Neuromuscular Junction & Excitation-Contraction Coupling

30 vocabulary flashcards covering key structures, molecules, and events involved in neuromuscular transmission, excitation-contraction coupling, and cross-bridge cycling.

Action Potential (AP)

A self-propagating electrical signal that travels along the sarcolemma and down T-tubules, initiating muscle fiber activation.

Sarcolemma

The plasma membrane of a muscle fiber; conducts action potentials and forms the motor end plate.

T-Tubule

Invagination of the sarcolemma that carries the action potential deep into the muscle fiber.

Voltage-Sensitive Tubule Protein

Protein in the T-tubule membrane that changes shape when depolarized, mechanically opening Ca2+ release channels on the SR.

Sarcoplasmic Reticulum (SR)

Specialized endoplasmic reticulum in muscle cells that stores and releases Ca2+ for contraction.

Ca2+ Release Channel (Ryanodine Receptor)

Channel in the SR terminal cistern that opens to flood the sarcoplasm with Ca2+ during excitation-contraction coupling.

Motor Neuron

Nerve cell whose axon branches to innervate skeletal muscle fibers, delivering the stimulus for contraction.

Neuromuscular Junction (NMJ)

Chemical synapse between a motor neuron’s axon terminal and the muscle fiber’s motor end plate.

Synaptic Cleft

Tiny extracellular gap separating the axon terminal from the motor end plate where neurotransmitter diffuses.

Synaptic Vesicle

Membranous sac in the axon terminal containing the neurotransmitter acetylcholine (ACh).

Acetylcholine (ACh)

Neurotransmitter released at the NMJ that binds sarcolemmal receptors to start an end plate potential.

Acetylcholine Receptor

Chemically gated ion channel on the motor end plate that opens upon ACh binding to allow Na+ influx.

Acetylcholinesterase

Enzyme in the synaptic cleft that rapidly breaks down ACh, terminating its signal.

End Plate Potential (EPP)

Local depolarization of the motor end plate caused by Na+ influx through ACh-gated channels.

Voltage-Gated Sodium Channel

Membrane protein that opens when threshold depolarization is reached, propagating the action potential.

Depolarization

Phase in which the membrane potential becomes less negative due to Na+ entry.

Repolarization

Return of the membrane potential to resting negativity, driven by K+ efflux.

Refractory Period

Brief time during repolarization when the muscle fiber cannot generate another action potential.

Sodium/Potassium Pump

Active transporter restoring ionic gradients after an action potential by moving 3 Na+ out and 2 K+ in.

Excitation-Contraction (E-C) Coupling

Sequence linking sarcolemmal excitation to Ca2+ release and subsequent contraction.

Latent Period

Short delay between the action potential and onset of tension development while E-C coupling occurs.

Troponin

Regulatory protein that binds Ca2+, causing a shape change that shifts tropomyosin off actin sites.

Tropomyosin

Rod-shaped protein that blocks myosin-binding sites on actin in a relaxed muscle fiber.

Cross Bridge

Connection formed when a myosin head attaches to an exposed actin binding site.

Power Stroke

Pivot of the myosin head to a low-energy state, pulling actin toward the M line and generating force.

ATP (Adenosine Triphosphate)

Energy molecule that detaches myosin from actin and re-cocks the myosin head after hydrolysis.

Myosin Head

Motor domain of thick filament that binds actin, hydrolyzes ATP, and produces movement.

Cross Bridge Cycle

Repeating sequence of attachment, power stroke, detachment, and cocking of myosin heads as long as Ca2+ and ATP are present.

Rigor Mortis

Post-mortem stiffening when ATP is absent, preventing myosin heads from detaching from actin.

Ca2+ ATPase (SR Pump)

Active transport protein that sequesters Ca2+ back into the SR, allowing muscle relaxation.