Joints and the Muscular System

Comprehensive vocabulary flashcards covering joint classifications, movements, and disorders, as well as the microscopic anatomy, excitation-contraction coupling, energy pathways, and physiology of the muscular system.

Synarthroses

A functional classification of joints that are completely immovable (providing no movement).

Amphiarthroses

A functional classification of joints that permit a slight amount of movement (some movement).

Diarthroses

A functional classification of joints that are freely movable; all synovial joints belong to this category.

Fibrous Joints

Joints where articulating bones are united by dense fibrous connective tissue without a joint cavity; most are immovable.

Suture

A fibrous joint held together by very short interconnecting fibers with interlocking bone edges; found exclusively in the skull, where they ossify and fuse with age.

Syndesmosis

A fibrous joint held together by a ligament or cord of fibrous tissue longer than in sutures; fiber length varies, and movement ranges from immovable to slightly movable.

Gomphosis

A "peg in socket" fibrous joint where a tooth is anchored in its alveolar bone socket by the periodontal ligament.

Synchondroses

Cartilaginous joints where bones are united by a bar or plate of hyaline cartilage; almost all are synarthrotic, such as epiphyseal plates and the immovable joint between the first rib and manubrium of the sternum.

Symphyses

Cartilaginous joints where bones are united by fibrocartilage sandwiched between hyaline cartilage; amphiarthrotic joints designed for strength and shock absorption, such as intervertebral discs and the pubic symphysis.

Synovial Joints

Freely movable (diarthrotic) joints characterized by an articular capsule (fibrous layer and inner synovial membrane), an articular cartilage-lined joint cavity, synovial fluid, and reinforcing ligaments.

Articular Discs (Menisci)

Pads of fibrocartilage that separate articulating surfaces within certain synovial joints to stabilize the joint and reduce wear and tear.

Bursae

Flattened, fluid-filled sacs lined with synovial membrane containing synovial fluid that roll between adjacent structures to reduce friction.

Tendon Sheaths

Elongated bursae that wrap completely around tendons subjected to friction, common where several tendons crowd together in narrow canals.

Plane Joint

A synovial joint with flat articular surfaces that allows nonaxial, gliding movements across one another, such as in intercarpal, intertarsal, and vertebral articular joints.

Hinge Joint

A uniaxial synovial joint in which a cylindrical projection of one bone fits into a complementary trough-shaped surface of another, permitting flexion and extension (such as the elbow and interphalangeal joints).

Condylar Joint

A biaxial synovial joint featuring oval articular surfaces where both surfaces are complementary ellipses, permitting flexion, extension, abduction, and adduction (e.g., metacarpophalangeal and wrist joints).

Saddle Joint

A biaxial synovial joint where both articulating surfaces have concave and convex areas shaped like a saddle, allowing abduction, adduction, flexion, and extension (exemplified by the carpometacarpal joint of the thumb).

Ball-and-Socket Joint

A multiaxial synovial joint where the spherical head of one bone articulates with the cup-like socket of another, permitting multiaxial movement including flexion, extension, abduction, adduction, and rotation (e.g., shoulder and hip joints).

Gliding Movement

A simple joint movement that occurs when the flat surfaces of two bones slide across each other without significant angulation or rotation.

Flexion and Extension

Angular movements in the sagittal plane: flexion decreases the angle between articulating bones, whereas extension increases the angle between articulating bones.

Abduction and Adduction

Angular movements in the frontal (coronal) plane: abduction moves a limb away from the median (midsagittal) plane, while adduction moves a limb toward the median plane.

Rotation

The turning of a bone around its own long axis toward the midline (medial rotation) or away from the midline (lateral rotation).

Pronation and Supination

Special movements of the forearm: pronation rotates the radius diagonally over the ulna, whereas supination rotates the radius and ulna parallel to each other.

Dorsiflexion and Plantar Flexion

Special up-and-down movements of the foot: dorsiflexion moves the superior surface of the foot toward the shin (standing on the heel), while plantar flexion points the toes downward away from the shin.

Elevation and Depression

Special nonangular vertical movements: elevation moves a body part superiorly (such as closing the mouth by moving the mandible up), while depression moves it inferiorly (such as opening the mouth by moving the mandible down).

