Muscular and Skeletal Systems: Histology, Physiology, and Articulations

Comprehensive vocabulary flashcards covering muscular system histology and physiology, bone tissue and development, fracture classifications and repair, and joint articulations and movements.

Excitability

The special functional ability of muscle tissue to receive and respond to electrical, chemical, or mechanical stimuli.

Contractility

The unique characteristic of muscle cells to forcibly shorten when adequately stimulated.

Extensibility

The ability of muscle tissue to extend or stretch beyond its resting length without sustaining damage.

Elasticity

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

Epimysium

An external sheath of dense regular connective tissue that envelops the entire perimeter of a skeletal muscle.

Perimysium

A layer of fibrous connective tissue that surrounds each individual fascicle (bundle of muscle fibers).

Endomysium

A delicate sheath of fine areolar connective tissue investing each single skeletal muscle fiber.

Muscular Fascia

A dense connective tissue sheet situated external to the epimysium that binds muscles together and organizes them into functional compartments.

Satellite Cells

Undifferentiated skeletal muscle stem cells located between the sarcolemma and basal lamina that can undergo mitosis to repair or replace damaged muscle fibers.

Aponeurosis

A broad, flat sheet of dense fibrous connective tissue that anchors a muscle to bone, deep fascia, or other muscles.

Sarcomere

The smallest functional and structural contractile unit of a muscle myofibril, defined as the region between two consecutive Z discs.

Z Disc

A coin-shaped protein disc that serves as the boundary of a sarcomere, anchoring thin actin myofilaments and interconnecting myofibrils.

H Zone

The pale central region of an A band where thick myosin filaments do not overlap with thin actin filaments in resting muscle.

M Line

A dark band of structural protein (myomesin) situated in the center of the H zone that anchors adjacent thick filaments.

Titin

An enormous elastic filament protein composed of peptide chains extending from the Z disc through the thick filament to the M line, conferring muscle elasticity and extensibility.

Myosin

The principal contractile protein constituting thick filaments, featuring two interwoven polypeptide chains forming tails and globular heads with actin-binding, ATP-binding, and ATPase catalytic sites.

Tropomyosin

A rod-shaped regulatory protein spiraling around the actin core that blocks myosin-binding sites on actin in a relaxed muscle fiber.

Troponin

A globular three-polypeptide regulatory protein complex attached to tropomyosin and actin that binds $\text{Ca}^{2+}$, initiating conformational displacement of tropomyosin from active sites.

Terminal Cisternae

Enlarged, flattened sacs of the sarcoplasmic reticulum positioned in pairs adjacent to transverse tubules that store and release concentrated $\text{Ca}^{2+}$.

T Tubules

Tubular invaginations of the sarcolemma traversing deeply into the interior of the muscle fiber, conducting action potentials to trigger calcium release from terminal cisternae.

Triad

A specialized three-part membrane assembly consisting of a central transverse tubule flanked symmetrically by two terminal cisternae of the sarcoplasmic reticulum.

Sliding Filament Model

The mechanism of muscle contraction stating that thin actin filaments slide past thick myosin filaments toward the center of the sarcomere, narrowing the H zone and I band without changing filament lengths.

Power Stroke

The working step of the cross-bridge cycle during which inorganic phosphate ($\text{P}_i$) and ADP are released, causing the activated myosin head to pivot and pull the thin filament toward the M line.

Neuromuscular Junction (NMJ)

The specialized synapse formed between the axon terminal of an alpha motor neuron and the motor end plate of a skeletal muscle fiber.

Acetylcholinesterase

An enzyme located in the synaptic cleft of the neuromuscular junction that hydrolyzes acetylcholine into acetic acid and choline, terminating excitation.

Isometric Contraction

A muscular contraction in which tension increases up to the muscle's peak capability while the muscle maintains a constant length without moving the load.

Concentric Contraction

A subtype of isotonic contraction in which the muscle generates sufficient tension to overcome external resistance and shortens to accomplish work.

Eccentric Contraction

An isotonic contraction in which a muscle lengthens while actively generating contractile tension to decelerate or control movement.

Motor Unit

A functional neuromuscular unit consisting of a single somatic motor neuron and all the skeletal muscle fibers innervated by its terminal axonal branches.

Muscle Twitch

The rapid, mechanical response of a muscle fiber or motor unit to a single, brief electrical or neural stimulus of threshold intensity.

Temporal (Wave) Summation

The progressive addition of contractile force occurring when consecutive stimuli arrive before the muscle fiber has completely relaxed from the preceding twitch.

Unfused (Incomplete) Tetanus

A sustained, vibrating muscle contraction resulting from repetitive stimulation at intermediate frequencies that allow only partial relaxation between consecutive twitches.

Fused (Complete) Tetanus

A smooth, steady maximal contraction without any observable relaxation phase, produced when stimulation frequency is sufficiently elevated.

Size Principle

The physiological recruitment rule stating that motor units containing the smallest muscle fibers are activated first, followed by progressively larger fibers as stimulus intensity increases.

Muscle Tone

A constant, low-level state of involuntary contraction present in relaxed skeletal muscles maintained by spinal reflex arcs, ensuring joint stability and posture.

