BIOL-2401 Chapter 9: Muscles & Muscle Tissue

Comprehensive vocabulary flashcards covering skeletal, cardiac, and smooth muscle histology, physiology of contraction, neuromuscular junction events, action potentials, crossbridge cycling, tension development, and metabolic pathways.

Five primary functions of skeletal muscle

Producing body movement, maintaining posture and stabilizing joints, protecting and supporting internal organs, regulating the passage of materials via sphincters, and producing heat.

Supportive roles of skeletal muscle beyond body movement

Stabilizing joint articulation, protecting abdominopelvic viscera, controlling voluntary elimination via sphincters, and generating heat to maintain normal body temperature.

Excitability

The ability of muscle tissue to respond to a stimulus by changing its membrane potential.

Conductivity

The ability of muscle tissue to propagate electrical signals (action potentials) along the cell membrane.

Contractility

The ability of muscle tissue to generate pulling force through interactions between actin and myosin.

Extensibility

The ability of muscle tissue to be stretched without sustaining damage.

Elasticity

The ability of muscle tissue to return to its original resting length after being stretched.

Cause of multinucleation in skeletal muscle fibers

During embryonic development, multiple single-nucleated myoblasts fuse together to form each mature, elongated muscle fiber.

Satellite cells

Adult muscle stem cells associated with skeletal muscle fibers that participate in cellular repair and limited regeneration.

Sarcoplasm

The cytoplasm of a muscle fiber containing typical cellular organelles, including abundant mitochondria and glycogen stores.

Sarcolemma

The plasma membrane of a muscle fiber that envelops the sarcoplasm and conducts action potentials along its length.

Transverse tubules (T-tubules)

Deep invaginations of the sarcolemma that extend into the interior of the muscle fiber to rapidly convey action potentials to the sarcoplasmic reticulum.

Sarcoplasmic reticulum (SR)

A specialized smooth endoplasmic reticulum network surrounding myofibrils that stores and releases $Ca^{2+}$ to regulate contraction.

Myofibrils

Long, cylindrical intracellular bundles of contractile and regulatory myofilaments that span the length of a skeletal muscle fiber.

Myosin

The motor protein that forms thick filaments, featuring heads that bind actin and generate mechanical pulling force using ATP.

Actin

The primary contractile protein of thin filaments that possesses binding sites for myosin heads during muscle contraction.

Tropomyosin

A regulatory filamentous protein on thin filaments that physically blocks myosin-binding sites on actin when a muscle is at rest.

Troponin

A globular regulatory protein on thin filaments that binds $Ca^{2+}$, causing a conformational shift in tropomyosin to expose actin's myosin-binding sites.

Sarcomere

The basic functional and contractile unit of skeletal muscle, consisting of repeating arrays of organized myofilaments delimited between two Z discs.

Motor unit

A functional group consisting of a single somatic motor neuron and all the skeletal muscle fibers that it innervates.

Neuromuscular junction (NMJ)

The specialized communication site where a somatic motor neuron meets a skeletal muscle fiber to stimulate contraction.

Structural relationship between a motor unit and neuromuscular junctions

A single somatic motor neuron branches its axon terminals so that each individual terminal forms a distinct neuromuscular junction with one skeletal muscle fiber.

Sequence of events at the neuromuscular junction (NMJ)

1. An action potential reaches the synaptic knob. 2. Voltage-gated $Ca^{2+}$ channels open and $Ca^{2+}$ enters the synaptic knob. 3. Synaptic vesicles exocytose acetylcholine (ACh) into the synaptic cleft. 4. ACh binds to receptors on the motor end plate, causing rapid $Na^+$ influx and an end-plate potential.

Difference between motor end plate ACh receptors and sarcolemma ion channels

ACh receptors at the motor end plate are chemically gated (ligand-gated) cation channels opened by acetylcholine binding, whereas channels along the sarcolemma are voltage-gated $Na^+$ and $K^+$ channels opened by changes in membrane potential.

Dual physiological roles of $Ca^{2+}$ in muscle excitation-contraction

At the motor neuron synaptic knob, $Ca^{2+}$ entry triggers the exocytosis of acetylcholine; inside the muscle fiber sarcoplasm, $Ca^{2+}$ binds to troponin to shift tropomyosin and expose myosin-binding sites on actin.

Resting membrane potential (RMP) of skeletal muscle

The electrical charge difference across the sarcolemma of an unstimulated, resting skeletal muscle fiber, established at $-90\,\text{mV}$.

End-plate potential (EPP)

A local depolarization at the motor end plate caused by the influx of $Na^+$ through chemically gated ACh receptors, shifting membrane potential from $-90\,\text{mV}$ to the threshold of $-65\,\text{mV}$.

Depolarization (sarcolemma)

The phase of an action potential characterized by the opening of voltage-gated $Na^+$ channels, causing a rapid influx of $Na^+$ that reverses membrane polarity from $-65\,\text{mV}$ to $+30\,\text{mV}$.

Repolarization (sarcolemma)

The phase of an action potential where voltage-gated $Na^+$ channels close and voltage-gated $K^+$ channels open, allowing $K^+$ to flow out and restoring membrane polarity from $+30\,\text{mV}$ back to $-90\,\text{mV}$.

Sequence of excitation-contraction coupling

An end-plate potential initiates an action potential that travels along the sarcolemma and down T-tubules to triads, stimulating voltage-sensitive receptors to open sarcoplasmic reticulum $Ca^{2+}$ release channels and flood the sarcoplasm with $Ca^{2+}$.

Triad

A microscopic membrane assembly composed of one central transverse tubule (T-tubule) flanked by two terminal cisternae of the sarcoplasmic reticulum.

