Muscular Tissue Structure, Contraction, and Physiology

Vocabulary flashcards covering muscular tissue anatomy, skeletal muscle fiber components, contraction mechanisms, bioenergetics, twitch dynamics, and muscle tissue types based on Chapter 11 notes.

Endomysium

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

Perimysium

A connective tissue sheath that bundles muscle fibers together into a fascicle.

Epimysium

A fibrous connective tissue sheath surrounding an entire skeletal muscle.

Fascia

A sheet of connective tissue that separates neighboring muscles or muscle groups from each other.

Fascicle

A bundle of parallel muscle fibers within a muscle, enclosed by perimysium.

Tendon

A collagenous band of connective tissue continuous with muscle sheaths that attaches muscle to the connective tissue of bone matrix.

Skeletal Muscle

A type of voluntary, striated muscle tissue usually attached to bones, composed of elongated multinucleate cells as long as $30\,cm$.

Striations

Alternating light and dark transverse bands in skeletal and cardiac muscle resulting from the internal arrangement of contractile proteins.

Myoblasts

Embryonic mesodermal cells that fuse together during development to form multinucleate skeletal muscle fibers.

Myosatellite Cells

Unfused myoblast cells that persist within adult muscle tissue to assist in muscle regeneration and repair.

Sarcolemma

The specialized plasma membrane surrounding a muscle fiber.

Sarcoplasm

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

Myoglobin

A reddish, oxygen-binding pigment unique to muscle fibers that temporarily stores oxygen for muscle activity.

Myofibrils

Long, cylindrical protein cords occupying most of the sarcoplasm, composed of bundles of myofilaments.

Sarcoplasmic Reticulum (SR)

The smooth endoplasmic reticulum of a muscle fiber that forms a network around each myofibril and functions as a reservoir for calcium ions ($Ca^{2+}$).

Terminal Cisternae

Dilated end-sacs of the sarcoplasmic reticulum that cross the muscle fiber from one side to the other.

Transverse Tubules (T tubules)

Tubular infoldings of the sarcolemma that penetrate into the sarcoplasm to carry electrical action potentials.

Triad

A cellular structure composed of one central T tubule flanked on each side by a terminal cisterna of the sarcoplasmic reticulum.

Contractile Proteins

Proteins, specifically myosin and actin, that directly generate tension during muscle contraction.

Regulatory Proteins

Proteins, specifically tropomyosin and troponin, that act like a switch to determine if and when contractile proteins can generate tension.

Structural Proteins

Proteins, such as titin and dystrophin, that anchor, align, and maintain the structural relationships of myofilaments.

Myosin

The primary protein forming thick filaments, consisting of a shaft-like tail and heads with binding sites for actin and ATP as well as ATPase activity.

Fibrous (F) Actin

A protein strand forming thin filaments composed of two intertwined chains of globular (G) actin subunits.

Tropomyosin

A regulatory protein on thin filaments that blocks the myosin-binding sites on actin when a muscle fiber is at rest.

Troponin

A regulatory protein complex bound to tropomyosin that contains a binding site for $Ca^{2+}$ ions.

Titin

A giant springy structural protein forming elastic filaments that runs through the core of thick filaments to prevent overstretching and provide elastic recoil.

Sarcomere

The functional, contractile unit of a myofibril, defined as the segment extending from one Z disc to the next Z disc.

A Band

The dark band in a sarcomere corresponding to the entire length of the thick myofilaments.

H Band

A lighter region in the center of the A band where thin filaments do not reach or overlap with thick filaments.

I Band

The light band spanning adjacent sarcomeres that contains thin and elastic filaments but no thick filaments.

Z Disc

A protein structure that bisects the I band and anchors thin filaments and titin.

Dystrophin

A clinically important structural protein that links outermost actin filaments to membrane proteins linked to the endomysium, transferring forces to tendons.

Denervation Atrophy

The shrinkage and wasting of paralyzed muscle tissue resulting from severed or disconnected motor nerves.

Neuromuscular Junction (NMJ)

The functional synapse formed where an axon terminal of a motor neuron meets a muscle fiber target (motor end plate).

Synaptic Knob

The swollen terminal end of an axon at the neuromuscular junction containing synaptic vesicles filled with acetylcholine.

Synaptic Vesicles

Membrane-bound spheres within the synaptic knob containing the neurotransmitter acetylcholine.

Synaptic Cleft

The narrow fluid-filled gap separating the synaptic knob from the sarcolemma of the muscle fiber.

Junctional Folds

Infoldings of the sarcolemma beneath the synaptic knob that maximize the surface area for acetylcholine receptors.

