Chapter 09: Muscular System Histology and Physiology

Comprehensive vocabulary flashcards covering the anatomy, histology, physiological mechanisms, energy pathways, and clinical conditions of the muscular system from Chapter 09.

Contractility

The ability of a muscle to shorten with force.

Excitability

The capacity of muscle tissue to respond to an electrical or chemical stimulus, usually originating from nerves.

Extensibility

The ability of a muscle to be stretched beyond its normal resting length and still retain the capacity to contract.

Elasticity

The ability of a muscle to recoil to its original resting length after being stretched.

Skeletal Muscle Tissue (Histology)

Striated muscle tissue characterized by very long, cylindrical fibers ($1\,\text{mm}\text{--}4\,\text{cm}$, up to $30\,\text{cm}$ in length; $10\,\mu\text{m}\text{--}100\,\mu\text{m}$ in diameter) with multiple, peripherally located nuclei under voluntary control.

Smooth Muscle Tissue (Histology)

Nonstriated muscle tissue composed of spindle-shaped cells ($15\,\mu\text{m}\text{--}200\,\mu\text{m}$ in length; $5\,\mu\text{m}\text{--}8\,\mu\text{m}$ in diameter) with a single, centrally located nucleus and gap junctions joining visceral cells; operates under involuntary control.

Cardiac Muscle Tissue (Histology)

Striated, branched muscle tissue found only in the heart ($100\,\mu\text{m}\text{--}500\,\mu\text{m}$ in length; $12\,\mu\text{m}\text{--}20\,\mu\text{m}$ in diameter) containing single, centrally located nuclei, intercalated disks, and autorhythmic cells under involuntary control.

Epimysium

A dense layer of connective tissue that surrounds the entire muscle organ and merges with muscular fascia.

Muscular Fascia

The layer of connective tissue located between adjacent muscles and between muscles and the overlying skin.

Perimysium

A loose connective tissue layer that surrounds a group of muscle fibers, organizing them into bundles known as fascicles, and serves as a passage for blood vessels and nerves.

Fascicle

A bundle of individual muscle fibers within a whole muscle, surrounded and defined by the perimysium.

Endomysium

A delicate layer of loose connective tissue that encases and separates individual muscle fibers within each fascicle.

Myoblasts

Precursor cells that fuse together during embryonic development to produce large, multinucleated skeletal muscle cells.

Hypertrophy

An increase in the size of a whole muscle caused by the enlargement of existing muscle fibers, characterized by an increase in myofibrils and satellite cell nuclear fusion, rather than an increase in fiber count.

Sarcolemma

The plasma membrane of a muscle fiber that envelops the sarcoplasm and cellular contents.

Transverse Tubules (T Tubules)

Invaginations or inward tube-like folds of the sarcolemma that project into the interior of the muscle fiber to carry action potentials deep inside.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum in muscle fibers that actively stores and releases calcium ions ($\text{Ca}^{2+}$).

Terminal Cisternae

Enlarged portions of the sarcoplasmic reticulum situated immediately adjacent to the transverse tubules.

Triad

A specialized skeletal muscle structure composed of a central transverse tubule (T tubule) and its two flanking terminal cisternae.

Myofibril

A bundle of contractile protein filaments (myofilaments) running longitudinally within an individual muscle fiber.

Sarcomere

The basic structural and functional unit of a muscle fiber, extending from one Z disk to the next; it represents the smallest portion capable of contracting.

Z Disk

A filamentous network of protein forming a disc-shaped boundary that serves as the attachment point for actin myofilaments.

I Band

A light-staining region of the sarcomere containing only thin (actin) myofilaments and bisected by a Z disk, extending to the ends of the thick myofilaments.

A Band

The central dark-staining region of a sarcomere spanning the entire length of the myosin myofilaments, containing overlapping actin and myosin filaments except at its center.

H Zone

A lighter central region within the A band of a relaxed sarcomere that consists solely of myosin (thick) myofilaments without actin overlap.

M Line

A delicate protein line located in the center of the H zone that holds thick myosin myofilaments in place.

Titin Filaments

Elastic protein chains of amino acids within myofibrils that impart extensibility and elasticity to muscle tissue.

