10.1 - 10.5 Muscle Tissue Structure, Physiology, Energetics, and Mechanics

Comprehensive vocabulary flashcards covering skeletal muscle organization, excitation-contraction coupling, cross-bridge mechanics, bioenergetics, motor control, and fiber types.

Excitability

The property of muscle tissue wherein plasma membranes can alter their electrical state from polarized to depolarized and propagate an action potential along the membrane.

Contractility

The quality that enables muscle tissue to pull on its attachment points and shorten with force.

Extensibility

The quality of muscle tissue that allows it to stretch or extend beyond its resting length.

Elasticity

The ability of muscle tissue to recoil back to its original resting length due to elastic fibers.

Syncytium

A single functional unit formed by cardiac muscle fibers that are physically and electrically connected, allowing the entire heart to contract in unison.

Muscle Tissue Types

The three distinct types of muscle tissue in the human body: (a) skeletal muscle, (b) smooth muscle, and (c) cardiac muscle.

Epimysium

A outer sheath of dense, irregular connective tissue that encloses the entire skeletal muscle organ, maintaining structural integrity during forceful contractions.

Perimysium

The middle layer of connective tissue that bundles individual skeletal muscle fibers into organized structures called fascicles.

Endomysium

A delicate inner connective tissue layer of collagen and reticular fibers surrounding each single muscle fiber, providing extracellular fluid and nutrients.

Aponeurosis

A broad, flat sheet of dense connective tissue into which the connective tissue layers (mysia) of a muscle fuse to attach to fascia or bone.

Sarcolemma

The specialized plasma membrane surrounding a skeletal muscle fiber.

Sarcoplasm

The cytoplasm of a muscle cell or muscle fiber.

Sarcoplasmic Reticulum (SR)

The specialized smooth endoplasmic reticulum in muscle fibers that stores, releases, and retrieves calcium ions ($Ca^{++}$).

Sarcomere

The repeating functional contractile unit of a striated muscle fiber, bounded by Z-discs and measuring approximately $2\,\mu\text{m}$ in length.

Myofibril

A cylindrical organelle approximately $1.2\,\mu\text{m}$ in diameter that extends the full length of a muscle fiber, composed of linearly arranged sarcomeres.

Neuromuscular Junction (NMJ)

The site of synaptic contact where a motor neuron's axon terminal meets a skeletal muscle fiber sarcolemma.

Excitation-Contraction Coupling

The sequence of events in which a membrane action potential ("excitation") triggers the release of $Ca^{++}$ from the SR, causing myofilaments to slide ("contraction").

Acetylcholine (ACh)

The neurotransmitter chemical released by motor neuron terminals into the synaptic cleft at the NMJ to initiate muscle fiber depolarization.

Acetylcholinesterase (AChE)

An enzyme in the synaptic cleft that rapidly hydrolyzes acetylcholine to prevent prolonged and unwanted muscle excitation.

Triad

A three-membrane structure consisting of a transverse tubule (T-tubule) situated between two adjacent terminal cisternae of the sarcoplasmic reticulum.

Sliding Filament Model of Muscle Contraction

The mechanism whereby thin actin filaments are pulled by myosin heads and slide past thick myosin filaments toward the center of the sarcomere, shortening the sarcomere.

Tropomyosin

A regulatory protein that covers the myosin-binding sites on actin strands in relaxed muscle, preventing cross-bridge interaction.

Troponin

A regulatory protein bound to tropomyosin that contains binding sites for $Ca^{++}$; binding of $Ca^{++}$ induces a conformational shift that exposes myosin-binding sites on actin.

Cross-Bridge Cycle

The cyclical attachment, power stroke, detachment, and re-cocking of myosin heads along actin filaments during muscle contraction.

Power Stroke

The step in the cross-bridge cycle where inorganic phosphate ($P_i$) is released, causing the myosin head to pivot toward the M-line and pull actin approximately $10\,\text{nm}$.

Rigor Mortis

Post-mortem muscle stiffness caused by the absence of ATP, preventing myosin heads from detaching from actin binding sites.

Creatine Phosphate

A high-energy molecule in muscle cells that donates a phosphate group to ADP via creatine kinase, powering the first approximately $15\,\text{seconds}$ of muscle contraction.

Glycolysis

An anaerobic metabolic pathway breaking down glucose to produce $2\text{ ATP}$ and $2$ pyruvic acid molecules per glucose.

Aerobic Respiration

An oxygen-requiring metabolic process occurring in mitochondria that breaks down glucose or fatty acids to produce carbon dioxide, water, and approximately $36\text{ ATP}$ per glucose.

Myoglobin

An oxygen-storing protein present in muscle fibers that provides an internal oxygen reserve for mitochondrial aerobic respiration.

Oxygen Debt

The amount of oxygen required post-exercise to convert accumulated lactic acid back to pyruvic acid or glucose and re-establish ATP, creatine phosphate, and oxygen reserves.

Duchenne Muscular Dystrophy (DMD)

An inherited X-linked progressive muscle disorder caused by a defect in dystrophin production, leading to sarcolemmal damage, excessive $Ca^{++}$ entry, and muscle degeneration.

Dystrophin

A protein that connects thin actin filaments of myofibrils to the sarcolemma, maintaining cell membrane integrity during contraction.

Hypertrophy

An increase in muscle mass and volume driven by the increased production of myofibrils and sarcomeres within individual muscle fibers.

Atrophy

A reduction in muscle size and strength caused by the loss of sarcomeres and myofibrils within muscle fibers due to disuse or pathology.

Concentric Contraction

An isotonic contraction in which the muscle shortens while generating constant tension to move a load.

Eccentric Contraction

An isotonic contraction in which the muscle lengthens while tension decreases in a controlled fashion to lower a load.

Isometric Contraction

A muscle contraction that develops tension without changing muscle length or joint angle because the load exceeds the generated muscle force.

Motor Unit

A structural and functional unit consisting of a single motor neuron and all the specific skeletal muscle fibers it innervates.

Recruitment

The activation of additional and progressively larger motor units by the nervous system to increase total muscle contraction force.

Length-Tension Relationship

The principle stating that maximal muscle tension is generated when sarcomere length is between $80\%$ and $120\%$ of its resting length, optimizing thick and thin filament overlap.

Muscle Twitch

A single, rapid contraction and relaxation cycle produced by a single action potential in a motor unit.

Latent Period

The initial time delay during a muscle twitch between motor neuron excitation and tension development, during which $Ca^{++}$ is released from the SR.

Wave Summation

The addition of successive motor unit contractions triggered before the previous relaxation phase ends, resulting in elevated total muscle tension.

Complete Tetanus

A continuous, sustained muscle contraction produced when high stimulus frequency eliminates the relaxation phase entirely.

Treppe

A progressive, staircase-like increase in contraction tension produced when a dormant muscle is repeatedly stimulated, driven by gradual $Ca^{++}$ buildup in the sarcoplasm.

Muscle Tone

A continuous, baseline state of partial muscle contraction produced by cyclical neural activation of alternating motor units, stabilizing joints and posture.

Slow Oxidative (SO) Fibers

Muscle fibers that contract slowly, rely on aerobic respiration, possess high mitochondria and myoglobin levels (red appearance), and display high fatigue resistance.

Fast Oxidative (FO) Fibers

Intermediate muscle fibers that contract rapidly, produce ATP aerobically, contain abundant mitochondria, and generate high tension with moderate fatigue resistance.

Fast Glycolytic (FG) Fibers

Large-diameter muscle fibers with high glycogen reserves that contract rapidly via anaerobic glycolysis, producing high force but fatiguing quickly (white appearance).