Muscles and Muscle Tissue: Gross, Microscopic, and Physiological Principles

Comprehensive practice flashcards defining foundational anatomical, microscopic, physiological, and clinical terms covered in Human Anatomy and Physiology Chapter 9: Muscles and Muscle Tissue.

Excitability

The functional ability of muscle cells to respond to certain stimuli by producing electrical signals.

Contractility

The functional ability of muscles to shorten forcefully when stimulated.

Extensibility

The functional characteristic allowing a muscle to be stretched or extended beyond its resting length.

Elasticity

The functional ability of a muscle fiber to recoil and resume its resting length after contracting or extending.

Epimysium

An overcoat of dense irregular connective tissue that surrounds the entire skeletal muscle.

Perimysium

A layer of fibrous connective tissue that surrounds and bundles muscle fibers into a fascicle.

Endomysium

A wispy sheath of areolar connective tissue that encloses each individual muscle fiber.

Fascia

Connective tissue situated outside of the epimysium that is continuous with tendons.

Tendon

A rope-like fibrous connective tissue structure that connects skeletal muscles to the origin or insertion point of bones or cartilage.

Aponeurosis

A sheet-like fibrous connective tissue membrane resembling a flattened tendon that binds muscles together or connects muscle to bone.

Origin

The end of a muscle attached to an immovable or less movable bone that remains stationary during contraction.

Insertion

The movable attachment end of a muscle that is pulled toward the origin during muscle contraction.

Sarcolemma

The plasma membrane of a skeletal or smooth muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber, notable for containing large amounts of glycosomes and myoglobin.

Glycosomes

Granules of stored glycogen located within the sarcoplasm that provide glucose during muscle cell activity.

Myoglobin

A red, oxygen-binding pigment found in the sarcoplasm of muscle fibers that stores oxygen for ATP production by mitochondria.

Sarcoplasmic Reticulum (SR)

An elaborate smooth endoplasmic reticulum surrounding each myofibril that stores and releases high concentrations of calcium ions ($\text{Ca}^{2+}$).

Terminal Cisterns

Dilated end sacs of the sarcoplasmic reticulum forming large, perpendicular cross channels flanking T tubules.

Transverse (T) Tubules

Tunnel-like elongated infoldings of the sarcolemma that penetrate into the cell interior perpendicular to muscle fibers, carrying extracellular fluid.

Triad

A specialized three-unit skeletal muscle structure formed by one transverse tubule and the two flanking terminal cisterns of the sarcoplasmic reticulum.

Myofibrils

Densely packed, cylindrical contractile elements that run the entire length of the muscle fiber and constitute most of the muscle volume.

Thick Filaments (Myosin Filaments)

Contractile protein filaments composed of about $200$ myosin molecules whose protruding globular heads bear actin- and ATP-binding sites.

Thin Filaments (Actin Filaments)

Cytoskeletal filaments composed of two twisted strands of actin subunits containing active sites for myosin attachment, along with regulatory proteins.

Tropomyosin

A rod-shaped regulatory protein in thin filaments that spirals around actin to block myosin-binding active sites in a relaxed muscle.

Troponin

A small, globular protein complex spaced along tropomyosin filaments that binds calcium ions and alters position to expose active binding sites on actin.

Titin (Elastic Filaments)

A structural protein that extends from the Z disc to the thick filament and attaches to the M line, stabilizing and anchoring the myosin filament.

Sarcomere

The fundamental contractile unit of a skeletal muscle fiber, bounded by one Z disc on each end.

Z Disc

A coin-shaped or zig-zag sheet of proteins that anchors thin actin filaments and elastic filaments, defining the boundaries of a sarcomere.

A Band

The dark central region of a sarcomere extending the complete length of the thick filaments, including regions where thick and thin filaments overlap.

H Zone

The lighter, narrow central region in a relaxed sarcomere's A band containing thick filaments only, with no overlapping thin filaments.

M Line

A dark vertical line in the center of the H zone formed by proteins that hold adjacent thick filaments together.

I Band

The light, less dense region of a sarcomere that contains only thin filaments and is bisected by the Z disc.

Neuromuscular Junction

A specialized synapse where an axon terminal of a somatic motor neuron meets the motor end plate of a muscle fiber.

Synaptic Cleft

The fluid-filled microscopic space that separates the axon terminal of a motor neuron from the sarcolemma of the muscle fiber.

Acetylcholine (ACh)

The specific neurotransmitter enclosed in synaptic vesicles that is released via exocytosis into the synaptic cleft at the neuromuscular junction.

Motor End Plate

The trough-like junctional folded section of the muscle fiber sarcolemma containing specific acetylcholine receptors.

Local Depolarization

A change in sarcolemma membrane potential resulting from acetylcholine binding, causing sodium ($\text{Na}^+$) to enter and making the inside more positive than the outside.

Excitation-Contraction (E-C) Coupling

The sequence of events by which the transmission of an action potential along the sarcolemma and down the T tubules leads to the release of $\text{Ca}^{2+}$ and sliding of myofilaments.

Power Stroke

The mechanical step of cross bridge cycling in which ADP and $\text{P}_i$ are released, causing the myosin head to pivot and pull the thin filament toward the M line.

Sliding Filament Model of Muscle Contraction

The theory stating that during contraction, thin filaments slide past thick filaments, increasing overlap and shortening the sarcomere without shortening the filaments themselves.

Muscle Tension

The mechanical force exerted by a contracting muscle on an object.

Load

The opposing weight or resistance exerted on a muscle by an object that is to be moved.

