Muscular System: Histology and Physiology

A comprehensive set of vocabulary flashcards summarizing essential terms and definitions from Chapter 9 ‑ Muscular System: Histology and Physiology.

Skeletal Muscle

Voluntary or reflexive striated muscle attached to bones; long, multinucleated fibers that move the body and generate heat.

Smooth Muscle

Involuntary, non-striated, spindle-shaped muscle found in walls of hollow organs and vessels; controls movement of substances and maintains tone.

Cardiac Muscle

Involuntary, striated, branched muscle of the heart; contains intercalated disks and is autorhythmic to pump blood.

Endomysium

Loose connective tissue surrounding each individual skeletal muscle fiber.

Perimysium

Loose connective tissue that encases a bundle (fascicle) of skeletal muscle fibers.

Epimysium

Dense connective tissue sheath enclosing an entire skeletal muscle; merges with fascia.

Tendon

Cordlike bundle of collagen connecting muscle to bone, formed by fusion of connective tissue layers.

Aponeurosis

Broad, flat sheet of connective tissue that attaches muscle to bone or other muscles.

Motor Neuron

Nerve cell that stimulates muscle fibers to contract.

Motor Unit

One motor neuron and all the muscle fibers it innervates; determines precision and force of contraction.

Sarcolemma

Plasma membrane of a muscle cell.

Transverse (T) Tubule

Invagination of sarcolemma that carries action potentials into the muscle fiber interior.

Sarcoplasmic Reticulum (SR)

Specialized smooth ER in muscle fibers that stores and releases Ca²⁺.

Terminal Cisternae

Enlarged SR sacs adjacent to a T-tubule; part of the triad.

Triad

Structure consisting of a T-tubule flanked by two terminal cisternae; site of excitation–contraction coupling.

Myofibril

Cylindrical bundle of myofilaments within a muscle fiber responsible for contraction.

Myofilament

Protein filament (thick or thin) that slides during muscle contraction.

Thick Myofilament

Filament composed mainly of myosin molecules with protruding heads.

Thin Myofilament

Filament made of actin, tropomyosin, and troponin proteins.

Myosin

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

Actin

Globular protein forming thin filaments; contains active sites for myosin binding.

Tropomyosin

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

Troponin

Regulatory protein with subunits binding actin, tropomyosin, and Ca²⁺ to control contraction.

Sarcomere

Basic contractile unit of striated muscle extending from one Z disk to the next.

Z Disk

Protein structure anchoring thin filaments and defining sarcomere boundaries.

M Line

Central line in a sarcomere that stabilizes thick filaments.

H Zone

Region of thick filaments with no thin filament overlap; disappears on full contraction.

A Band

Dark band encompassing the entire length of thick filaments, including overlap areas.

I Band

Light band containing only thin filaments and the Z disk.

Sliding Filament Model

Concept that thick and thin filaments slide past one another, shortening sarcomeres during contraction.

Excitability

Ability of muscle tissue to respond to stimuli by generating action potentials.

Contractility

Capacity of muscle to shorten forcefully.

Extensibility

Ability of muscle to be stretched beyond resting length and still contract.

Elasticity

Capability of muscle to recoil to original length after stretching.

Resting Membrane Potential

Polarized electrical charge across a cell membrane at rest (≈ –85 mV in muscle).

Depolarization

Phase of action potential where Na⁺ influx makes the inside of the membrane less negative.

Repolarization

Return of membrane potential toward negative values due to K⁺ efflux.

Hyperpolarization

Membrane potential becomes briefly more negative than resting level because of slow K⁺ channel closure.

Ligand-Gated Ion Channel

Membrane channel opened by chemical messengers (e.g., ACh).

Voltage-Gated Ion Channel

Channel that opens or closes in response to changes in membrane voltage.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and a skeletal muscle fiber.

Acetylcholine (ACh)

Neurotransmitter released at NMJ to trigger muscle action potentials.

Cross-Bridge Formation

Attachment of myosin heads to actin active sites during contraction.