Slow Oxidative Fibers

Red muscle fibers rich in myoglobin and mitochondria that split ATP slowly, depend on aerobic respiration, and exhibit high resistance to fatigue.

Fast Glycolytic Fibers

White muscle fibers with large glycogen reserves and fast myosin ATPases that generate rapid, powerful contractions via anaerobic glycolysis but fatigue quickly.

Oxygen Debt

The extra volume of oxygen required following vigorous exertion to restore oxygen reserves on myoglobin, convert accumulated lactic acid to pyruvic acid, and resynthesize glycogen, ATP, and creatine phosphate.

Delayed-Onset Muscle Soreness (DOMS)

Post-exercise muscle tenderness and stiffness peaking $24\text{--}72\text{ hours}$ after unaccustomed strenuous activity, stemming from microscopic muscle fiber damage rather than lactic acid accumulation.

Rigor Mortis

The generalized post-mortem stiffening of skeletal muscles that commences $3\text{--}4\text{ hours}$ after death and peaks at $12\text{ hours}$, caused by calcium leakage and the complete absence of ATP needed to detach cross bridges.

Varicosities

Bulbous autonomic nerve swellings along smooth muscle fibers that store and release neurotransmitters into broad synaptic clefts termed diffuse junctions.

Caveoli

Pouchlike indentations of the smooth muscle sarcolemma that concentrate extracellular $\text{Ca}^{2+}$ adjacent to calcium channels.

Calmodulin

A cytosolic calcium-binding regulatory protein in smooth muscle that activates myosin light chain kinase upon binding $\text{Ca}^{2+}$.

Duchenne Muscular Dystrophy (DMD)

A lethal X-linked recessive genetic muscle disorder characterized by the progressive deterioration of muscle fibers due to a deficiency in the cytoskeletal stabilizing protein dystrophin.

Sarcopenia

The age-associated, progressive loss of skeletal muscle mass and strength, resulting in up to a $50\%$ reduction in muscle mass by age $80$.

Synergist

A muscle that assists the prime mover (agonist) by supplying supplementary force or suppressing unwanted, extraneous movement across adjacent joints.

Fixator

A stabilizing synergist muscle that immobilizes the skeletal origin of a prime mover or provides a solid base for movement.

First-Class Lever

A biomechanical lever system in which the fulcrum is positioned intermediate to the applied effort and the resisting load.

Second-Class Lever

A biomechanical lever system where the resistance load lies between the fulcrum and the effort, consistently operating at a mechanical advantage (power lever).

Third-Class Lever

A biomechanical lever system in which the applied effort is situated between the fulcrum and the load, operating at a mechanical disadvantage with high velocity and range.

Hematopoiesis

The physiological process of blood cell formation that takes place within red bone marrow cavities.

Chondroblasts

Immature, active cartilage progenitor cells that actively synthesize and secrete extracellular cartilage matrix.

Chondrocytes

Mature cartilage cells that have become encapsulated within small matrix spaces known as lacunae.

Perichondrium

A double-layered sheath of dense connective tissue enveloping cartilage (except at articular surfaces), comprised of an outer fibrous vascular layer and an inner chondrogenic layer.

Appositional Growth

Growth in diameter or thickness of cartilage or bone by the differentiation of new cells and deposition of matrix at the tissue periphery.

Interstitial Growth

Internal expansion of cartilage resulting from the mitotic division of preexisting chondrocytes and the deposition of matrix between them.

Osteoblasts

Bone-forming cells derived from osteoprogenitor cells that synthesize and secrete organic bone matrix (osteoid) and are stimulated by calcitonin.

Osteocytes

Mature, non-dividing bone cells residing within lacunae that maintain mineral homeostasis and communicate through gap junctions along canaliculi.

Osteoclasts

Giant, multinucleated cells derived from the fusion of blood monocytes that secrete hydrochloric acid and lysosomal enzymes at their ruffled borders to resorb bone matrix.

Osteoid

The unmineralized organic component of bone matrix secreted by osteoblasts, consisting of collagen fibers, proteoglycans, and glycoproteins ($33\%$ of bone mass).

Hydroxyapatite

Inorganic mineral salts, primarily calcium phosphate ($\text{CaPO}_4$) crystals, that make up $67\%$ of bone weight and provide exceptional hardness and compression resistance.

Diaphysis

The elongated, cylindrical central shaft of a long bone, consisting of a thick cylinder of compact bone surrounding the medullary cavity.

Epiphysis

The expanded bulbous terminus of a long bone, comprised internally of spongy bone capped by a thin veneer of compact bone and articular cartilage.

Medullary Cavity

The central tubular space within the diaphysis of a long bone containing yellow bone marrow (fat) in adults.

Epiphyseal Line

The distinct bony seam left behind in adult long bones when the hyaline cartilage of the epiphyseal plate completely ossifies at the conclusion of growth.

Periosteum

A white, double-layered fibrous membrane covering the outer surface of bone, anchored securely to the matrix by perforating (Sharpey's) collagen fibers.