Acetylcholine (ACh)

The neurotransmitter released by motor neurons at the neuromuscular junction to excite skeletal muscle fibers.

Acetylcholinesterase (AChE)

An enzyme present in the synaptic cleft and sarcolemma that breaks down acetylcholine to terminate muscle stimulation.

End-Plate Potential (EPP)

A rapid local depolarization at the motor end plate caused by $Na^+$ influx through ligand-gated acetylcholine receptor channels.

Resting Membrane Potential

The baseline voltage difference across a resting muscle cell membrane, approximately $-90\,mV$, maintained by the sodium-potassium pump.

Sliding-Filament Theory

The mechanism of muscle contraction in which thin filaments slide over thick filaments toward the M line, shortening the sarcomere without shortening individual filaments.

Cross-Bridge

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

Power Stroke

The action of a myosin head releasing ADP and $P_i$ and flexing to pull the thin filament along the thick filament.

Recovery Stroke

The recocking of a myosin head back into its high-energy extended position following ATP hydrolysis.

Phosphagen System

An immediate energy system comprising ATP and creatine phosphate ($CP$) that provides energy for short bursts of activity up to about $6\,seconds$.

Creatine Kinase

An enzyme that transfers a phosphate group from creatine phosphate to ADP to rapidly regenerate ATP.

Myokinase

An enzyme that transfers a phosphate group from one ADP molecule to another, producing ATP and AMP.

Glycogen-Lactic Acid System

An anaerobic metabolic pathway that converts glucose or glycogen to lactic acid via glycolysis, providing a net gain of $2\,ATP$ per glucose for $30$ to $40\,seconds$ of activity.

Excess Post-Exercise Oxygen Consumption (EPOC)

The elevated rate of oxygen consumption after exercise (oxygen debt) required to replenish ATP and myoglobin stores and process lactic acid.

Twitch

A single cycle of contraction and relaxation in a muscle fiber produced by a single electrical stimulus at threshold or higher.

Latent Period

The brief delay between the application of a stimulus and the onset of tension development in a muscle twitch.

Temporal Summation

The additive effect of consecutive stimuli arriving before a muscle fiber completely relaxes, producing higher tension.

Incomplete Tetanus

A state of sustained muscle contraction exhibiting partial relaxation cycles between high-frequency stimuli.

Complete Tetanus

An unnatural state of continuous, smooth maximal muscle contraction lacking relaxation, caused by extremely high stimulus frequency in laboratory conditions.

Length-Tension Relationship

The physiological principle stating that the amount of tension generated by a muscle depends on its resting sarcomere length before contraction.

Slow Oxidative (SO) Fibers

Red, fatigue-resistant skeletal muscle fibers rich in myoglobin, mitochondria, and capillaries, specialized for aerobic respiration and sustained posture or endurance.

Fast Glycolytic (FG) Fibers

White, easily fatigued skeletal muscle fibers adapted for rapid, powerful, anaerobic contractions with high glycogen content.

Motor Unit

A single motor neuron together with all the individual skeletal muscle fibers it innervates.

Recruitment

The process of activating additional motor units to produce stronger overall muscle contraction force.

Muscle Tone

A baseline level of involuntary, sustained activation of motor units maintained by the central nervous system.

Isometric Contraction

A muscle contraction in which internal tension increases but muscle length remains unchanged, resulting in no movement.

Isotonic Contraction

A muscle contraction in which muscle length changes while maintaining constant tension.

Concentric Contraction

An isotonic contraction in which the muscle shortens as it maintains tension.

Eccentric Contraction

An isotonic contraction in which the muscle lengthens while maintaining tension.

Muscle Fatigue

The progressive, temporary weakness and loss of contractility resulting from prolonged muscle exercise.

Intercalated Discs

Specialized intercellular junctions in cardiac muscle containing desmosomes for mechanical linkage and gap junctions for electrical coupling.

Autorhythmic

The ability of cardiac pacemaker cells and certain smooth muscle cells to contract spontaneously and rhythmically without external nervous stimulation.

Calmodulin

A calcium-binding protein in smooth muscle cells that replaces troponin to activate contraction.

Dense Bodies

Protein structures in smooth muscle cells that anchor intermediate filaments and thin filaments, functioning like Z discs of skeletal muscle.

Single-Unit Smooth Muscle

Visceral smooth muscle tissue whose cells are connected by gap junctions and regulated as a cohesive sheet.

Multi-Unit Smooth Muscle

Smooth muscle tissue in which individual muscle fibers form distinct motor units and contract independently.