Skeletal Muscle Physiology, Nutrient Uptake, and Motor Control

Vocabulary flashcards covering contraction-mediated nutrient uptake, muscle fiber types and mechanics, motor unit recruitment principles, muscle reflexes, and cramping theories from lecture notes.

GLUT4 transporters

Specialized glucose transporters that translocate to the muscle cell membrane in response to muscle contraction during exercise or insulin stimulation to clear glucose from blood plasma.

CD36 transporter

A specialized membrane transporter that facilitates fatty acid uptake into skeletal muscle cells, reaching maximal membrane translocation at approximately $60\%\,VO_2\text{ max}$.

AMPK (AMP-activated protein kinase)

An intracellular enzyme functioning as a low-energy sensor that is activated by elevated AMP concentrations to trigger signaling cascades that move GLUT4 and CD36 transporters to the cell surface.

Hypoglycemia

A dangerous drop in blood glucose that exercise suppresses by shutting down insulin secretion, thereby preventing excessive GLUT4-mediated glucose extraction away from the brain.

Myosin isoforms

Variations of the myosin thick-filament protein across fiber types that exhibit differing ATPase activity rates, determining the velocity and power of sarcomere shortening.

ATPase

The enzyme located on the myosin head that hydrolyzes ATP into ADP and $P_i$ to power muscle contraction, with activity rates highest in Type IIx fibers and lowest in Type I fibers.

Type I muscle fibers

Slow-twitch muscle fibers characterized by high mitochondrial density, high capillary density, high fatigue resistance, high lipid storage, and a primary reliance on aerobic metabolism.

Type IIx muscle fibers

Fast-twitch muscle fibers characterized by the fastest ATPase activity, rapid shortening velocity, high force and power generation, high glycogen content, and high glycolytic capacity.

Type IIa muscle fibers

Intermediate skeletal muscle fibers that exhibit biochemical, contractile, and metabolic properties falling between Type I and Type IIx fibers.

Hybrid muscle fibers

Muscle fibers containing multiple myosin isoforms (such as 1-2a or 2a-2x), accounting for about $15\%$ of muscle fibers, which can adapt their gene expression in response to training.

Specific force (Specific tension)

The pound-for-pound force generated per unit of cross-sectional area of a muscle fiber, which is highest in Type IIx fibers compared to Type IIa and Type I fibers.

Fatigue

The inability of a skeletal muscle to maintain force production during sustained or repeated contractions.

Mitochondrial network

A complex, power-grid-like reticulum of interconnected mitochondria in muscle cells positioned directly over sarcomeres to produce ATP close to cross-bridges.

Motor unit

An alpha motor neuron and all the specific skeletal muscle fibers it innervates, all of which share the same uniform muscle fiber type.

Size principle of recruitment

The orderly recruitment rule stating that smaller motor units with lower depolarization thresholds (Type I) are recruited first, followed sequentially by larger motor units (Type IIa, then Type IIx) as required force rises.

Length-tension relationship

The physiological principle demonstrating that maximal active muscle force occurs at an optimal sarcomere length of $80\%$ to $120\%$ of resting length due to optimal actin-myosin cross-bridge alignment.

Muscle spindle reflex

A protective stretch reflex initiated by spindle sensory receptors inside the muscle belly that excites alpha motor neurons to induce reflexive muscle contraction when rapid elongation is sensed.

Golgi tendon organ (GTO) reflex

A protective tension reflex originating in tendon receptors that detects high mechanical tension and signals the spinal cord to cause reflexive muscle relaxation, preventing tendon ruptures.

Inhibitory interneuron

A spinal cord interneuron activated by sensory afferents from the Golgi tendon organ that suppresses efferent motor neuron discharge to trigger reflexive muscle relaxation.

Electrolyte depletion theory

A cramping hypothesis asserting that heavy sweat losses cause fluid and electrolyte imbalances that disrupt neuromuscular transmission and motor control, provoking involuntary contractions.

Neuromuscular control theory

A cramping hypothesis proposing that exercise-associated muscle cramps result from altered reflex activity characterized by increased muscle spindle excitation and reduced Golgi tendon organ inhibition.