Sliding Filament Mechanism & Neuromuscular Junction

Vocabulary flashcards covering key terms in the sliding filament mechanism and neuromuscular junction processes described in the lecture.

Sliding Filament Mechanism

Process where thick (myosin) and thin (actin) filaments slide past one another, shortening the sarcomere and causing muscle contraction.

Myosin

Motor protein that forms thick filaments; its heads bind actin, hydrolyze ATP, and generate the power stroke.

Actin

Protein that forms thin filaments; contains binding sites for myosin heads.

Troponin

Regulatory protein that binds Ca²⁺, changes shape, and moves tropomyosin off actin’s binding sites.

Tropomyosin

Rod-shaped protein that covers actin’s myosin-binding sites when the muscle is at rest.

Sarcomere

Functional contractile unit of muscle fiber, bordered by Z discs and containing overlapping thick and thin filaments.

Z Disc

Protein boundary of a sarcomere where thin filaments anchor.

M Line

Central line in a sarcomere that stabilizes thick filaments.

Zone of Overlap

Region where actin and myosin filaments overlap; increases during contraction.

Myosin Head

Pivoting portion of myosin that binds actin, hydrolyzes ATP, and performs the power stroke.

Cross-Bridge

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

Power Stroke

Pivot of the myosin head that pulls actin toward the M line.

ATP (Adenosine Triphosphate)

Energy molecule that resets myosin heads and enables their release from actin.

ADP + Pi

Products of ATP hydrolysis that remain bound to myosin until the power stroke occurs.

Hydrolysis (of ATP)

Chemical splitting of ATP into ADP and inorganic phosphate, releasing energy for myosin head cocking.

Sarcoplasmic Reticulum (SR)

Specialized endoplasmic reticulum in muscle fibers that stores and releases Ca²⁺.

Terminal Cisternae

Dilated end sacs of the SR that release Ca²⁺ upon stimulation.

Transverse (T) Tubules

Invaginations of the sarcolemma that conduct action potentials into the cell interior.

Sarcolemma

Plasma membrane of a muscle fiber.

Cytosol

Intracellular fluid where Ca²⁺ binds troponin during contraction.

Calcium Ions (Ca²⁺)

Ions released from SR that bind troponin and initiate contraction.

Sodium Ions (Na⁺)

Ions that enter the muscle fiber via ligand-gated channels, generating the muscle action potential.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and a muscle fiber that initiates muscle action potentials.

Synaptic Bulb (End Bulb)

Swollen axon terminal containing synaptic vesicles filled with neurotransmitter.

Synaptic Vesicle

Membrane-bound sac in the synaptic bulb that stores neurotransmitter (acetylcholine).

Synaptic Cleft

Narrow space between synaptic bulb and muscle fiber where neurotransmitter diffuses.

Acetylcholine (ACh)

Neurotransmitter released at the NMJ that binds receptors on the motor end plate.

Acetylcholinesterase (AChE)

Enzyme in the synaptic cleft that rapidly breaks down acetylcholine, ending stimulation.

Motor End Plate

Specialized region of sarcolemma with ACh receptors and ligand-gated Na⁺ channels.

Action Potential

Rapid electrical signal traveling along neuron or sarcolemma, triggering Ca²⁺ release.

Voltage-Gated Calcium Channel

Channel in synaptic bulb that opens in response to an action potential, allowing Ca²⁺ entry.

Ligand-Gated Sodium Channel

Sarcolemma channel that opens when ACh binds, permitting Na⁺ influx.

Depolarization

Membrane potential change caused by Na⁺ influx, initiating the muscle action potential.

Calcium ATPase Pump

SR membrane pump that uses ATP to return Ca²⁺ to the SR, leading to relaxation.

Length-Tension Relationship

Correlation between sarcomere length at rest and the force it can generate during contraction.