F Actin (Fibrous Actin)

A filament formed by two coiled strands of globular (G) actin monomers arranged in a double helix extending the length of the thin myofilament.

G Actin (Globular Actin)

A globular protein subunit that polymerizes to form F actin; each monomer features an active site that binds a myosin head during contraction.

Tropomyosin

An elongated filamentous protein that lies along the groove of the F actin double helix, covering active sites in resting muscle.

Troponin

A three-subunit protein complex bound to actin and tropomyosin with binding sites for G actin, tropomyosin, and calcium ions ($\text{Ca}^{2+}$).

Cross-Bridge

A physical connection formed when an energized myosin head attaches directly to an exposed active binding site on a G actin molecule.

Myosin ATPase

An enzymatic activity located on the myosin head that hydrolyzes adenosine triphosphate (ATP) into ADP and phosphate ($\text{P}_i$) to release energy for hinge movement.

Neuromuscular Junction

The specialized synapse or point of functional contact between an axon branch of a motor neuron and the sarcolemma of a skeletal muscle fiber.

Synaptic Cleft

The microscopic extracellular space separating the presynaptic terminal of a motor neuron from the postsynaptic motor end-plate.

Motor End-Plate

The folded region of the sarcolemma directly opposite the presynaptic terminal containing ligand-gated receptors for acetylcholine.

Acetylcholinesterase

An enzyme present in the synaptic cleft that rapidly hydrolyzes acetylcholine into acetic acid and choline, terminating synaptic stimulation.

Sliding Filament Model

The physiological concept describing how actin myofilaments slide past stationary myosin myofilaments, pulling Z disks closer together and shortening the sarcomere without changing individual filament lengths.

Resting Membrane Potential

The baseline electrical charge difference across the plasma membrane of an unstimulated cell, approximately $-85\,\text{mV}$ in skeletal muscle, maintained by ion gradients and the $\text{Na}^+\text{--}\text{K}^+$ pump.

Depolarization

A rapid shift in membrane voltage toward a less negative value (changing from roughly $-85\,\text{mV}$ to $+20\,\text{mV}$) driven by the influx of $\text{Na}^+$ through voltage-gated channels once threshold is reached.

Repolarization

The return of the membrane potential toward resting values (from approximately $+20\,\text{mV}$ back to $-85\,\text{mV}$) caused by the closure of voltage-gated $\text{Na}^+$ channels and the opening of voltage-gated $\text{K}^+$ channels.

All-or-None Principle

The physiological rule stating that if an electrical stimulus reaches or exceeds the threshold voltage, an action potential is produced completely; if threshold is not reached, no action potential occurs.

Excitation-Contraction Coupling

The sequential process linking the generation of an electrical action potential on the sarcolemma to the mechanical shortening of sarcomeres via intracellular $\text{Ca}^{2+}$ release.

Power Stroke

The mechanical event occurring when the hinged region of an attached myosin head bends, pulling the thin actin filament toward the center (H zone) of the sarcomere while releasing ADP and $\text{P}_i$.

Recovery Stroke

The repositioning of the myosin head back to its energized, high-energy state following the breakdown of a freshly bound ATP molecule into ADP and $\text{P}_i$.

Muscle Twitch

The mechanical contraction response of a single muscle fiber triggered by a single action potential along its motor neuron.

Lag Phase (Latent Phase)

The brief time interval between the application of an electrical stimulus and the initial onset of measurable muscle contraction.

Isometric Contraction

A type of muscle contraction in which internal tension increases while the total length of the muscle remains unchanged.

Isotonic Contraction

A muscle contraction characterized by a constant level of tension while the length of the muscle changes.

Concentric Contraction

An isotonic contraction in which tension generated by the muscle overcomes the opposing load, resulting in muscle shortening.

Eccentric Contraction

An isotonic contraction where tension is maintained but the opposing resistance is large enough to cause the contracting muscle to lengthen.

Motor Unit

A functional unit consisting of a single somatic motor neuron and every individual skeletal muscle fiber that its axon branches innervate.

Treppe

A graded increase in contraction force observed when a muscle is stimulated repeatedly with maximal stimuli at low frequencies after complete relaxation between twitches, partly caused by residual sarcoplasmic $\text{Ca}^{2+}$.

