Muscular System - Histology and Physiology (Seeley’s 13th Edition)

Vocabulary flashcards covering the major concepts of muscle tissue types, structure, contraction mechanisms, neuromuscular junctions, excitation-contraction coupling, energy metabolism, and aging/disease aspects from Seeley’s Muscular System notes.

Skeletal muscle

Voluntary, striated muscle that moves the skeleton and is controlled by the somatic nervous system.

Smooth muscle

In walls of hollow organs and vessels; involuntary, autonomic/hormonal control; some autorhythmicity.

Cardiac muscle

Heart muscle; striated and branched; autorhythmic; involuntary control.

Epimysium

Connective tissue layer that surrounds an entire muscle.

Perimysium

CT sheath around a fascicle (group of muscle fibers).

Endomysium

Loose CT surrounding individual muscle fibers.

Fascicle

Bundle of muscle fibers within a muscle.

Tendon

Cord-like tissue formed by dense CT that attaches muscle to bone.

Aponeurosis

Flat sheet of CT attaching muscle to bone or skin.

Muscular fascia

CT layer between adjacent muscles and between muscles and skin.

Motor neuron

Nerve cell that stimulates muscle fibers to contract.

Motor unit

One motor neuron and all the muscle fibers it innervates.

Actin

Thin filament; covers active sites on myosin; forms part of the sarcomere.

Myosin

Thick filament with heads that form cross-bridges with actin; ATPase activity.

Sarcomere

Basic contractile unit of a muscle fiber; Z disk to Z disk.

Z disk

Anchors actin filaments at each sarcomere boundary.

I band

Region with only thin filaments; moves during contraction but band itself remains related to sarcomere boundaries.

A band

Region where thick and thin filaments overlap; length constant during contraction.

H zone

Central part of A band with no actin-myosin overlap.

M line

Center of the sarcomere; holds myosin in place.

Titin

Elastic protein that adds elasticity and recoil to the sarcomere.

Sarcolemma

Plasma membrane of a muscle fiber.

Transverse tubules (T-tubules)

Inward folds of the sarcolemma that transmit action potentials into the cell.

Sarcoplasmic reticulum (SR)

Calcium-storage organelle in muscle; smooth ER variant.

Terminal cisternae

Enlarged SR regions adjacent to T-tubules.

Triad

One T-tubule plus two terminal cisternae; site of excitation-contraction coupling.

Sarcomere shortnening

Process by which actin slides past myosin to contract the muscle.

G actin

Globular actin monomers that polymerize to form F actin.

F actin (filamentous actin)

Polymerized actin forming the thin filament with active sites.

Tropomyosin

Elongated protein that blocks myosin-binding sites on actin when muscle is relaxed.

Troponin

Three-subunit complex that regulates actin-myosin interaction in presence of Ca2+.

Actin active sites

Locations on G actin that myosin heads bind during contraction.

Cross-bridge

Interaction between myosin heads and actin active sites during contraction.

Myosin head

Part of the thick filament that binds actin and performs the power stroke.

ATPase

Enzyme activity in myosin heads that hydrolyzes ATP to power contraction.

Neuromuscular junction (NMJ)

Synapse between a motor neuron and a skeletal muscle fiber.

Presynaptic terminal

Axon terminal containing synaptic vesicles with acetylcholine (ACh).

Acetylcholine (ACh)

Neurotransmitter released at NMJ to stimulate muscle fiber.

Synaptic cleft

Narrow gap between neuron and muscle where neurotransmitter diffuses.

Postsynaptic membrane (motor end-plate)

Muscle fiber membrane with ACh receptors.

Acetylcholinesterase

Enzyme that breaks down ACh in the synaptic cleft.

Sliding filament theory

Process where actin slides over myosin to shorten the sarcomere during contraction.

Resting membrane potential

Baseline polarized state of the cell membrane (inside more negative).

Depolarization

Membrane potential becomes less negative; Na+ enters the cell.

Repolarization

Return to resting membrane potential; K+ exits the cell.

Hyperpolarization

Membrane potential briefly more negative than resting after an impulse.

Sodium-potassium pump (Na+/K+ pump)

Active transport maintaining resting membrane potential by moving Na+ out and K+ in.