Power Stroke

Pivot of myosin head pulling actin filament toward sarcomere center.

ATP

Primary energy molecule required for cross-bridge cycling and detachment.

Hypertrophy

Increase in muscle fiber size due to exercise or other stimuli.

Atrophy

Decrease in muscle fiber size from disuse or disease.

Lag Phase

Time between stimulus application and onset of muscle contraction.

Contraction Phase

Period during which tension increases as cross-bridges form and power strokes occur.

Relaxation Phase

Period when tension decreases as Ca²⁺ is re-sequestered into SR and cross-bridges detach.

Isotonic Contraction

Muscle generates force while changing length.

Concentric Contraction

Type of isotonic contraction where the muscle shortens.

Eccentric Contraction

Isotonic contraction in which the muscle lengthens under tension.

Isometric Contraction

Muscle develops tension without changing length.

Twitch

Single, brief contraction–relaxation cycle of a muscle fiber.

Treppe

Stepwise increase in contraction strength with repeated stimuli (staircase effect).

Wave Summation

Addition of successive muscle twitches without full relaxation, increasing tension.

Incomplete Tetanus

Partial relaxation between high-frequency stimuli producing fluctuating tension.

Complete Tetanus

Sustained contraction with no relaxation due to very high stimulus frequency.

Slow-Twitch (Type I) Fiber

Fatigue-resistant muscle fiber with high myoglobin and oxidative capacity; suited for endurance.

Fast-Twitch (Type II) Fiber

Rapid, powerful muscle fiber with low myoglobin and glycolytic metabolism; fatigues quickly.

Creatine Phosphate

High-energy compound that rapidly donates phosphate to ADP to regenerate ATP.

Anaerobic Respiration

ATP production without oxygen, yielding lactic acid; supports short, intense activity.

Aerobic Respiration

Oxygen-requiring ATP production in mitochondria; efficient for prolonged activity.

Oxygen Deficit

Gap between onset of exercise and increased oxygen uptake; leads to post-exercise O₂ consumption.

Muscle Fatigue

Decline in muscle’s ability to generate force, often from ATP depletion or lactic acid buildup.

Physiological Contracture

State where lack of ATP prevents both contraction and relaxation of muscle fibers.

Psychological Fatigue

Perceived inability to continue activity originating from the CNS.

Reactive Oxygen Species (ROS)

Chemically reactive molecules causing oxidative stress and contributing to muscle fatigue.

Smooth Muscle Tone

Sustained, low-level contraction characteristic of smooth muscle walls.

Calmodulin

Ca²⁺-binding protein that activates myosin kinase in smooth muscle.

Myosin Kinase

Enzyme that phosphorylates myosin heads in smooth muscle to initiate contraction.

Myosin Phosphatase

Enzyme that dephosphorylates myosin heads, causing smooth muscle relaxation.

Caveolae

Sarcolemmal indentations in smooth muscle functioning similarly to T-tubules.

Dense Bodies

Cytoplasmic structures in smooth muscle that anchor actin filaments (analogous to Z disks).

Autorhythmic

Capability of certain muscle cells (e.g., cardiac) to generate spontaneous action potentials.

Intercalated Disk

Specialized junction connecting cardiac muscle cells, containing gap junctions and desmosomes.

Gap Junction

Protein channel allowing electrical coupling and ion flow between adjacent cells, prominent in cardiac and some smooth muscle.

Myoglobin

Oxygen-binding pigment in muscle that stores O₂, abundant in slow-twitch fibers.

Adenylate Kinase

Enzyme that converts 2 ADP into 1 ATP and 1 AMP, providing immediate energy.

Oxidative Stress

Cellular damage from excess ROS and free radicals, implicated in fatigue and soreness.

Rigor Mortis

Post-mortem muscle stiffness due to lack of ATP preventing cross-bridge detachment.

Aging Effects on Muscle

Includes reduced mass, decreased capillary density, slower contractions, and loss of fast-twitch fibers.