Human Anatomy and Physiology: Muscles and Muscle Tissue

Comprehensive vocabulary flashcards covering the structural hierarchy, microscopic anatomy, neuromuscular transmission, excitation-contraction coupling, and cross bridge cycling of skeletal muscle tissue.

Excitability (Responsiveness)

The ability of a muscle cell to receive and respond to electrical or chemical stimuli.

Contractility

The unique ability of muscle tissue to shorten forcibly when stimulated.

Extensibility

The ability of muscle cells to extend or stretch beyond their resting length.

Elasticity

The ability of a muscle fiber to recoil and resume its resting length after stretching.

Skeletal Muscle Tissue

Striated, voluntary, multinucleate muscle tissue packaged into organs attached to bones and skin; contracts rapidly, tires easily, and is capable of exerting powerful force.

Cardiac Muscle Tissue

Striated, involuntary muscle tissue found only in the walls of the heart, composed of branching chains of uni- or binucleate cells that contract at a steady rate regulated by an intrinsic pacemaker.

Smooth Muscle Tissue

Nonstriated, involuntary muscle tissue consisting of single, spindle-shaped, uninucleate cells found primarily in the walls of hollow visceral organs such as the stomach, airways, and urinary bladder.

Epimysium

An external overcoat of dense irregular connective tissue surrounding an entire skeletal muscle that may blend with deep fascia.

Fascicle

A discrete bundle of skeletal muscle fibers segregated from the rest of the muscle by a connective tissue sheath.

Perimysium

A layer of fibrous connective tissue that surrounds each individual fascicle within a muscle.

Endomysium

A fine sheath of areolar connective tissue surrounding each individual skeletal muscle fiber (cell).

Insertion

The site of attachment of a muscle to a movable bone.

Origin

The site of attachment of a muscle to an immovable or less movable bone.

Direct (Fleshy) Attachment

A muscle attachment where the epimysium is fused directly to the periosteum of a bone or perichondrium of a cartilage.

Indirect Attachment

A muscle attachment where connective tissue wrappings extend beyond the muscle as a ropelike tendon or flat, sheetlike aponeurosis anchoring to bone or fascia.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle cell, containing abundant glycosomes and myoglobin.

Glycosomes

Granules of stored glycogen located in the sarcoplasm that provide glucose for ATP production.

Myoglobin

A red pigment in muscle fibers that stores oxygen ($\text{O}_2$).

Myofibrils

Densely packed, rodlike contractile elements that account for approximately $80\%$ of muscle cell volume and contain thousands of sarcomeres.

Striations

Repeating bands of alternating dark A bands and light I bands along the length of each myofibril.

A Band

The dark region of a striation that extends the full length of the thick myofilaments.

I Band

The light region of a striation containing only thin (actin) myofilaments.

H Zone

The lighter midsection of each dark A band where thick and thin filaments do not overlap in relaxed muscle fibers.

M Line

A dark line formed by molecules of the protein myomesin that vertically bisects the H zone.

Z Disc (Z Line)

A coin-shaped protein sheet on the midline of an I band that anchors the thin actin filaments and marks the boundary of an individual sarcomere.

Sarcomere

The smallest contractile or functional unit of a muscle fiber, defined as the region between two successive Z discs.

Thick Filaments

Myofilaments composed of the protein myosin, each possessing a rodlike tail formed by two heavy polypeptide chains and globular heads formed by light polypeptide chains.

Thin Filaments

Myofilaments composed of two twisted strands of fibrous actin (F actin) built from globular actin (G actin) subunits, bound with the regulatory proteins troponin and tropomyosin.

Tropomyosin

A rod-shaped regulatory protein spiraling around actin that blocks myosin-binding active sites on actin when the muscle is in a relaxed state.

Troponin

A three-polypeptide regulatory protein complex that binds calcium ($\text{Ca}^{2+}$), causing a conformational change that moves tropomyosin away from myosin-binding sites on actin.