Acetylcholinesterase

The enzyme located in the synaptic cleft that rapidly degrades acetylcholine to terminate muscle stimulation and permit relaxation.

Rigor Mortis

The temporary state of skeletal muscle stiffness occurring hours after death due to ATP depletion, preventing myosin head detachment from actin filaments.

Myasthenia Gravis

An autoimmune disorder characterized by the destruction of acetylcholine receptors on the sarcolemma, frequently presenting as drooping eyelids and muscle weakness.

Motor Unit

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

Muscle Twitch

The laboratory-measured motor unit response to a single action potential from its motor neuron.

Latent Period

The initial interval of several milliseconds following stimulation during which excitation-contraction coupling occurs without measurable tension.

Period of Contraction

The middle phase of a muscle twitch during which cross bridges actively cycle and tension develops to a peak.

Period of Relaxation

The final phase of a muscle twitch during which calcium is actively transported back into the sarcoplasmic reticulum and tension decreases to zero.

Treppe (Staircase Effect)

A stepwise increase in contraction tension produced when identical stimuli are delivered after complete relaxation, caused by muscle warming and elevated enzyme efficiency.

Temporal (Wave) Summation

An increase in muscle contractile strength occurring when a second stimulus arrives before the muscle fiber has completely relaxed from the previous contraction.

Incomplete (Unfused) Tetanus

A state of sustained, quivering muscle contraction produced by intermediate stimulation frequencies with partial relaxation between stimuli.

Complete (Fused) Tetanus

A smooth, continuous, maximal muscle contraction with no relaxation phase whatsoever, induced by very high stimulation frequencies.

Recruitment (Multiple Motor Unit Summation)

The activation of additional motor units within a muscle to proportionally grade and increase the overall force of contraction.

Muscle Tone

A constant, slightly contracted resting state of skeletal muscles produced by involuntary spinal reflexes to maintain posture and joint firmness.

Isometric Contraction

A muscle contraction where internal tension increases to peak capacity, but the overall muscle length remains unchanged because the load is not moved.

Isotonic Contraction

A muscle contraction in which the muscle tension developed overcomes the opposing load, resulting in a change in muscle length.

Concentric Contraction

A category of isotonic contraction in which the muscle shortens while developing tension and performing work.

Eccentric Contraction

A category of isotonic contraction in which the muscle generates tension while actively lengthening.

Creatine Phosphate (CP)

A high-energy molecule stored in muscle cells that rapidly donates a phosphate to ADP via direct phosphorylation, supporting maximum contraction for about $15\,\text{seconds}$.

Anaerobic Respiration (Glycolysis)

The metabolic pathway converting glucose into two pyruvic acid molecules and $2\,\text{ATP}$ in the absence of oxygen, with pyruvic acid primarily converted to lactic acid.

Aerobic Respiration

The oxygen-requiring mitochondrial pathway breaking down glucose and fuels into carbon dioxide, water, and up to $32\text{ to }36\,\text{ATP}$, providing $95\%$ of energy at rest or moderate exercise.

Oxygen Debt (EPOC)

Excess postexercise oxygen consumption representing the volume of extra oxygen required after exercise to return glycogen, ATP, creatine phosphate, and myoglobin to resting states.

Slow Oxidative Fibers

Small, red, myoglobin-rich skeletal muscle fibers with high capillary density and abundant mitochondria that resist fatigue and support endurance activities.

Fast Oxidative Fibers

Intermediate-sized, red-to-pink skeletal muscle fibers with fast myosin ATPase activity that utilize primarily aerobic pathways and are suited for walking and sprinting.

Fast Glycolytic Fibers

Large-diameter, pale/white skeletal muscle fibers with high glycogen reserves and few mitochondria, designed for short-term, powerful anaerobic bursts but prone to rapid fatigue.

Disuse Atrophy

The progressive degeneration and loss of skeletal muscle mass resulting from immobilization or lack of neural stimulation, decreasing strength by about $5\%$ per day of complete rest.

Intercalated Discs

Complex cellular junctions connecting branching cardiac muscle cells, containing desmosomes for structural anchoring and gap junctions for electrical syncytium.

Dense Bodies

Cytoplasmic and sarcolemmal protein structures in smooth muscle attached to intermediate filaments that anchor actin thin filaments, serving a role analogous to Z discs.

Calmodulin

The cytosolic calcium-binding protein in smooth muscle cells that, when activated by $\text{Ca}^{2+}$, stimulates myosin light chain kinase.

Myosin Light Chain Kinase (MLCK)

An enzyme in smooth muscle activated by the calcium-calmodulin complex that phosphorylates myosin heads, enabling actin binding and contraction.

Varicosities

Bulbous swellings along autonomic nerve fibers that release neurotransmitters into wide synaptic clefts called diffuse junctions across smooth muscle sheets.

Peristalsis

Alternating waves of contraction and relaxation of opposing longitudinal and circular smooth muscle layers that propel substances through hollow organs.

Visceral (Single-Unit) Smooth Muscle

Sheets of smooth muscle in walls of hollow organs connected by gap junctions that contract synchronously as an autorhythmic electrical unit.

Multiunit Smooth Muscle

Smooth muscle tissue consisting of independent muscle fibers each supplied with its own autonomic nerve terminal and few gap junctions, found in large airways, large arteries, and intrinsic eye muscles.

Hyperplasia

The physiological capacity of certain smooth muscle cells to divide and increase in total cell number, such as during uterine enlargement under estrogen stimulation.

Duchenne Muscular Dystrophy (DMD)

An inherited, sex-linked muscle-wasting disorder carried by females and expressed in males, caused by a defective gene coding for the sarcolemmal stabilizing protein dystrophin.