Endosteum

A delicate internal connective tissue lining covering trabeculae, the medullary cavity, and canals within bone tissue.

Osteon (Haversian System)

The primary structural and functional unit of mature compact bone, consisting of concentric lamellae arranged around a neurovascular central canal.

Canaliculi

Microscopic, hairlike channels radiating from lacunae across bone lamellae, providing routes for osteocyte cytoplasmic processes and nutrient exchange.

Trabeculae

Interconnecting bony struts and plates forming the latticework of spongy (cancellous) bone aligned along principal lines of mechanical stress.

Wolff's Law

The physiological principle dictating that bone remodels, adapts, and thickens in direct response to the mechanical forces, stresses, and loads applied to it.

Parathyroid Hormone (PTH)

A hormone released by the parathyroid glands in response to hypocalcemia that stimulates osteoclasts to resorb bone matrix and restore blood calcium homeostasis ($9\text{--}11\,\text{mg}/100\,\text{ml}$).

Intramembranous Ossification

The process of bone development directly within fibrous connective tissue embryonic membranes, forming flat bones of the skull and clavicles.

Fontanelles

Unossified fibrous membrane areas present between the cranial bones of an infant skull during and shortly following birth.

Endochondral Ossification

The developmental mechanism wherein preexisting hyaline cartilage models break down and are systematically replaced by bone tissue to construct most of the skeleton.

Epiphyseal Plate

A hyaline cartilage plate situated at the junction of the diaphysis and epiphysis in growing children, organized into distinct functional zones enabling longitudinal bone elongation.

Fracture Hematoma

A localized collection of clotted extravasated blood that forms immediately at a fracture site due to ruptured periosteal and marrow blood vessels.

Fibrocartilaginous Callus

A temporary reparative bridge of granulation tissue, collagen fibers, and cartilage that splints broken bone ends within one week of fracture.

Greenstick Fracture

An incomplete pediatric fracture in which only one cortical margin of the bone shaft breaks while the convex side bends.

Comminuted Fracture

A complete bone fracture in which the bone shatters or fragments into three or more distinct pieces, commonly seen in elderly brittle bones.

Spiral Fracture

A ragged, helical bone break produced by excessive torsion or twisting stresses propagated along the longitudinal axis of a long bone.

Pott's Fracture

A specific ankle fracture that concurrently affects and fractures both bones of the leg (distal tibia and fibula).

Colles' Fracture

A characteristic fracture involving the distal metaphysis of the radius, typically caused by landing on an outstretched hand to cushion a forward fall.

Compression Fracture

A fracture occurring when a bone is crushed or collapsed under severe vertical axial stress, predominantly seen in osteoporotic vertebrae.

Osteomalacia

An adult skeletal disorder characterized by inadequate mineralization of osteoid, leading to soft, flexible, and painful bones, primarily caused by vitamin D or calcium deficiency.

Rickets

A childhood disease caused by a severe deficiency of vitamin D or calcium, resulting in poorly mineralized bone, epiphyseal deformities, and bowlegs.

Osteoporosis

A progressive metabolic bone disease in which osteoclastic bone resorption outpaces osteoblastic bone deposition, leaving porous, brittle bones vulnerable to pathological fractures.

Paget's Disease

A chronic bone condition characterized by frantic, excessive, and haphazard bone remodeling, creating localized patches of abnormally thick but mechanically weak woven bone.

Acromegaly

A hormonal disorder resulting from hypersecretion of growth hormone (GH) following puberty and epiphyseal closure, causing progressive enlargement of facial bones, hands, and feet.

Synarthrosis

A functional category of joints defined by an inability to permit movement (immovable joints, such as cranial sutures).

Amphiarthrosis

A functional category describing slightly movable joints, such as the pubic symphysis and intervertebral joints.

Diarthrosis

A functional classification denoting freely movable articulations, universally represented by synovial joints.

Suture

A synarthrotic fibrous joint found exclusively between adjacent bones of the skull, bonded tightly by dense fibrous connective tissue fibers.

Syndesmosis

A fibrous articulation where adjoining bones are connected solely by an interosseous ligament or sheet of fibrous tissue, permitting limited movement.

Gomphosis

A unique peg-in-socket fibrous joint securing the root of a tooth into its alveolar socket via the periodontal ligament.

Synchondrosis

An immovable cartilaginous joint wherein articulating bones are united by a rigid bar or plate of hyaline cartilage (e.g., epiphyseal plates).

Symphysis

An amphiarthrotic cartilaginous joint in which hyaline-capped bone surfaces fuse to an intervening fibrocartilage pad to resist compression.

Unhappy Triad

A severe composite knee injury characterized by concomitant tearing of the tibial (medial) collateral ligament, the medial meniscus, and the anterior cruciate ligament (ACL).

Rotator Cuff

A dynamic musculotendinous cuff that stabilizes the glenohumeral joint, comprised of the supraspinatus, infraspinatus, subscapularis, and teres minor muscles.

Anulus Fibrosus

The outer ring of fibrocartilage in an intervertebral disc that binds adjacent vertebral bodies and confines the central nucleus pulposus.