Muscle Excitation & Contraction

Vocabulary practice flashcards covering muscle tissue types, fiber structure, sarcomere components, sliding-filament mechanism, electrophysiology, crossbridge cycle, energy pathways, and whole muscle mechanics.

Resting Membrane Potential (RMP)

The voltage difference across a cell's plasma membrane when it is not being stimulated, resting at approximately $-80\,mV$ to $-85\,mV$ in a skeletal muscle fiber.

Electrochemical Gradient

The combined driving force of concentration and electrical gradients that determines the direction and rate of ion diffusion across a membrane.

$\text{Na}^+/\text{K}^+$ Pump

An active transport membrane protein that consumes ATP to pump $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell, maintaining normal resting ion gradients.

Action Potential (AP)

A rapid, temporary change in the electrical membrane potential that depolarizes from a negative RMP to a positive value and subsequently repolarizes back to RMP.

Depolarization

The phase of an action potential during which voltage-gated $\text{Na}^+$ channels open and $\text{Na}^+$ enters the cell, shifting the membrane potential to a more positive value.

Threshold

The critical membrane potential level that an initial stimulus must reach to trigger the opening of voltage-gated $\text{Na}^+$ channels and propagate an action potential.

Repolarization

The phase of an action potential where voltage-gated $\text{Na}^+$ channels close and voltage-gated $\text{K}^+$ channels open, allowing $\text{K}^+$ to exit and making the membrane potential negative again.

Neuromuscular Junction (NMJ)

The specialized synapse between a motor neuron's axon terminal and the motor end plate of a skeletal muscle fiber.

Axon Terminal

The swollen end (synaptic bulb) of a motor neuron axon containing synaptic vesicles filled with acetylcholine.

Acetylcholine (ACh)

The neurotransmitter released from synaptic vesicles at the neuromuscular junction to initiate excitation of a skeletal muscle fiber.

Motor End Plate

The specialized, folded region of the sarcolemma opposite the axon terminal that contains ligand-gated cation channels acting as acetylcholine receptors.

End-Plate Potential

A localized depolarization produced at the motor end plate of a muscle fiber when acetylcholine binds to ligand-gated cation channels, allowing cations to enter.

Excitation-Contraction Coupling

The sequence of events connecting the generation of a muscle action potential along the sarcolemma and down T-tubules to the release of calcium ions from the terminal cisternae.

Crossbridge

The physical bond formed between a cocked myosin head and an exposed active site on an actin filament.

Power Stroke

The event in which ADP and phosphate release from the myosin head, causing it to pivot from a high-energy cocked position to a relaxed position and pull actin toward the M line.

Acetylcholinesterase

The enzyme located in the synaptic cleft that degrades remaining acetylcholine, enabling repolarization and muscle relaxation.

Creatine Kinase

The enzyme that catalyzes the transfer of a phosphate group from creatine phosphate to ADP, producing ATP for up to approximately $10\text{ seconds}$ of maximal muscle activity.

Glycolytic Catabolism (Glycolysis)

An anaerobic series of enzymatic reactions in the cytosol that breaks down glucose into pyruvate, yielding a net of $2\text{ ATP}$ to support $30\text{--}40\text{ seconds}$ of contraction.

Oxidative Catabolism

An aerobic ATP-generating pathway in the mitochondria requiring oxygen and carbon fuels (such as glucose, fatty acids, and amino acids) that supplies energy for hours.