Titin (Elastic Filament)

A giant structural protein that extends from the Z disc to the thick filament and attaches to the M line; it holds thick filaments in place, resists excessive stretching, and aids in recoil.

Dystrophin

A structural protein that links thin actin filaments directly to integral proteins of the sarcolemma, anchoring contractile elements and stabilizing the membrane.

Sarcoplasmic Reticulum (SR)

An elaborate network of smooth endoplasmic reticulum tubules surrounding each myofibril that regulates intracellular $\text{Ca}^{2+}$ levels by storing and releasing calcium ions.

Terminal Cisterns

Large, perpendicular cross channels of the sarcoplasmic reticulum located at the A-I band junctions that store calcium ions.

T Tubules (Transverse Tubules)

Deep, tube-like invaginations of the sarcolemma that project into the interior of the muscle cell at each A-I band junction to conduct electrical nerve impulses deep inside the fiber.

Triad

A three-unit complex comprising a central T tubule flanked on each side by a terminal cistern of the sarcoplasmic reticulum.

Sliding Filament Model of Contraction

The mechanism of muscle contraction stating that thin filaments slide past thick filaments, increasing actin-myosin overlap without shortening the filaments themselves.

Neuromuscular Junction (Motor End Plate)

The specific region where an axon terminal of a somatic motor neuron meets the junctional folds of a skeletal muscle fiber sarcolemma.

Synaptic Cleft

A narrow, gel-filled space separating the motor neuron axon terminal from the muscle fiber sarcolemma.

Synaptic Vesicles

Membrane-bound sacs located inside the motor neuron axon terminal containing the neurotransmitter acetylcholine (ACh).

Junctional Folds

Trough-like infoldings of the muscle fiber sarcolemma beneath the axon terminal that contain millions of acetylcholine receptors.

End Plate Potential (EPP)

A local, graded depolarization of the sarcolemma at the neuromuscular junction resulting from the opening of chemically gated ion channels, causing more $\text{Na}^+$ to enter than $\text{K}^+$ to exit.

Acetylcholinesterase

An enzyme located in the synaptic cleft that rapidly breaks down acetylcholine into acetic acid and choline, terminating synaptic stimulation.

Refractory Period

A brief period during repolarization when a muscle fiber cannot be stimulated by another action potential until electrical resting conditions are restored.

Excitation-Contraction (E-C) Coupling

The sequence of events by which transmission of an action potential along the sarcolemma and down T tubules triggers the release of $\text{Ca}^{2+}$ from the sarcoplasmic reticulum, leading to myofilament sliding.

Cross Bridge Formation

The initial step of the cross bridge cycle where an energized, high-energy myosin head binds to an exposed active site on an actin filament.

Power (Working) Stroke

The phase of the cross bridge cycle where ADP and inorganic phosphate ($\text{P}_i$) are released, causing the myosin head to pivot and pull the thin actin filament toward the M line.

Cross Bridge Detachment

The step in the cross bridge cycle where a new molecule of ATP binds to the myosin head, weakening the actin-myosin link and causing the head to detach.

Cocking of the Myosin Head

The phase in which ATP is hydrolyzed into ADP and $\text{P}_i$ by the myosin ATPase, restoring the myosin head to its prestroke, high-energy conformation.

Duchenne Muscular Dystrophy (DMD)

A sex-linked recessive muscle-destroying disease occurring in $1$ in $3600$ male births, caused by a defective gene for dystrophin, which leads to membrane tearing, excess $\text{Ca}^{2+}$ influx, and loss of muscle mass.

Myasthenia Gravis

An autoimmune disorder characterized by drooping eyelids, difficulty swallowing, and generalized muscle weakness, caused by autoantibodies destroying ACh receptors at the neuromuscular junction.

Rigor Mortis

The postmortem stiffening of skeletal muscles that begins $3$ to $4$ hours after death and peaks at about $12$ hours, caused by the absence of ATP, which prevents cross bridge detachment and prevents $\text{Ca}^{2+}$ reuptake into the sarcoplasmic reticulum.