Ligand-gated channel

Ion channel opened by binding of a chemical messenger (e.g., ACh).

Voltage-gated channel

Ion channel opened by changes in membrane potential.

Action potential

Rapid electrical impulse that propagates along the sarcolemma and into T-tubules.

Excitation-contraction coupling

Link between the action potential and the mechanical contraction via Ca2+ release.

Calcium release from SR

Ca2+ released into the sarcoplasm triggers contraction by exposing actin sites.

Cross-bridge cycle

Cycle of attachment, power stroke, detachment, and reattachment of myosin to actin.

Power stroke

Myosin head pivots, pulling actin toward the center of the sarcomere.

ATP detachment

ATP binds myosin head causing it to detach from actin.

Recovery stroke

Myosin head re-energizes in preparation for another cycle.

Muscle twitch

Response of a muscle fiber to a single action potential: latent, contraction, relaxation phases.

Latent (lag) phase

Delay between stimulus and the start of contraction.

Isometric contraction

Contraction with no change in muscle length; tension rises.

Isotonic contraction

Contraction with a change in muscle length while tension stays constant.

Motor unit recruitment

Process of activating more motor units to increase force.

Treppe

Increase in force with each successive stimulus during warm-up (staircase effect).

Wave summation

Increased muscle tension with higher stimulus frequency due to incomplete relaxation.

Incomplete tetanus

Contractions with partial relaxation between stimuli.

Complete tetanus

Sustained contraction with no relaxation between stimuli.

Length-tension relationship

Active tension depends on muscle length; optimal overlap yields max force.

Size principle

Small motor units are recruited first, larger ones recruited as needed.

Slow-twitch fibers (Type I)

Oxidative, fatigue-resistant fibers with high myoglobin and mitochondria.

Fast-twitch fibers (Type II)

Glycolytic or oxidative-glycolytic fibers; fast, powerful contractions.

Hypertrophy

Increase in muscle size due to more myofibrils and satellite cell activity.

Atrophy

Decrease in muscle size from disuse or aging.

Satellite cells

Muscle stem cells that fuse to existing fibers for growth/repair.

Phosphocreatine

High-energy phosphate reservoir that donates phosphate to ADP to form ATP.

Creatine kinase

Enzyme transferring phosphate from phosphocreatine to ADP to form ATP.

Adenylate kinase

Enzyme converting 2 ADP to ATP and AMP.

Anaerobic respiration

Glycolysis without oxygen producing ATP and lactic acid.

Aerobic respiration

Oxygen-dependent production of ATP, CO2, and water.

Lactic acid

Product of anaerobic glycolysis; contributes to muscle fatigue.

Oxygen debt (EPOC)

Extra oxygen required after exercise to restore baseline conditions.

Fatigue

Decreased ability to sustain exercise due to metabolic factors.

Rigor mortis

Postmortem muscle stiffening from Ca2+ influx and cross-bridge formation.

Smooth muscle latch state

Sustained, low-energy contraction due to slow dephosphorylation of myosin.

Calmodulin

Ca2+-binding protein that activates myosin kinase in smooth muscle.

Myosin kinase

Enzyme activated by Ca2+-calmodulin that phosphorylates myosin for contraction.

Myosin phosphatase

Enzyme that dephosphorylates myosin to promote relaxation.

Visceral smooth muscle

Sheets of smooth muscle with gap junctions; autorhythmic waves.

Multiunit smooth muscle

Independent smooth muscle units; fewer gap junctions.

Intercalated discs

Specialized junctions between cardiac muscle cells for synchronized contraction.

Cardiac autorhythmic cells

Pacemaker cells in the heart that generate rhythmic impulses.

Dystrophin

Protein associated with Duchenne muscular dystrophy; links cytoskeleton to ECM.

Duchenne muscular dystrophy

X-linked muscular dystrophy caused by dystrophin deficiency; progressive weakness.

Aging effects on skeletal muscle

Decreased muscle mass and endurance; slower contraction; fewer fast-twitch fibers.

Oxygen deficit

Lag between onset of exercise and increased oxygen uptake for metabolism.

Excess post-exercise oxygen consumption (EPOC)

Increased respiration after exercise to restore homeostasis.