Muscle Tissue and the Muscular System

Comprehensive practice flashcards covering skeletal muscle histology, sarcomere structure, contractile and regulatory proteins, fiber types, and gross muscular anatomy principles including origins, insertions, fascicle architectures, and coordinated muscle actions.

Muscle Belly

The fleshy body of a skeletal muscle connected to the skeleton by tendons; it contains hundreds to thousands of muscle fibers, is highly vascular, and can be arranged in a variety of shapes depending on function.

Tendon

A minimally vascular structure composed of dense regular connective tissue with parallel bundles of collagen fibers and few fibroblasts that connects a muscle belly to the skeleton.

Endomysium

A reticular connective tissue covering that surrounds each individual muscle fiber, binding fibers together while allowing free movement and carrying a small blood supply of capillaries.

Perimysium

A dense irregular connective tissue covering that surrounds each muscle fascicle, permitting some freedom of movement and conveying blood vessels.

Fascicle

A discrete bundle of skeletal muscle fibers wrapped by perimysium.

Epimysium

A dense irregular connective tissue layer that surrounds the entire muscle belly and binds fascicles together.

Myoblasts

Embryonic progenitor cells that fuse in groups of $100+$ during development to form a single, mature, multinucleated skeletal muscle fiber that is terminally differentiated and incapable of cell division.

Satellite Cells

Myoblasts retained in mature skeletal muscle that retain the ability to divide and fuse, providing limited regeneration after muscle damage before fibrous scar tissue forms.

Hypertrophy

The enlargement of existing muscle fibers through increased synthesis of myofibrils, mitochondria, and other organelles, resulting in an increased fiber diameter and more forceful contractions.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber containing substantial amounts of glycogen (stored glucose), myoglobin, and rows of mitochondria arranged throughout the fiber.

Myoglobin

An oxygen-binding protein located in the sarcoplasm of muscle fibers.

Transverse Tubules (T Tubules)

Deep, fluid-filled invaginations of the sarcolemma that tunnel from the fiber surface toward the center; they open to the exterior and are filled with interstitial fluid.

Sarcoplasmic Reticulum (SR)

A fluid-filled system of membranous sacs similar to smooth endoplasmic reticulum that encircles myofibrils and stores calcium ions.

Terminal Cisterns

Dilated end sacs of the sarcoplasmic reticulum positioned against transverse (T) tubules.

Triad

A structural grouping in skeletal muscle fibers composed of $1$ transverse tubule and $2$ terminal cisterns abutting it on either side.

Myofibrils

The striated, contractile elements of a muscle fiber composed of cylindrical bundles of protein filaments.

Thin Filaments

Contractile filaments $8\,\text{nm}$ in diameter composed primarily of the protein actin.

Thick Filaments

Contractile filaments $16\,\text{nm}$ in diameter composed primarily of the motor protein myosin.

Sarcomere

The basic functional and contractile unit of a myofibril, extending from one Z-disc to the next.

Z-Discs

Narrow, plate-shaped regions of dense protein material that separate one sarcomere from the next.

A Band

The dark, central region of a sarcomere that spans the entire length of the thick filaments.

I Band

The light region of a sarcomere composed exclusively of thin filaments, bisected by a Z-disc.

H Zone

A narrow, lighter region in the center of the A band that contains thick filaments only and lacks thin filaments.

M Line

A region in the middle of the H zone consisting of structural proteins that hold thick filaments together.

Zone of Overlap

The peripheral regions of the A band where thick and thin filaments lie parallel and overlap one another.

Myosin

The motor contractile protein that constitutes thick filaments ($\sim 300$ molecules per thick filament); converts chemical energy from ATP into mechanical energy, featuring tails pointing toward the M line and outward-extending heads.

Actin

The primary contractile protein of thin filaments; each actin molecule has a myosin-binding site where a myosin head can attach.

Tropomyosin

A regulatory protein in thin filaments that covers the myosin-binding sites on actin in a relaxed muscle fiber.

Troponin

A regulatory protein complex that holds tropomyosin in place; upon binding calcium ions, it undergoes a conformational change that moves tropomyosin away from the myosin-binding sites.

Titin

The third most plentiful protein in skeletal muscle and the largest known protein; it extends from the Z disc to the M line, anchoring thick filaments and accounting for the elasticity and extensibility of myofibrils.

Myomesin

A structural protein that forms the M line and binds to titin.

Nebulin

A structural protein in myofibrils that anchors thin filaments to Z discs.

Dystrophin

A structural protein that links thin filaments of the actin cytoskeleton to integral membrane proteins of the sarcolemma, transmitting mechanical tension to tendons.

Sliding Filament Mechanism

The model of muscle contraction in which myosin heads walk along thin filaments toward the M line, drawing the thin filaments inward and shortening the sarcomere.

Slow Oxidative (SO) Fibers

Type I skeletal muscle fibers that appear dark red due to high myoglobin content, generate ATP aerobically, contract slowly, and resist fatigue to sustain long-term endurance contractions.

Fast Oxidative-Glycolytic (FOG) Fibers

Type IIa skeletal muscle fibers that appear red due to moderate myoglobin content, generate ATP both aerobically and anaerobically via glycolysis, and contract faster with moderate fatigue resistance.

Fast Glycolytic (FG) Fibers

Type IIb skeletal muscle fibers that appear white due to low myoglobin content, store large amounts of glycogen, generate ATP anaerobically by glycolysis, contract rapidly and forcefully, and fatigue quickly.

Origin

The attachment point of a muscle tendon to a stationary bone that remains fixed and is stabilized by other muscles during contraction.

Insertion

The attachment point of a muscle tendon to the movable bone that is displaced toward the origin during muscle contraction.

Parallel Fascicle Arrangement

A fascicle arrangement in which fascicles run parallel to the long axis of the muscle, forming a straplike muscle shape.

Fusiform Fascicle Arrangement

A fascicle organization in which fascicles run nearly parallel to the long axis and terminate in flat tendons, with a muscle belly that is wider in the middle than at its ends.

Circular Fascicle Arrangement

A concentric ring pattern of fascicles that surrounds body openings to form sphincters.

Convergent Fascicle Arrangement

A broad, triangular muscle architecture in which fascicles converge from a wide origin toward a single, thick central tendon.

Pennate Fascicle Arrangement

A feather-like structural pattern where short fascicles attach obliquely to a central tendon; classified as unipennate (fascicles on one side), bipennate (fascicles on both sides), or multipennate (multiple feather-like units attached to one central tendon).

Agonist (Prime Mover)

The primary muscle directly responsible for producing a specific desired movement.

Antagonist

A muscle that relaxes and stretches to yield to the prime mover; its active contraction directly opposes the action of the agonist.

Synergist

A muscle that assists the prime mover by generating additional force or by stabilizing intermediate joints to prevent unwanted movements.

Fixator

A muscle that stabilizes the origin of the agonist so that the prime mover acts from a firm, stable base.