Muscle Structure and Contraction - Vocabulary

Vocabulary flashcards covering key components of muscle structure, contraction mechanism, Ca2+ handling, membranes, and neuromuscular junction processes.

Fascicle

A group of muscle fibers wrapped in a connective tissue called perimysium.

Perimysium

The connective tissue sheath surrounding a fascicle.

Epimysium

The connective tissue wrapping the entire muscle.

Endomysium

The connective tissue surrounding each individual muscle fiber.

Muscle belly

A group of fascicles wrapped in a connective tissue called epimysium.

Tendon

Extensions of perimysium and epimysium that attach muscle to bone; interwoven within the muscle; very strong.

Origin

The attachment of a muscle that stays stationary during contraction.

Insertion

The attachment that moves toward the origin during contraction.

Sarcomere

The functional unit of a muscle; contains thick (myosin) and thin (actin) filaments, titin; boundaries are Z-disc and M-line with I-band, A-band, and H-zone.

Z-disc

The boundary line of a sarcomere where actin filaments anchor.

M-line

The center line of the sarcomere where myosin filaments align.

I-band

Region containing only thin actin filaments within a sarcomere.

A-band

Region that includes the length of thick myosin filaments and overlapping actin.

H-zone

Center portion of the sarcomere where only myosin is present; narrows during contraction.

Myofibril

A bundle of sarcomeres inside a muscle fiber.

Actin

Thin filament; includes G-actin (globular) and F-actin (filamentous); provides binding sites for myosin (exposed when tropomyosin moves).

Myosin

Thick filament; has heads that form cross-bridges with actin; contains ATPase.

G-Actin

Globular actin monomer that polymerizes to form F-actin.

F-Actin

Filamentous actin; polymerized strand of actin forming the backbone of thin filaments.

Tropomyosin

Rope-like protein that blocks myosin-binding sites on actin in relaxed muscle.

Troponin complex

Three-protein complex (Troponin C, I, T) that regulates tropomyosin and exposes myosin-binding sites when Ca2+ binds.

Calcium (Ca2+)

Ion stored in the sarcoplasmic reticulum; released to bind troponin and expose actin binding sites to enable contraction.

Sarcoplasmic Reticulum

Calcium reservoir surrounding myofibrils; releases Ca2+ for contraction and reabsorbs it for relaxation.

T-tubule

Invagination of the sarcolemma that carries action potentials into the interior of the muscle fiber.

Triad

Arrangement of T-tubule with terminal cisternae of the SR surrounding a sarcomere; site of Ca2+ release.

Terminal cisterna

Dilated ends of the sarcoplasmic reticulum in the triad that store Ca2+.

Acetylcholine (ACh)

Neurotransmitter at the neuromuscular junction that initiates muscle contraction by depolarizing the sarcolemma.

Acetylcholinesterase

Enzyme that degrades acetylcholine in the synaptic cleft to terminate the signal.

Neuromuscular junction

Synapse between a motor neuron and a skeletal muscle fiber where ACh is released.

End plate potential

Local depolarization of the motor end plate due to Na+ influx through ACh receptor channels.

Action potential

Electrical impulse that travels along the sarcolemma and into T-tubules to trigger Ca2+ release from the SR.

Resting membrane potential

Electrical potential with inside of the cell negative relative to outside, maintained by ion gradients.

Sodium-Potassium Pump (Na+/K+ pump)

Membrane pump that moves 3 Na+ out and 2 K+ in, maintaining the resting membrane potential.

Depolarization

Change in membrane potential making the inside more positive due to Na+ influx.

Repolarization

Return to the resting membrane potential after depolarization; Ca2+ is pumped back to the SR and ions are balanced.

Cross-bridge

Link formed between a myosin head and actin during contraction.

Cross-bridge cycle

Series of steps by which myosin heads pull actin filaments toward the center of the sarcomere.

Myosin head

The site on the myosin molecule that forms a cross-bridge with actin and hydrolyzes ATP to power the stroke.

ATPase site

Part of the myosin head that hydrolyzes ATP to provide energy for the contraction cycle.

Myosin-binding site

Site on actin where myosin head binds during contraction; exposed when Ca2+ moves tropomyosin away.

Sliding Filament Theory

Contraction occurs when actin slides past myosin, shortening the sarcomere without changing the length of the filaments.