Bio 203: Muscle, Locomotion, and Motor Control

Comprehensive vocabulary flashcards covering the structural hierarchy, cellular and molecular physiology of skeletal, cardiac, and smooth muscle, neuromuscular junction signaling, excitation-contraction coupling, sarcomere mechanics, cross-bridge cycling, motor unit recruitment, and muscle fiber classification from Bio 203.

Tunicate

A marine chordate that possesses a free-swimming larval tadpole form with a nervous system; upon attaching to a surface as an adult, it loses mobility and its nervous system largely disintegrates.

Somatic Nervous System

The branch of the peripheral nervous system that communicates with sense organs and voluntary muscles, comprising sensory (afferent) and motor (efferent) divisions.

Autonomic Nervous System

The structural branch of the peripheral nervous system that communicates with internal organs and glands, subdivided into sympathetic, parasympathetic, and enteric divisions.

Enteric Nervous System

The division of the autonomic nervous system dedicated to regulating gastrointestinal (GI) tract function.

Muscle Fiber

A single elongated muscle cell; in skeletal muscle, it is a fused, multinucleated cell.

Myo-

A prefix derived from the Greek mys and Latin musculus, meaning "mouse."

Sarco-

A prefix derived from the Greek sarx, meaning "flesh" or "meat."

Skeletal Muscle

Striated muscle tissue attached to bones and select sphincters composed of large, multinucleated cells controlled by the somatic nervous system via motor units.

Cardiac Muscle

Striated, involuntary muscle tissue found in the heart composed of smaller, branched, uninucleated cells electrically coupled through gap junctions and intercalated disks.

Smooth Muscle

Non-striated, involuntary muscle tissue found in internal hollow organs and tubes, consisting of small, uninucleated cells coordinated by gap junctions and motor units under autonomic control.

Intercalated Disk

A specialized junctional structure between individual cardiac muscle cells containing gap junctions that facilitate synchronized electrical depolarization and contraction.

Epimysium

A sheath of connective tissue enclosing an entire skeletal muscle that provides structural integrity and converges to form the tendon connecting muscle to bone.

Perimysium

Connective tissue surrounding individual muscle fascicles that permits different segments of a muscle to contract independently.

Endomysium

The fine layer of connective tissue and blood vessels directly surrounding each individual muscle fiber.

Fascicle

A bundle of skeletal muscle fibers enclosed by perimysium, typically containing $\text{20 to 100}$ muscle fibers per bundle.

Myofibril

An intracellular contractile cylindrical organelle within a muscle fiber composed of organized arrays of thick (myosin) and thin (actin) myofilaments.

Sarcolemma

The specialized plasma membrane of a muscle cell or fiber that conducts action potentials.

Sarcoplasm

The cytoplasm of a muscle fiber, heavily populated with mitochondria and glycogen stores.

Sarcoplasmic Reticulum (SR)

A modified smooth endoplasmic reticulum in muscle fibers that wraps around myofibrils and stores calcium ions ($\text{Ca}^{2+}$).

Transverse Tubule (T-tubule)

Regularly-spaced invaginations of the sarcolemma that penetrate into the interior of a muscle fiber to conduct action potentials deep to the sarcoplasmic reticulum.

Terminal Cisternae

Enlarged end regions of the sarcoplasmic reticulum that associate directly with transverse tubules.

Triad

A three-unit anatomical structure in skeletal muscle consisting of one central transverse tubule flanked on either side by terminal cisternae of the sarcoplasmic reticulum.

Myosin

The motor protein composing the thick filament, possessing a tail, hinge region, and dual globular heads that have ATPase activity and binding sites for actin.

Actin

The primary protein of the thin filament, composed of globular G-actin monomers polymerizing into F-actin chains containing specific myosin-binding sites.

Tropomyosin

A regulatory filamentous protein running parallel to actin chains that covers the myosin-binding sites on G-actin when the muscle is at rest.

Troponin

A regulatory protein complex associated with tropomyosin that binds cytosolic $\text{Ca}^{2+}$, triggering a conformational shift that pulls tropomyosin off the myosin-binding sites on actin.

Sarcomere

The basic contractile functional unit of striated muscle, bounded at each end by a Z disk.

Z Disk (Z Line)

The protein boundary at each end of a sarcomere that anchors thin (actin) filaments.

I Band

The light band of the sarcomere containing thin filaments (actin) only; it shortens during muscle contraction.

A Band

The dark region of the sarcomere containing the entire length of thick filaments (myosin) including areas of overlap with thin filaments; its length remains constant during contraction.

H Zone

The lighter region in the center of the A band consisting solely of thick filaments (myosin); it virtually disappears when the muscle contracts.

M Line

The midline protein structure in the center of the sarcomere to which thick filaments attach.

Sliding Filament Theory

The model of muscle contraction stating that actin and myosin myofilaments slide past one another to shorten the sarcomere without changing their own physical lengths.

Neuromuscular Junction (NMJ)

The chemical synapse formed between the axon terminal of a somatic motor neuron and the motor end plate of a skeletal muscle fiber.

Motor End Plate

The specialized, folded region of the muscle fiber sarcolemma opposite the presynaptic motor neuron terminal, densely concentrated with nicotinic acetylcholine receptors.

