Muscular System: Contraction of Skeletal Muscle

Vocabulary flashcards covering the cross-bridge cycle, sliding filament theory, excitation-contraction coupling, motor units, and contraction disorders from Lecture 17.

Contraction Cycle Initiation

The start of the contraction cycle within the zone of overlap in a sarcomere, triggered by the arrival of calcium ions ($\text{Ca}^{2+}$).

Zone of Overlap

The region in a sarcomere where thick and thin filaments overlap and where calcium ions ($\text{Ca}^{2+}$) arrive to trigger cross-bridge cycling.

Troponin

A regulatory protein on the thin filament that binds $\text{Ca}^{2+}$, causing a positional shift that pulls tropomyosin away from actin active sites.

Tropomyosin

A regulatory protein that covers the active sites on actin in a resting muscle, preventing interaction with energized myosin heads.

Active Site

The specific binding location on an actin molecule where an energized myosin head attaches once the site is exposed.

Active-Site Exposure

The process whereby $\text{Ca}^{2+}$ binds troponin, weakening the bond to actin and rolling tropomyosin away from actin active sites.

Cross-Bridge

The physical connection formed when an energized myosin head binds to an exposed active site on actin.

Cross-Bridge Formation

The third step of the cross-bridge cycle in which energized myosin heads bind to exposed actin active sites.

Power Stroke

The pivoting motion of the myosin head toward the M line that releases stored energy and drags the actin filament past myosin.

M Line

The center line of a sarcomere toward which the myosin heads pivot during the power stroke.

Myosin Head Pivoting

The release of stored energy as the myosin head tilts toward the M line, accompanied by the dissociation of bound ADP and phosphate.

Cross-Bridge Detachment

The breaking of the bond between the myosin head and the active site on actin when an ATP molecule binds to the myosin head.

ATP (Adenosine Triphosphate)

The molecule required for cross-bridge detachment and whose hydrolysis provides the energy to recock the myosin head.

Myosin Reactivation

The step in which the free myosin head splits ATP into ADP and P, using the released energy to recock the head into its energized state.

Recocking

The repositioning of the myosin head back to its energized, resting-state angle using energy derived from ATP splitting.

Contraction Cycle Cessation

The end of the cross-bridge cycle occurring when $\text{Ca}^{2+}$ is pumped back into the sarcoplasmic reticulum and active sites are re-covered.

Sliding Filament Theory

The theory stating that muscle contraction occurs when myosin cross-bridges tilt in power strokes, pulling actin filaments past myosin to shorten the muscle.

A Band

The dark sarcomere region containing thick filaments that maintains a constant width during muscle contraction.

I Band

The light sarcomere region containing only thin filaments that shortens and gets smaller during contraction.

H Band

The lighter zone in the center of the A band that narrows as thin filaments slide toward the M line during contraction.

Z Line

The boundary structure of a sarcomere that moves closer to neighboring Z lines during muscle shortening.

Relaxed Sarcomere Structure

The organization of a resting sarcomere showing the designated locations of the A band, Z lines, H band, and I bands.

Sarcomere

The functional repeating contractile unit of a muscle fibre located between two adjacent Z lines.

Myofibril

A cylindrical intracellular structure composed of repeating sarcomeres that shortens when muscle fibres contract.

Bilateral Free End Shortening

The contraction pattern where both ends of a myofibril are free to move, causing both ends to pull inward toward the center.

Unilateral Fixed End Shortening

The contraction pattern where one end of a myofibril is anchored in place, causing the free end to move toward the fixed end.

Motor Unit

A single alpha motor neuron and all the skeletal muscle fibres it innervates.

Alpha Motor Neuron

A motor neuron with its cell body in the central nervous system that sends action potentials to stimulate skeletal muscle fibers.

Axon Hillock

The junction between the alpha motor neuron cell body and the axon where action potentials originate.

Dendrites

Branching extensions of the alpha motor neuron cell body that receive signals from other neurons.

Terminal Branches

The distal axonal arborizations of a motor neuron that branch out to innervate individual muscle fibres.

Motor End Plate

The specialized, folded region of the muscle fibre sarcolemma located directly opposite the axon terminal branches.

Synaptic Cleft

The narrow extracellular fluid space separating the axon terminal of a motor neuron from the sarcolemma of a muscle fibre.

Acetylcholine (ACh)

The neurotransmitter released at the neuromuscular junction that binds to receptors on the sarcolemma to trigger excitation.

ACh Receptor

A membrane receptor protein located on the sarcolemma that binds acetylcholine released from the axon terminal.

Sarcolemma

The excitable plasma membrane of a muscle fibre along which action potentials spread.

Transverse Tubule (T Tubule)

Tubular invaginations of the sarcolemma that conduct electrical action potentials deep into the interior of the muscle fibre.

Sarcoplasmic Reticulum (SR)

A specialized membrane network in muscle fibres that stores, releases, and reabsorbs calcium ions ($\text{Ca}^{2+}$).

