lecture 10: Skeletal Muscle Structure and Contraction

Vocabulary flashcards covering skeletal muscle organization, protein filaments, neuromuscular junctions, and excitation-contraction coupling.

Sarcomere

The functional unit of skeletal muscle located between two Z lines, consisting of an organized arrangement of actin and myosin filaments.

Myosatellite cells

Unfused embryonic myoblasts that persist as stem cells in skeletal muscle tissue after birth to assist in muscle repair and regeneration.

Sarcolemma

The plasma membrane of a muscle fiber that is continuous with transverse tubules and surrounding connective tissue layers.

Nebulin

A slender protein component of thin filaments that holds strands of G-actin molecules together.

Titin

Strands of elastic protein extending through the myosin core to each Z line that maintain filament alignment and assist the sarcomere in returning to its resting length.

Troponin

A regulatory protein on thin filaments that binds calcium ions ($\text{Ca}^{2+}$), causing a shape change that shifts tropomyosin away from active sites on G-actin.

Tropomyosin

A protein that coils around thin filaments and covers myosin-binding active sites on G-actin in resting muscle fibers.

G-actin

Globular actin molecules containing specific active sites where myosin cross-bridges attach.

Sarcoplasmic Reticulum

A network of membranous channels surrounding each myofibril that stores and releases calcium ions ($\text{Ca}^{2+}$) to initiate muscle contraction.

Transverse Tubules (T-tubules)

Membranous channels continuous with the sarcolemma that extend inward to conduct action potentials deep into the muscle fiber.

Terminal Cisternae

Expanded regions of the sarcoplasmic reticulum situated on either side of a transverse tubule.

Triad

A structural unit inside a muscle fiber consisting of one central T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.

Neuromuscular Junction

The specialized synapse where the axon terminal of a motor neuron communicates with a skeletal muscle fiber.

Motor End Plate

The specialized region of the sarcolemma at the neuromuscular junction containing neurotransmitter receptors and high concentrations of mitochondria.

Motor Unit

A single motor neuron along with all the muscle fibers it innervates and controls.

Acetylcholine (ACh)

The neurotransmitter released from motor neuron vesicles into the synaptic cleft to excite muscle fibers and stimulate contraction.

Acetylcholinesterase

An enzyme located in the synaptic cleft that breaks down acetylcholine to terminate excitation and allow muscle relaxation.

Cross-Bridge

The connection formed when a myosin head attaches to an exposed active binding site on actin.

Power Stroke

The pivoting movement of a bound myosin head that pulls actin filaments toward the center (M line) of the sarcomere.

Botulism

A condition caused by a toxin that blocks acetylcholine release from motor neurons, resulting in flaccid paralysis of muscles.

Unipennate Muscle

A muscle architecture in which fascicles attach obliquely to a tendon from one side only.

Length-Tension Relationship

The principle stating that optimal muscle tension is generated when resting sarcomere length allows maximum cross-bridge formation, typically $75\%$ to $130\%$ of optimal length.