End Plate Potential

Local depolarization of sarcolemma produced by ACh-gated ion flow.

Depolarization

Membrane potential becomes less negative (toward +); initiated by sodium influx.

Repolarization

Return of membrane potential to resting level, mainly via potassium efflux.

Refractory Period

Time during repolarization when a fiber cannot be restimulated.

Motor Unit

One motor neuron and all the muscle fibers it innervates.

Recruitment (Motor Unit Summation)

Increasing force by activating additional motor units.

Muscle Twitch

Response of a muscle to a single threshold stimulus; has latent, contraction, and relaxation phases.

Temporal (Wave) Summation

Increased force from rapid stimulus frequency before muscle fully relaxes.

Tetanus (Complete)

Smooth, sustained contraction produced by high-frequency stimulation with no relaxation.

Isotonic Contraction

Muscle changes length while tension overcomes load; concentric or eccentric.

Isometric Contraction

Muscle develops tension but does not change length; load not moved.

Muscle Tone

Constant, slight contraction of muscles at rest maintained by spinal reflexes.

Creatine Phosphate

High-energy molecule that donates phosphate to ADP for rapid ATP regeneration (≈15 s).

Anaerobic Glycolysis

ATP production without oxygen; converts glucose to lactic acid, yields 2 ATP (≈30–60 s).

Aerobic Respiration

ATP generation using oxygen in mitochondria; yields ~32 ATP per glucose, supports prolonged activity.

Muscle Fatigue

Physiological inability to contract due to ionic imbalances or ATP deficit.

Excess Post-exercise Oxygen Consumption (EPOC)

Extra oxygen needed after exercise to restore metabolic conditions (formerly “oxygen debt”).

Slow Oxidative Fiber

Red, fatigue-resistant fiber; slow contraction, high myoglobin, aerobic ATP production.

Fast Oxidative Fiber

Intermediate fiber; fast contraction, aerobic metabolism, moderate fatigue resistance.

Fast Glycolytic Fiber

White, fast-fatigable fiber; fast contraction, low myoglobin, anaerobic ATP supply.

Peristalsis

Alternating contraction of longitudinal and circular smooth muscle layers that propels contents.

Caveolae

Pouch-like plasma membrane infoldings in smooth muscle that store extracellular Ca²⁺.

Dense Bodies

Protein plaques in smooth muscle that anchor thin filaments; analogous to Z discs.

Calmodulin

Calcium-binding protein in smooth muscle that activates myosin light chain kinase.

Myosin Light Chain Kinase (MLCK)

Enzyme activated by Ca²⁺-calmodulin; phosphorylates myosin to initiate smooth muscle contraction.

Stress-Relaxation Response

Smooth muscle’s ability to stretch briefly then adapt to new length (e.g., bladder filling).

Hyperplasia (Smooth Muscle)

Increase in smooth-muscle cell number by mitosis, enlarging organs (e.g., uterus).

Intercalated Disc

Specialized junction connecting cardiac muscle cells; contains desmosomes and gap junctions.

Duchenne Muscular Dystrophy

X-linked disorder caused by lack of dystrophin; progressive skeletal muscle degeneration in males.

Sarcopenia

Age-related loss of skeletal muscle mass and strength; partially reversible with exercise.