Chapter 10: Muscle Tissue

Comprehensive vocabulary flashcards covering skeletal, cardiac, and smooth muscle tissue anatomy, histology, electrophysiology, excitation-contraction coupling, sliding filament mechanism, motor units, energy metabolism, and muscle mechanics based on Chapter 10 lecture material.

Excitability

The universal muscle characteristic of responsiveness to chemical signals, stretch, and other stimuli by generating electric voltage changes across the plasma membrane.

Conductivity

The universal muscle characteristic wherein a local electrical change initiates a traveling wave of excitation (action potential) along a muscle fiber.

Contractility

The universal muscle characteristic that enables muscle cells to shorten substantially and pull on bones or other tissues when stimulated.

Extensibility

The universal muscle characteristic that allows a muscle cell to stretch between contractions without being damaged.

Elasticity

The universal muscle characteristic that allows a muscle cell to recoil to its original shorter resting length after being stretched.

Epimysium

The fibrous connective tissue sheath that surrounds an entire skeletal muscle organ and is continuous with tendons.

Perimysium

The thicker connective tissue sheath that wraps around bundles of muscle fibers, organizing them into individual fascicles.

Endomysium

A thin sleeve of loose connective tissue that surrounds each individual skeletal muscle fiber.

Fascicle

A discreet bundle of skeletal muscle fibers enclosed by the perimysium.

Muscle Fiber

A single elongated, multinucleated skeletal muscle cell containing densely packed myofibrils.

Myofibril

A long, cylindrical bundle of protein myofilaments occupying most of the sarcoplasm within a muscle fiber.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber, containing glycogen, myoglobin, and myofibrils.

Glycogen

An intracellular carbohydrate polymer stored within the sarcoplasm that provides energy during heightened muscular activity.

Myoglobin

A red oxygen-binding pigment protein located in the sarcoplasm that provides supplemental oxygen during muscular activity.

Sarcoplasmic Reticulum

The smooth endoplasmic reticulum of a muscle fiber that forms a network around each myofibril and functions as a reservoir for calcium ions.

Terminal Cisterns

Dilated cross-channels or end sacs of the sarcoplasmic reticulum that flank transverse tubules.

Transverse Tubules

Tubular infoldings of the sarcolemma that penetrate through the cell and across the myofibrils to carry electrical signals into the cell interior.

Triad

A three-unit anatomical complex consisting of one transverse tubule flanked on each side by a terminal cistern of the sarcoplasmic reticulum.

Thick Filaments

Myofilaments composed of several hundred molecules of myosin, each featuring a double-helical tail and two globular heads projecting outward.

Thin Filaments

Myofilaments composed primarily of intertwined strands of fibrous actin ($\text{F actin}$) along with tropomyosin and troponin complexes.

Tropomyosin

A regulatory protein on thin filaments that blocks the active binding sites of actin subunits when the muscle fiber is relaxed.

Troponin

A three-protein regulatory complex attached to tropomyosin that contains binding sites for calcium ions.

Sarcomere

The functional contractile unit of a muscle fiber, extending from one Z disc to the next adjacent Z disc.

Z Disc

A protein plaque providing an anchorage point for thin and elastic filaments that bisects the I band and marks the boundary of a sarcomere.

A Band

The dark-staining striation band consisting of parallel thick filaments lying side by side, overlapping partially with thin filaments.

H Band

The lighter region in the center of an A band where only thick filaments are present without overlapping thin filaments.

M Line

A dark transverse protein line in the middle of the H band where thick filaments are linked together.

I Band

The light-staining striation band composed solely of thin filaments, elastic filaments, and bisected by a Z disc.

Titin

A gigantic elastic protein that forms elastic filaments running through the core of thick filaments to anchor them to the Z disc and M line.

Dystrophin

A large protein beneath the sarcolemma that links the outermost actin myofilaments to transmembrane proteins and the endomysium.

Motor Unit

A functional unit consisting of a single somatic motor neuron and all the skeletal muscle fibers innervated by its collateral axon branches.

Somatic Motor Fibers

The axons of somatic motor neurons that lead from the brainstem and spinal cord to skeletal muscles, supplying an average of $\sim 200$ muscle fibers each.

