Muscular System: Skeletal Muscle Structure and Function

Vocabulary flashcards covering skeletal muscle structure, motor control, energy use, and contraction mechanisms.

Skeletal muscle

Voluntary, striated muscle attached to bones; responsible for movement and posture.

Smooth muscle

Involuntary muscle found in walls of internal organs; non-striated and spindle-shaped.

Cardiac muscle

Involuntary, striated muscle of the heart; branched and uninucleated.

Epimysium

Connective tissue surrounding the entire muscle.

Perimysium

Connective tissue surrounding a fascicle (bundle) of muscle fibers.

Endomysium

Connective tissue surrounding each individual muscle fiber.

Fascicle

Bundle of muscle fibers within a skeletal muscle.

Sarcolemma

Cell membrane of a muscle fiber.

Sarcoplasmic reticulum (SR)

Organelle that stores and releases Ca2+ to trigger contraction.

T-tubule

Transverse tubule; invagination of the sarcolemma that propagates action potentials into the muscle fiber.

Myofibril

Rod-like structures containing sarcomeres; drive muscle contraction.

Sarcomere

Basic contractile unit of a muscle, between two Z-discs; contains actin and myosin filaments.

Z-disc (Z-line)

Boundaries of a sarcomere; marks the start/end of a sarcomere.

A band

Region with thick filaments; overlaps with thin filaments; length remains constant during contraction.

I band

Region with only thin filaments; shortens during contraction.

M line

Center line of the sarcomere; anchors thick filaments.

Actin

Thin filament protein; provides active sites for myosin binding.

Myosin

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

Tropomyosin

Protein that blocks actin active sites at rest.

Troponin

Calcium-binding protein that moves tropomyosin to expose actin sites during contraction.

Cross-bridge

Interaction where myosin heads bind to actin and pull during the power stroke.

Sliding Filament Theory

Contraction occurs as thick and thin filaments slide past each other, shortening the sarcomere.

Excitation-Contraction Coupling

Process linking a nerve signal to muscle contraction via NMJ and Ca2+ release.

Neuromuscular junction

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

Alpha motor neuron

Nerve cell that innervates multiple muscle fibers at the NMJ.

Motor unit

One alpha motor neuron and all the muscle fibers it innervates.

Acetylcholine (ACh)

Neurotransmitter released at the NMJ to stimulate the muscle.

Acetylcholine receptor

Receptor on the motor end-plate that binds ACh to trigger an action potential.

Calcium (Ca2+)

Ion released from the SR to signal contraction by binding troponin.

Sarcoplasmic reticulum

Calcium-storage organelle in muscle fibers.

ATP

Adenosine triphosphate; primary energy source for muscle contraction.

ATPase

Enzyme on myosin heads that hydrolyzes ATP to power the contraction cycle.

Type I fiber

Slow-twitch muscle fibers; high aerobic endurance; fatigue resistant.

Type IIa fiber

Fast-twitch oxidative fibers; intermediate speed and fatigue resistance.

Type IIx (IIb) fiber

Fast-twitch glycolytic fibers; high force, quick to fatigue.

Motor unit recruitment

Orderly activation of motor units from small to large as needed.

Size principle

Small motor units activated first; larger units recruited as force increases.

Isometric contraction

Contraction with tension but no change in muscle length.

Concentric contraction

Muscle shortens while producing force.

Eccentric contraction

Muscle lengthens while producing force.

Muscle fiber

A single muscle cell; many fibers form a muscle, wrapped in connective tissue.

Fascia

Connective tissue surrounding muscles and groups of muscles.

Fasciculus

Another term for a muscle fascicle; a bundle of fibers.

Myofilaments

Actin (thin) and myosin (thick) filaments that drive contraction.

G-actin vs F-actin

G-actin is single actin units; polymerizes to form F-actin in thin filaments.

Sarcomere length

Distance between two Z-discs; determines force potential.

Power stroke

Myosin head pivots, pulling actin filaments past each other.

Heat production in muscle

Contraction generates heat; skeletal muscle is a major source of body heat.

Muscle fiber arrangement

Muscle fibers grouped into fascicles, forming bundles within the muscle.

Contraction prerequisites

Adequate Ca2+, ATP, intact neuromuscular signaling, and proper sarcomere structure.