Anatomy and Physiology: Articulations and the Muscular System

Comprehensive practice flashcards defining foundational vocabulary for joint classifications, anatomical movements, skeletal muscle histology, the sliding filament model, neuromuscular junctions, and muscle action dynamics.

Joint (Articulation)

The location where two or more bones meet.

Fibrous Joint

A joint united by fibrous connective tissue that lacks a joint cavity and allows little to no mobility; includes sutures, syndesmoses, and gomphoses.

Cartilaginous Joint

A joint united by a pad of cartilage (hyaline or fibrocartilage) without a joint cavity, allowing little to slight mobility; includes synchondroses and symphyses.

Synovial Joint

A complex joint characterized by the presence of a joint cavity containing lubricating synovial fluid, which permits a substantial degree of mobility.

Synarthrosis

A functional classification describing an immovable joint.

Amphiarthrosis

A functional classification describing a slightly movable joint.

Diarthrosis

A functional classification describing a freely movable joint.

Suture

An immobile fibrous joint found between skull bones where adjacent bones interdigitate via interlocking finger-like processes for stability.

Fontanel

An un-ossified, membranous region within cranial sutures present in newborns.

Syndesmosis

A fibrous joint in which bones are separated further than in a suture and joined by ligaments to allow slight movement, such as the radioulnar interosseous membrane.

Gomphosis

A specialized immobile fibrous joint consisting of a peg that fits into a socket, exemplified by periodontal ligaments anchoring teeth into alveolar bone.

Synchondrosis

A cartilaginous joint united by hyaline cartilage that exhibits little to no movement; may be permanent or temporary (e.g., epiphyseal line, first sternocostal joint, sphenooccipital synchondrosis).

Symphysis

A slightly movable cartilaginous joint joined by fibrocartilage, such as the pubic symphysis and intervertebral disks.

Synostosis

A bony fusion resulting from the ossification of a temporary cartilaginous joint, such as when an epiphyseal plate becomes an epiphyseal line.

Articular Cartilage

Hyaline cartilage that covers the ends of articulating long bones within a synovial joint, providing a smooth, low-friction surface.

Joint Capsule

A two-layered envelope enclosing a synovial joint cavity, consisting of an outer fibrous capsule of dense irregular connective tissue continuous with the periosteum and an inner synovial membrane.

Synovial Membrane

A thin, delicate inner layer of the joint capsule that lines the synovial cavity (except over articular cartilage) and secretes synovial fluid.

Synovial Fluid

A slippery fluid made of polysaccharides (such as hyaluronic acid), serum filtrate, proteins, lipids, and cells that lubricates synovial joints and minimizes friction.

Bursa

A fluid-filled sac lined with synovial membrane that extends from a joint to cushion structures and reduce friction, commonly found in the shoulder and knee.

Flexion

An angular movement that decreases the angle between articulating bones at a joint.

Extension

An angular movement that increases the angle between articulating bones at a joint.

Hyperextension

The extension of a body part beyond its normal anatomical position, commonly observed at the head and hand.

Dorsiflexion

Movement of the foot superiorly toward the shin, as performed during heel walking.

Plantar Flexion

Movement of the foot inferiorly toward the ground, as performed during toe walking.

Abduction

Movement of a limb or anatomical structure away from the median plane of the body.

Adduction

Movement of a limb or anatomical structure toward the median plane of the body.

Rotation

The turning of an anatomical structure about its long axis (e.g., rotation of the head, humerus, or whole body).

Pronation

A rotational movement of the forearm that causes the palm of the hand to face posteriorly.

Supination

A rotational movement of the forearm that causes the palm of the hand to face anteriorly.

Circumduction

A compound movement combining flexion, extension, abduction, and adduction such that the moving appendage describes the outline of a cone.

Elevation

A superior movement that lifts a structure upward, such as shrugging the shoulders.

Depression

An inferior movement that lowers a structure downward, such as relaxing the shoulders or opening the jaw.

Protraction

An anterior gliding movement that brings a structure forward.

Retraction

A posterior movement that pulls an anteriorly translated structure back to anatomical position or further posteriorly.

Opposition

The movement of the thumb across the palm to touch the tips of the other fingers on the same hand.

Contractility

The fundamental property of muscle tissue characterized by the ability to forcefully shorten and generate mechanical force.

Excitability

The capacity of muscle fibers and neurons to detect and respond to electrical stimuli by generating an action potential.

Extensibility

The property of muscle tissue that permits it to be stretched beyond its normal resting length without experiencing damage.

Elasticity

The ability of muscle tissue to recoil and return to its original resting length after being stretched.

Epimysium

The outer connective tissue sheath that wraps around the entirety of a muscle organ.

Perimysium

The connective tissue membrane that bundles individual muscle fibers into discrete compartments called fascicles.

Endomysium

The delicate connective tissue layer that surrounds each individual muscle fiber.

Fascicle

A discrete bundle of multiple muscle fibers bound together by perimysium.

Muscle Fiber

An individual, elongated skeletal muscle cell containing multiple nuclei, many mitochondria, and densely packed myofibrils.

Tendon

A dense cord-like connective tissue band connecting muscle to the periosteum of bone.

