BIOL 2401: Skeletal System and Articulations

Articulation (Joint)

A site where two bones meet and body movement occurs, characterized by an inverse trade-off between structural strength and mobility.

Synarthrosis

An immovable, very strong functional joint type where the edges of articulating bones touch or interlock, which may be fibrous or cartilaginous.

Amphiarthrosis

A slightly movable functional joint type that is structurally stronger than a diarthrosis and more mobile than a synarthrosis.

Diarthrosis (Synovial Joint)

A freely movable functional joint characterized by a joint capsule lined with a synovial membrane and filled with synovial fluid, typically located at the ends of long bones.

Suture

A fibrous synarthrosis located strictly between bones of the skull where bone edges interlock and are bound by dense fibrous connective tissue.

Gomphosis

A specialized fibrous synarthrotic joint that binds each tooth into its bony socket via the periodontal ligament.

Synchondrosis

A rigid cartilaginous synarthrosis where bones are joined by hyaline cartilage, such as between vertebrosternal ribs and the sternum, or at the epiphyseal plate of growing long bones.

Synostosis

A totally rigid, bony synarthrotic joint created when two separate bones fuse together, such as the metopic suture of the frontal bone or epiphyseal lines in adult long bones.

Syndesmosis

A fibrous amphiarthrosis where two adjacent bones are connected by a ligament or an interosseous membrane, such as between the radius and ulna.

Symphysis

A cartilaginous amphiarthrosis where articulating bones are united by a resilient pad of fibrocartilage, such as the pubic symphysis and intervertebral discs.

Synovial Fluid

A viscous fluid secreted by the synovial membrane that has the consistency of egg yolk, contains proteoglycans, and functions in lubrication, nutrient distribution, and shock absorption.

Meniscus

A pad of fibrocartilage situated between opposing articular surfaces in a synovial joint that cushions and stabilizes the joint.

Bursa

A small, fluid-filled pocket of synovial fluid that cushions areas where tendons, ligaments, skin, or muscle rub against underlying bone.

Monaxial Joint

A synovial joint that permits movement in only one plane, exemplified by the elbow joint.

Biaxial Joint

A synovial joint that permits movement across two planes, exemplified by the radiocarpal wrist joint.

Triaxial Joint (Multiaxial Joint)

A synovial joint that allows movement across three planes: anterior–posterior, medial–lateral, and rotational, exemplified by the hip and shoulder joints.

Plane Joint (Gliding Joint)

A nonaxial synovial joint featuring flattened or slightly curved surfaces that slide past each other with limited movement, such as between carpal or tarsal bones.

Condylar Joint (Ellipsoid Joint)

A biaxial synovial joint characterized by an oval articular face nesting within an opposing depression, such as the radiocarpal joint.

Saddle Joint

A biaxial synovial joint where articular surfaces fit together like a rider in a saddle, exemplified by the articulation between the trapezium and the first metacarpal bone.

Pivot Joint

A monaxial synovial joint that permits rotation around a longitudinal axis only, such as the articulation between the $C_1$ and $C_2$ vertebrae.

Flexion

An angular movement in the anterior–posterior (sagittal) plane that reduces the angle between articulating bones.

Extension

An angular movement in the anterior–posterior (sagittal) plane that increases the angle between articulating bones.

Abduction

An angular movement in the frontal (coronal) plane that moves a limb laterally away from the longitudinal axis or midline of the body.

Adduction

An angular movement in the frontal (coronal) plane that draws a limb medially toward or across the longitudinal midline of the body.

Circumduction

A complex angular movement of a limb tracing a continuous circular cone without rotation, formed through sequential flexion, abduction, extension, and adduction.

Pronation

A rotational motion of the forearm where the radius rolls across the ulna, resulting in the palm facing posteriorly.

Supination

A rotational motion of the forearm where the palm is turned anteriorly, placing the radius and ulna in a parallel alignment in anatomical position.

Dorsiflexion

A special movement at the ankle joint that lifts the superior surface of the foot and elevates the toes toward the anterior leg.

Plantar Flexion

A special movement at the ankle joint that extends the foot downward, lifting the heel and pointing the toes toward the ground.

Protraction

A non-angular special movement that pushes a body structure anteriorly forward in the horizontal plane, such as thrusting the mandible outward.

Retraction

A non-angular special movement that pulls a body structure posteriorly back in the horizontal plane, returning it from protraction.

