Chapter Nine

Skeletal Muscle

Muscle tissue characterized by multinucleated fibers, many mitochondria, transverse tubules, myofibrils, and sarcomeres.

Sarcolemma

Plasma membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber.

Sarcoplasmic reticulum

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

Myofibrils

Structures in skeletal muscle that give it a striated appearance due to the orderly arrangement of thick and thin filaments.

Thick Filaments

Composed almost entirely of myosin proteins, which have two large polypeptide chains, two globular heads, and two intertwined tails.

Myosin

A protein that forms thick filaments in muscle fibers, characterized by globular heads that extend out to form cross bridges.

Thin Filaments

Primarily composed of actin, along with troponin and tropomyosin.

Actin

Protein that forms thin filaments, with each G actin subunit having a binding site for myosin.

Tropomyosin

Regulatory protein that overlaps binding sites on actin for myosin, inhibiting interaction when the muscle fiber is relaxed.

Troponin

Regulatory protein that forms a complex with actin and tropomyosin and binds Ca2+.

Sarcomere

The basic contractile unit of muscle fibers, containing thick and thin filaments.

Titin

Spring-like protein in sarcomeres that prevents overstretching.

Motor Units

A motor neuron and all the skeletal muscle fibers it innervates.

Neuromuscular Junction

The site where a motor neuron stimulates a skeletal muscle fiber, releasing acetylcholine (Ach).

Excitation-Contraction Coupling

The process by which an action potential stimulates the release of Ca2+ from the sarcoplasmic reticulum, leading to muscle contraction.

Sliding Filament Mechanism

The process where thick and thin filaments slide past each other to produce muscle contraction.

Cross-Bridge Cycle

The cycle of interactions between myosin heads and actin filaments during muscle contraction.

Isotonic contraction

Muscle contraction where the muscle changes length while producing tension.

Concentric contraction

A type of isotonic contraction where the muscle shortens while producing tension.

Eccentric contraction

A type of isotonic contraction where the muscle lengthens while producing tension.

Isometric contraction

Muscle contraction where the muscle creates tension but does not change length.

Muscle Fatigue

The decrease in tension developed by a muscle fiber after repeated stimulation.

Muscle Adaptation to Exercise

The increase in muscle fiber size and ATP production capacity due to increased contractile activity.

Smooth Muscle

Muscle tissue that does not have organized myofibrils, no sarcomeres, and contracts via the sliding-filament mechanism.

Cardiac Muscle

Striated muscle that uses the sliding filament mechanism to contract, characterized by branching cells and intercalated discs.