Size Principle of Motor Units

The physiological principle stating that smaller, more sensitive motor units are recruited first, whereas larger, less excitable motor units require stronger central nervous system stimulation.

Neuromuscular Junction

The specialized synapse where the axon terminal of a somatic motor neuron meets a target skeletal muscle fiber.

Motor End Plate

The specialized, folded depression of the sarcolemma directly across from the axon terminal at the neuromuscular junction.

Acetylcholine

The neurotransmitter synthesized by somatic motor neurons and released from synaptic vesicles to stimulate muscle fibers.

Junctional Folds

Accordion-like invaginations of the sarcolemma at the motor end plate that substantially increase surface area for acetylcholine receptors.

Synaptic Cleft

The microscopic narrow space separating the synaptic knob of the motor neuron from the motor end plate of the muscle fiber.

Acetylcholinesterase

An enzyme located in the synaptic cleft and sarcolemma that rapidly degrades acetylcholine into acetic acid and choline to stop muscle stimulation.

Resting Membrane Potential

The stable electrical potential difference across the sarcolemma of an unstimulated resting muscle cell, equal to approximately $-90\text{ mV}$.

Action Potential

A rapid, transient shift in electrical voltage across a cell membrane triggered by the sequential opening and closing of voltage-gated ion channels.

End-Plate Potential

A rapid local depolarization of the motor end plate from $-90\text{ mV}$ to roughly $-60\text{ mV}$ caused by the opening of ligand-regulated ion channels upon acetylcholine binding.

Excitation-Contraction Coupling

The sequence of physiological events connecting the action potential on the sarcolemma and down T tubules to the activation of cross-bridge cycling by calcium.

Sliding Filament Theory

The model of muscle contraction asserting that myofilaments do not change in length; rather, thin filaments slide past thick filaments toward the center of the sarcomere.

Recovery Stroke

The process where the hydrolysis of ATP to ADP and inorganic phosphate ($\text{P}_i$) recocks the myosin head into a high-energy position.

Cross-Bridge

The physical link formed when a cocked myosin head binds to an exposed active site on an actin molecule.

Power Stroke

The mechanical movement in which the myosin head releases ADP and $\text{P}_i$, bending toward its low-energy position and dragging the thin filament toward the center of the sarcomere.

Muscle Tone

A state of partial, steady sustained contraction maintained by the central nervous system to keep resting muscles near their optimal length.

Muscle Twitch

A quick cycle of contraction and relaxation in a muscle fiber produced by a single action potential, typically lasting $7-100\text{ ms}$.

Myogram

A graphical recording that plots the timing and strength of a muscle twitch or contraction over time.

Latent Period

The brief interval between the delivery of a stimulus and the onset of mechanical tension, encompassing excitation and excitation-contraction coupling.

Wave Summation

The phenomenon whereby each new muscle twitch adds to the previous one to produce greater aggregate tension when stimuli arrive before relaxation is complete.

Incomplete Tetanus

A sustained muscular contraction pattern produced by high-frequency stimulation where the muscle relaxes only partially between twitches.

Complete Tetanus

A smooth, sustained contraction devoid of any intermediate relaxation twitches, achieved only at unnaturally high stimulation frequencies.

Isometric Contraction

A muscle contraction where internal tension develops without a change in overall muscle length or joint angle.

Isotonic Contraction

A muscle contraction in which internal tension remains constant while the muscle changes length.

Concentric Contraction

A form of isotonic contraction wherein the muscle shortens while maintaining internal tension against a load.

Eccentric Contraction

A form of isotonic contraction wherein the muscle lengthens while maintaining internal tension to control the descent of a load.

Anaerobic Fermentation

A pathway of cellular energy generation that produces ATP ($2\text{ ATP}$ per glucose) in the absence of oxygen, yielding lactate as a byproduct.

Aerobic Respiration

An oxygen-dependent metabolic pathway producing substantial ATP ($\sim 30\text{ ATP}$ per glucose) within mitochondria, yielding carbon dioxide and water with no lactate.