Mechanism of actin-myosin inhibition at rest and relief by $Ca^{2+}$

At rest, tropomyosin physically covers myosin-binding sites on actin; when $Ca^{2+}$ binds troponin, troponin undergoes a conformational shift that pulls tropomyosin away, exposing the actin binding sites.

Crossbridge formation

The initial step of crossbridge cycling where an energized (cocked) myosin head attaches to an exposed myosin-binding site on actin.

Power stroke

The mechanical step in crossbridge cycling where the myosin head swivels toward the center of the sarcomere, pulling the thin filament inward while releasing ADP and $P_i$.

Detachment (crossbridge cycling)

The step in the crossbridge cycle where a new ATP molecule binds to the myosin head, triggering its release from the actin filament.

Resetting of myosin head (reactivation)

The hydrolytic step in crossbridge cycling where ATP is cleaved into ADP and $P_i$ by myosin ATPase, re-cocking the myosin head into its energized conformation.

Consequence of ATP absence on actin-myosin interaction

Without ATP binding, myosin heads remain locked to actin in a permanent crossbridge state, preventing muscle relaxation (the physiological basis of rigor mortis).

Muscle twitch

A single, brief contraction-relaxation event produced by a muscle fiber or motor unit in response to a single electrical stimulus.

Latent period

The brief delay between the application of a stimulus and the initial onset of force generation, during which excitation-contraction coupling takes place without measurable tension.

Contraction period

The twitch phase where crossbridge cycling actively occurs, causing internal tension to rise up to its peak value.

Relaxation period

The twitch phase where $Ca^{2+}$ is actively transported back into the sarcoplasmic reticulum, causing crossbridges to detach and tension to return to baseline levels.

Motor unit recruitment

The progressive activation of increasing numbers of motor units by the nervous system to produce greater muscle tension.

Size principle

The physiological order of motor unit recruitment stating that smaller, lower-threshold motor units are recruited first, followed by progressively larger units as force requirements increase.

Compatibility of graded muscle tension with the all-or-none principle

While each individual muscle fiber contracts completely once threshold is reached (all-or-none), whole-muscle force is graded through recruiting varying numbers of motor units and modulating stimulation frequency.

Wave summation

The cumulative increase in muscle tension observed when successive stimuli arrive before a muscle fiber has completely relaxed from the previous contraction.

Incomplete tetany

A state of sustained, undulating muscle contraction resulting from high stimulation frequency with only partial relaxation occurring between stimuli.

Complete tetany

A steady, maximal muscle contraction with smooth, continuous tension and no visible relaxation phase between very high-frequency stimuli.

Muscle fatigue

A physiological reduction or decline in a muscle's capacity to develop and maintain contractile tension during prolonged or repetitive activity.

Direct phosphorylation

An anaerobic metabolic reaction where creatine kinase transfers a high-energy phosphate from creatine phosphate (CP) to ADP, supplying $1\,\text{ATP}$ per CP for about $15\,\text{seconds}$.

Anaerobic pathway (glycolysis)

An oxygen-independent metabolic pathway that converts glucose into pyruvate and lactate, generating a net gain of $2\,\text{ATP}$ per glucose for about $30\text{--}40\,\text{seconds}$ of strenuous activity.

Aerobic pathway (cellular respiration)

An oxygen-dependent metabolic process occurring in mitochondria that oxidizes glucose, fatty acids, or amino acids to yield about $32\,\text{ATP}$ per glucose, supplying energy for hours of continuous activity.

Oxygen debt (EPOC)

Excess post-exercise oxygen consumption required to replenish myoglobin stores, reload ATP and creatine phosphate reserves, metabolize accumulated lactate, and normalize body homeostasis.

Slow oxidative (SO) fibers

Type I muscle fibers characterized by slow contraction speed, low relative force, aerobic ATP production, abundant mitochondria, and high resistance to fatigue.

Fast oxidative (FO) fibers

Type IIa muscle fibers that exhibit fast contraction speed, moderate force production, aerobic and glycolytic ATP synthesis, and intermediate resistance to fatigue.

Fast glycolytic (FG) fibers

Type IIx muscle fibers that contract rapidly, generate high levels of force, rely mainly on anaerobic glycolysis, and fatigue quickly due to sparse mitochondria.

Fiber type suitability for posture versus high-force activities

Slow oxidative fibers excel at sustained posture due to fatigue-resistant aerobic metabolism, whereas fast glycolytic fibers excel at brief, high-force actions (like sprinting) due to rapid crossbridge cycling and glycolytic energy production.

Microscopic anatomy and features of cardiac muscle

Involuntary, striated muscle found in the myocardium featuring short, branched cells with 1 or 2 central nuclei, abundant mitochondria, and intercalated discs with desmosomes and gap junctions.

Microscopic anatomy and features of smooth muscle

Involuntary, non-striated muscle composed of fusiform (spindle-shaped) cells with a single central nucleus, no T-tubules (uses caveolae), and relies on calmodulin instead of troponin for contraction.

Intercalated discs

Specialized cell-to-cell junctions between adjacent cardiac muscle cells consisting of desmosomes for structural integrity and gap junctions for synchronous electrical propagation.

Calmodulin

A cytoplasmic calcium-binding regulatory protein in smooth muscle cells that forms a $Ca^{2+}$-calmodulin complex to activate myosin light-chain kinase.

Myosin light-chain kinase (MLCK)

An enzyme activated by the $Ca^{2+}$-calmodulin complex in smooth muscle that phosphorylates myosin heads, enabling them to bind actin filaments.