Wave Summation

An increase in muscle tension observed when the frequency of stimulation rises such that successive twitches begin before preceding twitches can fully relax.

Incomplete Tetanus

A state of sustained, undulating contraction produced by high-frequency stimulation in which the muscle fibers only partially relax between stimuli.

Complete Tetanus

A steady, smooth, maximal muscle contraction with no relaxation phase whatsoever, produced by extremely rapid action potential frequency.

Active Tension

The force applied to an object by a muscle when it actively contracts, which peaks when the sarcomere is at its optimal resting length ($80\%\text{--}120\%$ of resting length).

Passive Tension

The tension applied to a load when a muscle is stretched beyond its resting length without receiving electrical stimulation.

Multiple Motor-Unit Recruitment

The process of increasing whole muscle contraction force by activating an increasing number of individual motor units via progressively stronger stimuli.

Size Principle

The organizational rule stating that during motor unit recruitment, smaller motor units are activated first, followed sequentially by progressively larger motor units.

Muscle Tone

A continuous, partial state of tension exhibited by resting muscles for extended periods, maintained by asynchronous, out-of-phase contractions of motor units.

Slow-Twitch (Type I) Muscle Fibers

Small-diameter, fatigue-resistant muscle fibers that contract more slowly, possess abundant myoglobin, capillaries, and mitochondria, and primarily rely on aerobic respiration.

Fast-Twitch (Type II) Muscle Fibers

Large-diameter muscle fibers with fast myosin ATPase activity that contract rapidly, have fewer mitochondria and less blood supply, and fatigue quickly.

Adenylate Kinase

An enzyme that transfers a high-energy phosphate group between two ADP molecules to generate one ATP and one AMP, supplying energy for $2\text{--}3\,\text{seconds}$ of contraction.

Creatine Kinase

An enzyme that catalyzes the transfer of a phosphate group from creatine phosphate (CP) to ADP, generating ATP and creatine to support $5\text{--}6\,\text{seconds}$ of contraction.

Physiological Contracture

An extreme state of muscle fatigue caused by local ATP depletion in which cross-bridges cannot detach, preventing both contraction and relaxation.

Rigor Mortis

The rigid state of muscles developing several hours following death, caused by $\text{Ca}^{2+}$ leakage into the sarcoplasm and lack of ATP to detach cross-bridges, persisting until tissue deteriorates.

Excess Postexercise Oxygen Consumption (EPOC)

The prolonged elevated rate of oxygen intake occurring after physical activity ceases, required to repay the oxygen deficit and restore cellular homeostasis.

Caveolae

Shallow, pocket-like invaginations of the smooth muscle sarcolemma that function similarly to T tubules in sequestering extracellular $\text{Ca}^{2+}$.

Dense Bodies

Protein structures scattered throughout the sarcoplasm and attached to the sarcolemma of smooth muscle cells that serve as anchoring points for actin myofilaments, analogous to Z disks.

Calmodulin

A regulatory calcium-binding protein in smooth muscle cells that, when bound to $\text{Ca}^{2+}$, activates myosin kinase.

Myosin Kinase

An enzyme in smooth muscle activated by the $\text{Ca}^{2+}\text{-calmodulin}$ complex that phosphorylates myosin heads with ATP, allowing cross-bridge cycling to commence.

Myosin Phosphatase

An enzyme that dephosphorylates the myosin head in smooth muscle, causing muscle relaxation.

Latch State

A unique condition in smooth muscle where cross-bridges remain attached for prolonged periods after dephosphorylation, sustaining tension with minimal ATP consumption.

Visceral Smooth Muscle

A type of smooth muscle consisting of cell sheets coupled by numerous gap junctions that function as a single unit and frequently exhibit autorhythmic contractions.

Multiunit Smooth Muscle

A classification of smooth muscle found in sheets (blood vessels), bundles (arrector pili), or single cells where independent units contract separately with fewer gap junctions.

Intercalated Disks

Specialized cell junctions found exclusively in cardiac muscle tissue that interconnect adjacent cardiocytes through gap junctions and desmosomes.

Duchenne Muscular Dystrophy (DMD)

A genetic muscle disease characterized by muscle weakness, muscle atrophy, contractures, and skeletal deformities like kyphoscoliosis, with cardiac failure affecting up to $95\%$ of advanced cases.