Phosphagen System

An immediate ATP-generating system in muscle utilizing myokinase and creatine kinase to sustain maximum activity for roughly $10\text{ seconds}$.

Myokinase

An enzyme in the phosphagen system that transfers a phosphate group from one ADP to another to create ATP and AMP.

Creatine Kinase

An enzyme in the phosphagen system that transfers a high-energy phosphate group from creatine phosphate to ADP, forming ATP and creatine.

Glycogen-Lactate System

The anaerobic pathway that converts glycogen to glucose and then to lactate, producing enough ATP to sustain maximal muscle activity for $30-40\text{ seconds}$.

Excess Postexercise Oxygen Consumption

The elevated rate of oxygen consumption following exercise needed to repay oxygen debt, restore myoglobin and creatine phosphate, and clear lactate.

Type I Muscle Fibers

Slow-twitch, fatigue-resistant muscle fibers rich in myoglobin, mitochondria, and capillaries, specialized for aerobic cellular respiration.

Type II Muscle Fibers

Fast-twitch, low-endurance muscle fibers possessing thick diameters, abundant glycogen, and high creatine phosphate, specialized for anaerobic fermentation.

Type IIA Muscle Fibers

Rare intermediate muscle fibers combining rapid contraction velocity with oxidative fatigue resistance.

Multiple Motor Unit Summation

The recruitment of additional motor units within a nerve as stimulus voltage increases, resulting in greater contractile strength in the whole muscle.

Muscle Fatigue

The progressive loss of contractile strength and efficiency resulting from prolonged muscle exertion.

Central Fatigue

A decrease in motor signals sent from the central nervous system during prolonged exercise, influenced in part by ammonia release from working muscles.

$$\text{VO}_2\text{ max}$$

The maximum rate of oxygen consumption attained during exercise, marked by the point where oxygen uptake plateaus despite increasing workload.

Resistance Training

Exercise involving high-load contractions against heavy resistance that stimulates muscle hypertrophy by adding myofilaments and myofibrils.

Endurance Training

Low-intensity, long-duration conditioning that expands glycogen stores, enhances slow-twitch fibers, and increases mitochondrial and capillary density.

Cardiomyocytes

Striated, involuntary, uninucleated muscle cells of the heart that branch and connect end-to-end at intercalated discs.

Intercalated Discs

Complex intercellular junctions linking adjacent cardiomyocytes that consist of mechanical desmosomes and electrical gap junctions.

Myocytes

Involuntary, non-striated, fusiform-shaped single cells comprising smooth muscle tissue.

Dense Bodies

Protein structures scattered through the sarcoplasm and plasma membrane of smooth muscle cells that anchor actin myofilaments, replacing Z discs.

Caveolae

Numerous shallow invaginations of the smooth muscle sarcolemma containing gated calcium channels through which extracellular calcium enters the cell.

Latch-Bridge Mechanism

A specialized property of smooth muscle myosin heads that allows them to remain bound to actin for prolonged periods without consuming extra ATP.

Calmodulin

The calcium-binding regulatory protein in smooth muscle that replaces troponin and activates myosin light-chain kinase.

Myosin Light-Chain Kinase

An enzyme activated by the calcium-calmodulin complex that phosphorylates the regulatory protein on the smooth muscle myosin head, enabling ATP hydrolysis.

Multiunit Smooth Muscle

A category of smooth muscle found in large arteries and pulmonary passages where each myocyte forms an independent motor unit innervated by a nerve fiber.

Single-Unit Smooth Muscle

The common visceral type of smooth muscle where myocytes are electrically coupled by gap junctions and contract synchronously as a functional unit.

Varicosity

Periodic swollen beads along autonomic motor nerve fibers containing synaptic vesicles that release neurotransmitters into smooth muscle tissue.

Stress-Relaxation Response

The physiological mechanism in smooth muscle whereby stretching prompts a brief contraction followed by relaxation, permitting hollow organs to fill gradually without emptying.

Smooth Muscle Plasticity

The ability of smooth muscle to adjust its tension over a wide range of stretch lengths, preventing an emptied hollow organ from becoming flabby.