Temporomandibular Joint (TMJ)

The articulation where the mandibular condyle joins the mandibular fossa and articular tubercle of the temporal bone; prone to anterior dislocation, pain, and clicking or popping sounds.

Glenoid Labrum

A rim of fibrocartilage that deepens the shallow glenoid cavity of the scapula to help support the humerus in the shoulder joint.

Elbow Joint

A uniaxial hinge joint stabilized by the anular ligament around the radius head, the radial collateral ligament on the lateral side, and the ulnar collateral ligament on the medial side.

Hip Joint

A stable ball-and-socket joint where the head of the femur articulates with the acetabulum of the hip bone, deep-set with an acetabular labrum and reinforced by the iliofemoral, pubofemoral, and ischiofemoral ligaments.

Cruciate Ligaments

Intracapsular ligaments of the knee—the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL)—that cross each other like an X to prevent displacement between the tibia and femur.

Meniscus Tear

A common joint injury involving damage to the fibrocartilage pads (menisci) of the knee, visible upon arthroscopic examination between the femur and tibia.

Rheumatoid Arthritis

A chronic autoimmune disease characterized by bilateral inflammation of synovial membranes, pain, joint stiffness, and severe swelling leading to deformities.

Excitability

The functional characteristic of muscle tissue enabling it to receive and respond to stimuli by altering its electrical membrane potential.

Contractility

The specialized ability of muscle tissue to shorten forcibly when adequately stimulated.

Extensibility

The ability of muscle tissue to be extended or stretched beyond its resting length.

Elasticity

The ability of muscle fibers to recoil and resume their initial resting length after being stretched.

Epimysium

An external sheath of dense irregular connective tissue that surrounds the entire skeletal muscle.

Perimysium

A layer of fibrous connective tissue surrounding bundles of muscle fibers called fascicles.

Endomysium

A fine sheath of areolar connective tissue surrounding each individual skeletal muscle fiber.

Muscle Fiber

An elongated, multinucleate skeletal muscle cell bounded by the sarcolemma and filled with densely packed myofibrils.

Sarcomere

The smallest functional contractile unit of a muscle fiber, extending from one Z disc to the next, organized into dark A bands and light I bands.

Thick Filament

A myofilament composed of bundled myosin molecules, each containing an elongated tail and two flexible heads that bear actin-binding and ATP-binding sites.

Duchenne Muscular Dystrophy

An X-linked recessive genetic disorder caused by defective dystrophin, which fails to anchor the thin filaments to the sarcolemma, resulting in torn membranes, calcium overload, and muscle apoptosis.

Sarcoplasmic Reticulum (SR)

A modified smooth endoplasmic reticulum surrounding individual myofibrils that regulates intracellular levels of calcium by storing and releasing $\text{Ca}^{2+}$.

T Tubules

Invaginations of the sarcolemma that penetrate deep into the muscle fiber interior to conduct action potentials to the terminal cisterns of the sarcoplasmic reticulum.

Sliding Filament Model

The model of muscle contraction where thin filaments slide past thick filaments, increasing overlap at the A band, shortening I bands, drawing Z discs closer, and obliterating the H zone.

Chemically Gated Ion Channel

A transmembrane ion channel that opens or closes in direct response to the binding of a specific chemical messenger, such as acetylcholine (ACh).

Neuromuscular Junction (NMJ)

The synaptic connection between the axon terminal of a somatic motor neuron and the junctional folds of the skeletal muscle sarcolemma.

End Plate Potential (EPP)

A local depolarization produced at the neuromuscular junction when acetylcholine binds to sarcolemma receptors, opening chemically gated channels that allow rapid $\text{Na}^+$ influx.

Myasthenia Gravis

An autoimmune disorder characterized by destruction of acetylcholine (ACh) receptors, presenting with symptoms such as drooping eyelids, difficulty swallowing, speech impairments, and generalized weakness.

Excitation-Contraction (E-C) Coupling

The sequence of physiological events by which transmission of an action potential along the sarcolemma and down the T tubules triggers the release of $\text{Ca}^{2+}$ from the sarcoplasmic reticulum, leading to myofilament sliding.