Aponeurosis

A broad, flattened sheet of dense connective tissue that attaches a muscle to bone or surrounding tissue.

Hypertrophy

An increase in overall muscle mass achieved by the enlargement of individual muscle fibers via the accumulation of additional myofibrils, rather than an increase in cell count.

Sarcolemma

The specialized plasma membrane of a muscle fiber capable of generating and conducting action potentials.

Sarcoplasm

The specialized cytoplasm of a muscle fiber enclosing myofibrils, glycogen stores, and organelle networks.

Transverse Tubule (T-tubule)

A tube-like invagination of the sarcolemma that penetrates deep into the sarcoplasm to convey action potentials to the interior of the muscle fiber.

Sarcoplasmic Reticulum

A modified smooth endoplasmic reticulum within muscle fibers that sequesters and releases high concentrations of calcium ions ($Ca^{2+}$).

Myofibril

A rod-like contractile organelle within a muscle fiber composed of thousands of repeating, end-to-end sarcomeric units.

Sarcomere

The basic functional contractile unit of a muscle fiber, spanning from one Z disk to the adjacent Z disk.

Z Disk

A protein disc separating adjacent sarcomeres that anchors thin actin filaments and pulls inward toward the M line during contraction.

M Line

The stationary protein line in the exact center of a sarcomere that holds thick myosin filaments in place.

H Zone

The central zone of a resting sarcomere flanking the M line that contains thick myosin filaments exclusively, with no actin overlap.

A Band

The dark-staining band of a sarcomere that spans the full length of the thick myosin filaments, including regions of overlap with thin actin filaments.

I Band

The light-staining band of a myofibril that spans across the Z disk and contains thin actin filaments exclusively.

Actin Myofilament

A thin filament formed of a two-stranded double helix possessing active sites where myosin heads bind during cross-bridge cycling.

Tropomyosin

A long, ribbon-like protein strand wrapped along the actin double helix that blocks actin active sites during muscle relaxation.

Troponin

A regulatory protein complex bound to tropomyosin that contains specific binding sites for $Ca^{2+}$; upon calcium binding, it moves tropomyosin to uncover actin active sites.

Myosin Myofilament

A thick filament composed of bundled myosin proteins, featuring protruding globular heads that bind actin active sites and hydrolyze ATP.

Neuromuscular Junction (NMJ)

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

Motor End Plate

The specialized, folded portion of the sarcolemma directly beneath the motor nerve terminal that contains ligand-gated sodium channels.

Synaptic Cleft

The narrow extracellular gap separating the presynaptic neuronal membrane from the postsynaptic motor end plate.

Synaptic Vesicles

Membranous sacs stored in the presynaptic axon terminal that contain and exocytose the neurotransmitter acetylcholine (ACh).

Acetylcholine (ACh)

The neurotransmitter released at the neuromuscular junction that binds ligand-gated channels on the motor end plate to cause sarcolemma depolarization.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that rapidly degrades acetylcholine into acetic acid and choline, terminating the excitation signal.

Cross-Bridge Formation

The step in muscle contraction in which energized myosin heads bind to the exposed active sites on the actin filament.

Power Stroke

The mechanical event during cross-bridge cycling where the bound myosin head hinges toward the M line, pulling the actin filament and attached Z disks inward.

Cross-Bridge Release

The detachment of the myosin head from the actin active site upon the binding of a new molecule of ATP.

Recovery Stroke

The conformational reset of the detached myosin head returning to its high-energy, cocked position following ATP hydrolysis into ADP and $P_i$.

Motor Unit

A single motor neuron along with all the individual skeletal muscle fibers that it innervates.

Isometric Contraction

A muscle contraction wherein muscle length remains unchanged while tension increases without producing gross joint movement.

Isotonic Contraction

A muscle contraction wherein muscle tension remains constant as the muscle shortens, producing joint movement.

Muscle Twitch

A single, brief contraction and relaxation cycle of a muscle fiber elicited in response to a solitary action potential.

Tetanus

A sustained, continuous muscle contraction resulting from rapid and repeated action potentials that merge individual twitches together; can be incomplete (unfused) or complete (fused).

Muscle Tone

A constant, low-level resting tension in muscle tissue that stabilizes posture, supports upright head position, and flattens the abdomen.

Myoglobin

An oxygen-binding globular protein present in muscle sarcoplasm that possesses a higher affinity for oxygen than hemoglobin, acting as an intracellular oxygen reservoir for aerobic respiration.

Muscle Origin

The anatomical attachment site of a muscle tendon to the more stationary or immovable bone.

Muscle Insertion

The anatomical attachment site of a muscle tendon to the more movable bone where mechanical action takes place.

Muscle Belly (Gaster)

The thick, fleshy central region of a skeletal muscle situated between its origin and insertion.

Agonist (Prime Mover)

The principal muscle that contracts and shortens to execute the vast majority of a specific joint movement.

Synergist

A muscle that contracts alongside the agonist to assist in producing the primary movement.

Fixator

A specialized synergist that immobilizes a bone or joint to stabilize the origin of the prime mover.

Antagonist

A muscle located on the opposite side of a joint from an agonist that relaxes and lengthens while the agonist contracts, working in opposition.