Muscle Physiology and Contraction

Comprehensive vocabulary flashcards covering the cross-bridge cycle, neuromuscular junction, energy systems, muscle mechanics, fiber types, muscle tissues, and internal muscle organization.

Cross-bridge

A structure formed when an energized myosin head attaches to an exposed binding site on actin.

Power stroke

The phase of the cross-bridge cycle where the myosin head pivots, pulling actin toward the center of the sarcomere and releasing ADP and phosphate.

Detachment

The step in the cross-bridge cycle where a new ATP molecule binds to myosin, causing it to release from actin.

Reactivation

The step where myosin breaks down ATP into ADP and phosphate, using the released energy to re-cock the myosin head for another cycle.

Tropomyosin

A protein that blocks the myosin-binding sites on actin at rest and shifts away when calcium binds to troponin.

Troponin

A regulatory protein on actin that binds calcium, causing tropomyosin to move and expose binding sites for myosin.

Neuromuscular junction (NMJ)

The site where a motor neuron communicates with a skeletal muscle fiber.

Motor neuron

A nerve cell that sends electrical signals to control muscle activity.

Axon terminal

The end of a motor neuron that stores and releases neurotransmitters.

Synaptic cleft

The microscopic gap separating the axon terminal of a neuron from the muscle fiber.

Acetylcholine (ACh)

The neurotransmitter released by a motor neuron to stimulate an electrical signal in a muscle fiber.

Motor end plate

The specialized region of the sarcolemma containing receptors that bind acetylcholine.

Acetylcholinesterase

The enzyme in the synaptic cleft that degrades acetylcholine to terminate the muscle stimulation signal.

Creatine phosphate

A high-energy molecule (phosphocreatine) that rapidly donates a phosphate to ADP to regenerate ATP without requiring oxygen.

Anaerobic glycolysis

A metabolic pathway that breaks down glucose to generate ATP quickly without oxygen, converting excess pyruvate to lactate.

Aerobic respiration

An oxygen-dependent process in the mitochondria that breaks down nutrients to produce large amounts of ATP for prolonged activity.

Muscle twitch

The mechanical response of a single muscle fiber to a single electrical stimulus.

Latent period

The brief delay between the stimulus and the onset of measurable tension, during which the impulse spreads and calcium is released.

Contraction phase

The period of a muscle twitch where cross-bridge cycling causes tension to rise toward its peak.

Relaxation phase

The period of a twitch where $Ca^{2+}$ is actively transported back into the sarcoplasmic reticulum and muscle tension declines.

Tetanus (physiological)

A smooth, sustained muscle contraction produced by rapid, repetitive stimuli before relaxation can occur.

Incomplete (unfused) tetanus

A state of sustained muscle tension featuring slight fluctuations because the fiber relaxes partially between successive stimuli.

Complete (fused) tetanus

A continuous, steady contraction with no fluctuations, occurring when stimulus frequency is high enough to blend twitches entirely.

Temporal summation

The addition of muscle twitches resulting from increasing the frequency of stimulation to the same muscle fiber over time.

Spatial summation

The increase in contractile force resulting from the recruitment of additional motor units.

Motor unit

A functional entity consisting of a single motor neuron and all the muscle fibers it innervates.

Isometric contraction

A contraction in which tension increases but muscle length remains unchanged and the joint does not move.

Isotonic contraction

A contraction where muscle length changes while producing enough tension to move a load.

Concentric contraction

A subtype of isotonic contraction during which the muscle shortens as it generates force.

Eccentric contraction

A subtype of isotonic contraction during which the muscle lengthens under tension while controlling a load.

Muscle tone

The resting, involuntary, low-level tension maintained in skeletal muscles by the alternating activation of motor units.

Slow oxidative fibers (Type I)

Fatigue-resistant, small-diameter muscle fibers with abundant myoglobin and mitochondria, specialized for aerobic endurance activities.

Fast oxidative-glycolytic fibers (Type IIa)

Intermediate muscle fibers that contract rapidly and utilize both aerobic respiration and glycolysis for speed and moderate endurance.

Fast glycolytic fibers (Type IIx)

Large-diameter fibers that produce powerful, rapid contractions using glycolysis, but fatigue very quickly.

Skeletal muscle

Striated, voluntary muscle tissue attached to bones via tendons, responsible for body movement and maintaining posture.

Cardiac muscle

Striated, involuntary muscle tissue found exclusively in the myocardium of the heart, containing branched cells and intercalated discs.

Intercalated discs

Specialized junctional complexes between cardiac muscle cells that facilitate mechanical coupling and coordinated electrical signaling.

Smooth muscle

Non-striated, involuntary muscle tissue found in walls of hollow organs that regulates internal passage diameters and propels substances.

Fascicle

A distinct bundle of skeletal muscle fibers wrapped together inside a muscle organ.

Muscle fiber

An individual, elongated, multinucleated skeletal muscle cell packed with myofibrils.

Myofibril

A threadlike, cylindrical organelle within a muscle fiber composed of repeating segments called sarcomeres.

Sarcomere

The functional contractile unit of skeletal muscle defined as the region extending from one Z disc to the next Z disc.

Myofilaments

Protein filaments found within sarcomeres, specifically categorized into thin filaments (actin) and thick filaments (myosin).

Sarcoplasm

The specialized cytoplasm of a muscle fiber containing glycogen, enzymes, organelles, and myoglobin.

Sarcoplasmic reticulum (SR)

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

Myoglobin

An iron- and oxygen-binding pigment found in muscle sarcoplasm that serves as an internal oxygen reserve for aerobic ATP production.

T tubules (transverse tubules)

Deep invaginations of the sarcolemma that propagate action potentials from the cell surface into the fiber interior.

Triad

A structural assembly consisting of a central T tubule flanked on each side by a terminal cisterna of the sarcoplasmic reticulum.

Sliding filament theory

The mechanism of muscle contraction stating that thin actin filaments slide past thick myosin filaments without either changing length, shortening the sarcomere.

A band

The dark region of a sarcomere corresponding to the full length of the thick myosin filaments, which remains constant in length during contraction.