Cross Bridge Cycle

The cyclical series of four mechanical events—cross bridge formation, power stroke, cross bridge detachment, and cocking of the myosin head—that pulls thin filaments toward the M line.

Rigor Mortis

The postmortem stiffening of skeletal muscles that starts within $3\text{--}4\text{ hours}$ and peaks at $12\text{ hours}$, caused by calcium influx and an absence of ATP required to detach myosin cross bridges.

Motor Unit

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

Muscle Twitch

The response of a muscle to a single action potential, characterized on a myogram by a latent period, a period of contraction, and a period of relaxation.

Temporal Summation

The addition of muscle twitches that occurs when a second stimulus arrives before the muscle has completely relaxed from the previous one, generating greater tension.

Unfused Tetanus

A sustained, quivering muscle contraction produced when stimulation frequency increases so that the muscle only partially relaxes between successive stimuli.

Fused Tetanus

A smooth, sustained maximal muscle contraction resulting from high stimulation frequencies that allow no relaxation between stimuli.

Size Principle of Recruitment

The orderly recruitment of motor units during muscle activation, in which the smallest motor units are recruited first, followed by medium, and finally the largest motor units for maximal tension.

Isotonic Contraction

A muscle contraction where internal muscle tension exceeds the load, causing the muscle to change length (shortening in concentric contraction or lengthening in eccentric contraction).

Isometric Contraction

A muscle contraction in which tension increases to its peak capability but the muscle does not change length because the load exceeds the tension developed.

Direct Phosphorylation

An anaerobic ATP production pathway where creatine kinase transfers a high-energy phosphate from creatine phosphate to ADP, yielding $1\text{ ATP}$ per CP molecule for about $15\text{ seconds}$ of energy.

Anaerobic Pathway (Glycolysis)

An oxygen-independent cytosolic pathway that breaks down glucose into pyruvate and generates a net gain of $2\text{ ATP}$ along with lactate, providing energy for $30\text{--}40\text{ seconds}$.

Aerobic Pathway

An oxygen-dependent metabolic process occurring in mitochondria that breaks down glucose, glycogen, and fatty acids through the citric acid cycle and electron transport chain, providing sustained ATP for hours.

Excess Postexercise Oxygen Consumption (EPOC)

The additional amount of oxygen required after exercise to restore myoglobin oxygen reserves, convert accumulated lactate, replenish glycogen stores, and resynthesize ATP and creatine phosphate reserves.

Length-Tension Relationship

The principle that optimal muscle operating tension occurs within $80\%\text{ to }120\%$ of resting sarcomere length, where optimal thin and thick filament overlap occurs.

Slow Oxidative Fibers

Fatigue-resistant, red skeletal muscle fibers with high myoglobin content, many mitochondria and capillaries, and slow contraction speed; recruited first for endurance activities.

Fast Glycolytic Fibers

Fatigue-prone, pale skeletal muscle fibers with low myoglobin, few mitochondria, high glycogen content, and fast contraction speed; recruited third for powerful, short-duration activities.

Disuse Atrophy

The rapid wasting and loss of muscle mass resulting from immobilization or loss of neural stimulation, occurring at a rate of roughly $5\%$ per day and accompanied by fibrous connective tissue replacement.

Varicosities

Bulbous swellings along autonomic nerve fibers innervating smooth muscle that store and release neurotransmitters into wide synaptic clefts called diffuse junctions.

Stress-Relaxation Response

A functional feature of smooth muscle that allows hollow organs to stretch slowly without immediately contracting, accommodating volume changes while retaining the ability to contract on demand.

Unitary Smooth Muscle

Smooth muscle found in the walls of hollow organs (excluding the heart) organized into opposing sheets, coupled by gap junctions, exhibiting spontaneous action potentials, and responding as a single unit.

Multi-Unit Smooth Muscle

Smooth muscle located in the lungs, large arteries, arrector pili, and iris, composed of independent fibers with few gap junctions, regulated by the autonomic nervous system and hormones via graded contractions.

Sarcopenia

The age-related loss of skeletal muscle mass and strength that typically begins by age $30$, where muscle fibers decrease and connective tissue increases.