Muscle Physiology: Contraction Mechanics, Energy Systems, Fiber Types, and Smooth Muscle

Comprehensive vocabulary flashcards covering skeletal muscle twitch dynamics, motor unit recruitment, ATP regeneration pathways, fiber classification, and smooth muscle structure and crossbridge regulation.

Motor Unit

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

Muscle Twitch

The simplest contractile response of a muscle fiber to a single action potential delivered by its motor neuron, recorded graphically as a myogram.

Latent Period

The initial phase of a muscle twitch during which excitation-contraction coupling takes place and $Ca^{2+}$ is released into the sarcoplasm, but no measurable muscle tension is generated.

Period of Contraction

The phase of a muscle twitch in which $Ca^{2+}$ binds to troponin, active cross-bridge cycling occurs, and muscle tension rises to a peak.

Period of Relaxation

The final phase of a muscle twitch where $Ca^{2+}$ is actively transported back into the sarcoplasmic reticulum, cross bridges detach, active sites are covered, and muscle tension returns to zero.

Muscle Tone

The baseline state of slight resting tension in skeletal muscles generated by involuntary, subconscious, alternating activation of motor units, which stabilizes joints and maintains metabolic activity.

Wave (Temporal) Summation

The additive increase in contractile force that occurs when a second stimulus is applied before the muscle has completely relaxed from the previous contraction, caused by the release of additional $Ca^{2+}$.

Unfused (Incomplete) Tetanus

A sustained, quivering contraction produced when the frequency of stimulation increases, allowing only brief, partial relaxation periods between successive twitches.

Fused (Complete) Tetanus

A smooth, sustained maximal contraction plateau that results when stimulation frequency is high enough to eliminate any relaxation between stimuli.

Subthreshold Stimulus

A stimulus voltage that is too weak to reach the threshold required to elicit any observable muscle contraction.

Threshold Stimulus

The minimum stimulus strength required to initiate an action potential and produce the very first observable muscle contraction.

Maximal Stimulus

The strongest electrical stimulus that recruits all available motor units in a muscle, producing its maximal contractile force.

Size Principle

The physiological recruitment rule stating that motor units with the smallest muscle fibers are activated first, while larger, more powerful motor units are progressively recruited only as stimulus strength increases.

Direct Phosphorylation

An anaerobic ATP regeneration pathway in which creatine phosphate transfers a high-energy phosphate group directly to ADP via creatine kinase, powering muscle contraction for about $15\,\text{seconds}$.

Creatine Kinase

The enzyme that catalyzes the direct transfer of a phosphate group from creatine phosphate to ADP to synthesize ATP.

Anaerobic Pathway (Anaerobic Glycolysis)

The metabolic process in which glucose is split in the cytosol into two molecules of pyruvic acid to produce a net of $2\,\text{ATP}$, and pyruvic acid is converted into lactic acid when oxygen is limited, supplying energy for $30\text{--}40\,\text{seconds}$.

Aerobic Respiration

The mitochondrial metabolic pathway that consumes oxygen to break down glucose, pyruvic acid, free fatty acids, or amino acids, yielding $32\,\text{ATP}$ per glucose along with $CO_2$ and $H_2O$ for hours of sustained activity.

Aerobic Endurance

The length of time a skeletal muscle can sustain contraction while relying on aerobic pathways during light-to-moderate physical activity.

Anaerobic Threshold

The physiological tipping point of exercise intensity at which muscle cells can no longer meet ATP demands aerobically and shift predominantly to anaerobic glycolysis.

Muscle Fatigue

A physiological state of inability to contract despite ongoing stimulation, commonly caused by ionic imbalances ($K^+$, $Na^+$, $Ca^{2+}$) altering membrane potentials and the depletion of cellular energy stores.

Excess Postexercise Oxygen Consumption (EPOC)

Also called oxygen debt, the elevated volume of oxygen consumed following exercise that is required to restore myoglobin and hemoglobin reserves, replace glycogen stores, and resynthesize ATP and creatine phosphate.

Muscle Hypertrophy

An enlargement of skeletal muscle mass resulting from an increase in muscle fiber diameter and increased numbers of myofilaments, mitochondria, and glycogen stores following resistance training.

Disuse Atrophy

The degeneration and loss of muscle mass that occurs when muscles are immobilized, unexercised, paralyzed, or subjected to aging.

Slow Oxidative (SO) Fibers

Type I skeletal muscle fibers that are small in diameter, dark red from high myoglobin levels, have abundant mitochondria and capillaries, split ATP slowly, generate energy aerobically, and are highly fatigue-resistant.

Fast Oxidative (FO) Fibers

Type IIa skeletal muscle fibers that are intermediate in size, red to pink, contract rapidly and powerfully, primarily synthesize ATP aerobically, and possess intermediate resistance to fatigue.

Fast Glycolytic (FG) Fibers

Type IIx skeletal muscle fibers that have the largest diameter, appear pale or white due to low myoglobin content, rely primarily on anaerobic glycolysis, contract rapidly with high force, and fatigue quickly.

Longitudinal Layer

The outer smooth muscle layer of hollow organs where cells are oriented parallel to the long axis, shortening organ length when contracted.

Circular Layer

The inner smooth muscle layer of hollow organs where cells run circumferentially around the organ, constricting the internal lumen diameter upon contraction.

Varicosities

Bulbous swellings along autonomic motor axons that store and release neurotransmitters across a diffuse junction rather than forming defined neuromuscular junctions.

Diffuse Junction

The wide extracellular synaptic cleft onto which autonomic varicosities release neurotransmitters to stimulate broad sheets of smooth muscle cells.

Caveolae

Pouchlike infoldings of the smooth muscle sarcolemma that concentrate voltage-gated $Ca^{2+}$ channels, enabling rapid entry of extracellular calcium into the sarcoplasm.

Calmodulin

The intracellular calcium-binding regulatory protein found in smooth muscle that substitutes for troponin, forming a complex with $Ca^{2+}$ to activate myosin light-chain kinase.

Dense Bodies

Anchoring protein structures dispersed throughout the sarcoplasm and attached to the sarcolemma of smooth muscle cells that bind intermediate filaments and thin actin filaments, functioning analogously to Z discs.

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 and undergo cross-bridge cycling.

Smooth Muscle Tone

A state of continuous, moderate contraction maintained by smooth muscle organs over prolonged durations at low metabolic energy costs.