Terminal Cisternae

Expanded end sacs of the sarcoplasmic reticulum situated adjacent to T tubules that release $\text{Ca}^{2+}$ into the sarcoplasm.

Excitation-Contraction Coupling

The physiological sequence connecting electrical excitation of the sarcolemma with $\text{Ca}^{2+}$ release and cross-bridge cycling.

Action Potential Propagation

The rapid electrical impulse that spreads across the muscle fibre sarcolemma and travels deep along the T tubules.

Calcium Ions ($\text{Ca}^{2+}$)

The intracellular chemical messengers released by the sarcoplasmic reticulum that bind troponin to initiate cross-bridge cycling.

Acetylcholinesterase (AChE)

The enzyme located at the neuromuscular junction that breaks down acetylcholine, terminating action potential generation.

Sarcoplasmic Calcium Reabsorption

The process wherein the sarcoplasmic reticulum reabsorbs $\text{Ca}^{2+}$, causing calcium concentration in the sarcoplasm to decline.

Active Site Recovery

The return of tropomyosin to its resting conformation over actin active sites once $\text{Ca}^{2+}$ dissociates from troponin.

Muscle Relaxation

The passive lengthening process in which a muscle returns to its resting length after cross-bridge cycling ends.

Rigor Mortis

A fixed muscular contraction occurring after death caused by ATP depletion and calcium accumulation in the cytosol.

Cytosolic Calcium Buildup

The postmortem accumulation of $\text{Ca}^{2+}$ in the muscle cytosol occurring because ion pumps cease operating without ATP.

Malignant Hyperthermia

A condition caused by a genetic mutation in the sarcoplasmic reticulum resulting in uncontrolled $\text{Ca}^{2+}$ release, severe contractions, and extreme fever.

Halothane

An anesthetic agent that triggers severe calcium release from the mutated sarcoplasmic reticulum in malignant hyperthermia.

Zone of Overlap

The region in a sarcomere where thick and thin filaments interlock, and where the arrival of calcium ions ($Ca^{2+}$) initiates the contraction cycle.

Troponin

A regulatory protein on the thin filament that binds calcium ions ($Ca^{2+}$), causing a conformational change that shifts tropomyosin away from the active sites on actin.

Tropomyosin

A filamentous protein that covers the active sites on actin in resting muscle, preventing the binding of myosin heads until moved by troponin.

Cross-Bridge Formation

The step in the contraction cycle where energized myosin heads bind to the exposed active sites on the actin filament.

Power Stroke

The pivoting of the myosin head toward the M line that occurs after cross-bridge formation, dragging the actin filament and releasing bound ADP and phosphate.

Cross-Bridge Detachment

The breaking of the bond between the myosin head and the active site on actin when a new ATP molecule binds to the myosin head.

Myosin Reactivation

The step where the free myosin head hydrolyzes ATP into ADP and P, using the released energy to recock the head into its energized conformation.

Sliding Filament Theory

The concept explaining muscle contraction where thin actin filaments slide past thick myosin filaments toward the center of the sarcomere, shortening the muscle and producing tension without shortening the individual filaments.

A Band

The dark band of the sarcomere comprising the entire length of the thick filaments, which maintains a constant width during muscle contraction.

I Band

The light band of the sarcomere containing only thin actin filaments, which narrows and decreases in width as muscle contraction occurs.

Z Lines

The boundary lines demarcating each end of a sarcomere that are pulled closer together during muscle contraction.

Motor Unit

A functional unit consisting of a single alpha motor neuron and all the skeletal muscle fibers that its terminal branches innervate.

Excitation-Contraction Coupling

The sequence of electrical and cellular events by which an action potential arriving at the sarcolemma leads to calcium release from the sarcoplasmic reticulum and mechanical force production.

Acetylcholine (ACh)

The neurotransmitter released from the motor neuron axon terminal into the synaptic cleft that binds to receptors on the sarcolemma to trigger a muscle action potential.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that breaks down acetylcholine, terminating action potential generation and initiating relaxation.

Transverse (T) Tubules

Tubular extensions of the sarcolemma that conduct electrical action potentials deep into the interior of the muscle fiber toward the sarcoplasmic reticulum.

Sarcoplasmic Reticulum

A specialized smooth endoplasmic reticulum in muscle fibers that sequesters, releases, and actively reabsorbs calcium ions ($Ca^{2+}$).

Rigor Mortis

A state of fixed muscular contraction after death that occurs when ATP is exhausted, preventing cross-bridge detachment and causing $Ca^{2+}$ to accumulate in the cytosol.

Malignant Hyperthermia

A condition linked to a genetic mutation in the sarcoplasmic reticulum that causes hypersensitivity to halothane, leading to massive uncontrollable $Ca^{2+}$ release, severe contractions, and dangerous elevations in body temperature.