Opposition

A specialized thumb motion that brings the tip of the first digit toward the palm or pads of the other fingers on the same hand to grasp objects.

Glenoid Labrum

A rim of fibrocartilage that lines and deepens the shallow socket of the glenoid cavity in the scapula for the humeral head.

Rotator Cuff (SITS)

A stabilizing musculotendinous cuff supporting the shoulder joint composed of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis.

Unhappy Triad

A sequence of three joint injuries caused by a strong lateral impact to an extended knee: rupture of the tibial collateral ligament, tearing of the medial meniscus, and tearing of the anterior cruciate ligament.

Osteoarthritis

A degenerative joint disease caused by cumulative physical wear and tear or collagen defects that erodes articular cartilage, commonly affecting individuals over age $60$.

Gouty Arthritis

An acute inflammatory arthritis caused by the deposition and crystallization of uric acid within the synovial fluid of joints.

Osseous Tissue

A hard, dense connective tissue featuring specialized cells and a mineralized extracellular matrix that forms the primary support framework of the skeleton.

Hematopoiesis

The physiological formation and maturation of red blood cells, white blood cells, and platelets within red bone marrow.

Yellow Bone Marrow

A soft connective tissue filling medullary cavities and spongy bone spaces that contains adipocytes storing triglycerides as an energy reserve.

Sesamoid Bone

A small, round, flat bone that forms entirely embedded inside a tendon near joint surfaces to alter pull vectors, such as the patella.

Diaphysis

The central tubular shaft of a long bone composed of a thick wall of compact bone surrounding an internal medullary cavity.

Epiphysis

The expanded terminal head at each end of a long bone composed primarily of internal spongy (trabecular) bone covered by a thin outer layer of compact bone.

Metaphysis

The narrow transitional junction in a long bone between the diaphysis and epiphysis, containing the epiphyseal plate during skeletal growth.

Diploë

The internal layer of spongy bone sandwiched between two protective parallel plates of compact bone inside cranial flat bones.

Hydroxyapatite

A hard crystalline mineral complex, $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$, formed when calcium phosphate, $\text{Ca}_3(\text{PO}_4)_2$, interacts with calcium hydroxide, $\text{Ca(OH)}_2$, composing nearly two-thirds of bone mass.

Osteogenic Cell

An undifferentiated mesenchymal stem cell situated in the inner periosteum and endosteum that divides by mitosis to give rise to osteoblasts.

Osteoblast

A cuboidal bone-forming cell that produces and deposits unmineralized matrix (osteoid) during osteogenesis.

Osteoid

The uncalcified organic matrix composed mostly of collagen fibers secreted by active osteoblasts prior to mineral deposition.

Osteocyte

A mature bone cell residing inside a lacuna that does not divide, maintaining the mineral and protein content of surrounding matrix and responding to damage.

Osteoclast

A large, multinucleated cell derived from monocyte and macrophage lineages that secretes acids and proteolytic enzymes to dissolve mineralized matrix via osteolysis.

Canaliculi

Narrow micro-tunnels penetrating the mineralized matrix through which osteocyte cytoplasmic extensions pass to allow exchange of nutrients, ions, and waste products.

Osteon (Haversian System)

The functional microscopic structural unit of mature compact bone, comprising concentric lamellae surrounding a vascular central canal.

Perforating Canal (Volkmann's Canal)

A channel extending perpendicularly through compact bone that carries blood vessels from the periosteum deep into the central canals and medullary cavity.

Trabeculae

The open branching struts and plates that compose the lattice network of spongy bone, oriented along lines of mechanical stress.

Periosteum

A dual-layered membrane covering all external bone surfaces (except articular surfaces), consisting of an outer fibrous layer and an inner cellular layer of osteogenic cells.

Perforating Fibers (Sharpey's Fibers)

Collagen bundles originating in the outer periosteum that penetrate directly into the superficial bone matrix to firmly anchor tendons and ligaments.

Endosteum

An incomplete cellular monolayer containing osteoblasts, osteogenic cells, and osteoclasts that lines the medullary cavity, trabecular surfaces, and central canals.

Intramembranous Ossification

The embryonic bone formation process beginning at about the eighth week of development where mesenchymal cells differentiate into osteoblasts within fibrous connective tissue, producing dermal bones.

Endochondral Ossification

The progressive replacement of a pre-existing hyaline cartilage model with osseous tissue, responsible for developing the majority of the skeleton.

Epiphyseal Plate

A cartilaginous disc of hyaline