Nicotinic Acetylcholine Receptor (AChR)

A ligand-gated monovalent cation channel on the motor end plate that opens upon binding acetylcholine, facilitating $\text{Na}^+$ influx and $\text{K}^+$ efflux to generate a large depolarizing graded potential.

Acetylcholinesterase

An enzyme in the synaptic cleft of the neuromuscular junction that hydrolyzes acetylcholine into acetic acid and choline, terminating synaptic signaling.

Safety Factor (Neuromuscular Junction)

The high margin of physiological reliability ensuring that every single action potential arriving at a motor neuron terminal reliably depolarizes the end plate to threshold and generates a muscle action potential.

Dihydropyridine (DHP) Receptor

A voltage-sensing L-type calcium channel located in the transverse tubule membrane that undergoes a conformational shape change in response to an action potential.

Ryanodine Receptor (RyR)

A calcium release channel in the sarcoplasmic reticulum membrane that is opened via mechanical coupling with the DHP receptor, allowing $\text{Ca}^{2+}$ to flood into the sarcoplasm.

SERCA (Sarcoplasmic Endoplasmic Reticulum Calcium ATPase)

An ATP-driven calcium pump in the sarcoplasmic reticulum membrane that active transports $\text{Ca}^{2+}$ from the cytosol back into the SR lumen to allow muscle relaxation.

Cross-Bridge

The physical linkage formed when a cocked, energized myosin head binds to an accessible myosin-binding site on an actin monomer.

Rigor State

The phase of the cross-bridge cycle where the myosin head remains tightly locked to actin due to the complete absence of ATP and ADP.

Power Stroke

The rotational movement of the myosin neck while bound to actin, triggered by the release of inorganic phosphate ($\text{P}_i$), which pulls the actin filament approximately $11\,\text{nm}$ toward the center of the sarcomere.

Antagonist Muscle Pair

Pairs of opposing muscles working across a joint (such as biceps flexors and triceps extensors) where contraction of one muscle coordinates with the relaxation of the other to produce opposite movements.

Flexor

A muscle whose contraction decreases the angle of a joint (e.g., the biceps flexing the forearm).

Extensor

A muscle whose contraction increases the angle of a joint (e.g., the triceps extending the forearm).

Peristalsis

Rhythmic, alternating contractions of circular and longitudinal muscles traveling as a wave down an organism or tubular organ to produce locomotion or fluid propulsion.

Length-Tension Relationship

The principle stating that active isometric force generation depends on initial sarcomere length, with maximum tension occurring at an optimal resting length ($2.0\text{ to }2.3\,\mu\text{m}$) due to optimal actin-myosin overlap.

Muscle Twitch

A single, isolated contraction-relaxation cycle in a single muscle fiber triggered by a single motor neuron action potential.

Latent Period

The short delay ($10\text{ to }100\,\text{msec}$) between the initial muscle action potential and the onset of physical tension, primarily required for excitation-contraction coupling and $\text{Ca}^{2+}$ diffusion.

Temporal Summation

The cumulative increase in muscle tension that occurs when repeated action potentials arrive before the muscle fiber has completely relaxed from the previous contraction.

Unfused (Incomplete) Tetanus

A state of sustained, undulating muscle tension produced when repetitive stimuli arrive close enough together that the fiber only relaxes slightly between stimuli.

Complete (Fused) Tetanus

A state of steady, maximal muscle tension achieved when high-frequency stimulation prevents any relaxation between successive stimuli.

Muscle Fatigue

The reversible inability of a muscle fiber to maintain contractile tension during periods of sustained, repetitive activation, particularly common in glycolytic fibers.

Motor Unit

A single somatic motor neuron and all the individual skeletal muscle fibers that it innervates.

Motor Pool

The collection of all individual motor neurons that innervate a given single muscle.

Size Principle of Motor Unit Recruitment

The physiological recruitment hierarchy in which smaller, lower-threshold, weaker motor units are recruited first, and larger, stronger motor units are recruited as stimulus intensity increases.

Asynchronous Recruitment

A neural strategy during sustained submaximal muscle contractions where the CNS alternates active motor units to allow resting units to synthesize ATP and prevent fatigue.

Type I (Slow-Twitch Oxidative) Fiber

Red muscle fiber with slow myosin ATPase activity, high myoglobin content, abundant mitochondria, and high fatigue resistance, specialized for endurance and posture maintenance.

Type IIA (Fast-Twitch Oxidative-Glycolytic) Fiber

Red-to-intermediate muscle fiber with fast myosin ATPase activity, intermediate diameter, high capillary density, and fatigue resistance, used for standing and walking.

Type IIB / IIX (Fast-Twitch Glycolytic) Fiber

White, large-diameter muscle fiber with the fastest myosin ATPase activity, rapid $\text{Ca}^{2+}\text{-ATPase}$ activity, and high anaerobic glycolytic capacity; develops force rapidly but fatigues quickly.

Myoglobin

An intracellular oxygen-binding heme protein that imparts a dark red color to slow-twitch oxidative muscle fibers and provides an oxygen reserve for oxidative phosphorylation.

Myofibrillar Hypertrophy

The enlargement of muscle fibers driven by the increased synthesis and accumulation of actin and myosin contractile myofibrils.

Sarcoplasmic Hypertrophy

The expansion of skeletal muscle cell volume caused by increased sarcoplasmic fluid volume and glycogen storage capacity.