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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

Joints • A joint is the point at which two bones articulate with each other • Joints vary greatly in structure & function • Degree of movement at a joint is determined by the structure of the joint Classification of Joints Fibrous Joints Sutures • Found only in the skull • Tightly bound by a minimal amount of fibrous tissue • Fibrous tissue is continuous with the periosteum around these flat bones • During development, bones separated by short collagen fibres • Edges of the joining bones are wavy and interlocking • Knit bones together also allow growth so that skull can expand with brain during childhood • During middle age, fibrous tissue ossifies and the skull bones fuse together • At this stage the closed suture are called synostoses – bony junctions • Clinical Note – Hydrocephalus o Very young infants may develop a blockage in the ventricular drainage system o Leads to the excessive build up of CSF fluid o If untreated can lead to severe deformities of the infants skull, and lasting brain damage o If caught early – shunt is surgically implanted, bypasses blockage in brain (usually inaccessible), catheter allows for additional mechanisms of drainage Syndesmososes • Bones connected solely by ligaments, bands of fibrous tissue longer than those occurring in sutures • Amount of movement allowed depends on the length of connecting fibres o If fibers are short, as in distal tibiofibular articulation – little to no movement is allowed o If fibres are long, as in interosseous membrane between radius & ulna – a lot of movement Gomphoses • Peg-in-socket joint • Only example: articulation of a tooth within its socket • Connecting ligament: periodontal ligament Cartilaginous Joints • Articulating bones are united by cartilage • Lack a joint cavity and are not highly moveable • Two types o Synchondroses o Symphyses Synchodroses • Joint where hyline cartilage units the bone • Ex. Epiphyseal plates o Functionally, these plates are classified as immovable synarthroses • Ex. First rib’s costal cartilage & manubrium of sternum Symphyses • Fibrocartilage units the bone • Ex. intervertebral discs, pubic symphasis of the pelvis • Even though fibrocartilage is main element  hyaline cartilage is also present in form of articular cartilages on the bony surfaces o Articular cartilages function to reduce friction between the bones during movement • Fibrocartilage resists both tension and compression and can act as a resilient shock absorber • Slightly movable joints (amphiarthroses) • Provide strength with flexibility Synovial Joints • Most moveable joints of the body and are all diarthroses (freely moveable) • Contains fluid-filled joint cavity • Define most joints of the body & especially ones in the limbs General Structure • Articular cartilage o Ends of opposing bones are covered by articular cartilages composed of hyaline cartilage o Spongy cushions absorb compressive forces on joint & keep bone ends from being crushed • Joint (articular) cavity o Feature unique to synovial joints o Joint cavity is a potential space that holds a small amount of synovial fluid • Articular capsule o Joint cavity is enclosed by a two-layered articular capsule or joint capsule o Outer fibrous layer of dense irregular connective tissue is continuous with the periosteum layer of the joining bones & strengthen the joint so that the bones are not pulled o Inner synovial membrane composed of loose connective tissue and lines the joint capsule, and convers all the internal joint surfaces not covered by cartilage - function is to make synovial fluid • Synovial fluid o Viscous liquid inside the joint cavity o Resembles raw egg white (ovum = egg) o Primarily a filtrate of blood, arising from capillaries in synovial membrane o Also contains special glycoprotein molecules, secreted by fibroblasts in synovial membrane – make synovial fluid a slippery lubricant that eases the movement at the joint o Occupies joint cavity but also occurs within articular cartilages o Pressure placed on joints during normal movement squeezes synovial fluid into and out of articular cartilages o Mechanism: weeping lubrication – nourishes the cells in the articular cartilages (cartilage is avascular) and lubricates the free surfaces of these cartilages, allowing the adjoining bones ot move across each other with a minimum amount of friction • Reinforcing ligaments o Some are reinforced and strengthened by bandlike ligaments o Most often, ligaments are capsular – thickened parts of the fibrous layer of the articular capsule o In other cases, ligaments are extracapsular or intracapsular  Extracapsular: located just outside the capsule (ex. fibular & tibial collateral ligaments of the knee)  Intracapsular: located internal to capsule (ex. anterior & posterior cruciate ligaments in knee), covered with synovial membrane – separates them from cavity they run through • Nerve and vessels o Joints richly supplied with sensory nerve fibres that innervate the articular capsule  Some of these fibres detect pain, as anyone who has suffered a joint energy knows  Most of the fibres monitor how stretched the capsule is  Monitoring of stretching is a way by which nervous system senses posture & movements o Also have a rich blood supply  Most of the blood vessels supply the synovial membrane, where extensive capillary beds produce the blood filtrate that is the basis of synovial fluid o Each synovial joint is served by branches from several major nerves and blood vessels o Branches come from many different directions and supply overlapping areas of joint capsule  Overlap provides functional redundancy – when the normal movement at a joint compress a blood vessel, other vessels stay open and keep joint nourished  When injury to a joint destroys some vessels & nerves, others survive & keep joint functioning • Articular disc o Certain synovial joints contain a disc of fibrocartilage: articular disc or meniscus o Occur in the temporomandibular (jaw) joint, sternoclavicular joint, knee joint and a few mothers o Disc extends internally from the capsule and completely or partly divides joint cavity into 2 o Occur in joints whose articulating bone ends have somewhat different shapes o When 2 articulating surfaces fit poorly, they touch each other only at small points, where loading forces become highly concentrated – can damage articular cartilages & lead to osteoarthritis o Articular disc fills the gaps and improves the fit, thereby distributing the load more evenly & minimizing wear and damage o These discs may also allow two different movements at the same joint – distinct movement across each face of the disc, as in the case with jaw joint • Bursae and tendon sheaths o Contain synovial fluid & often are associated with synovial joints o Essentially closed bags of lubricant, these structures act like “ball bearings” to reduce friction between body elements that move over one another o Bursa  Flattened fibrous sac lined by synovial membrane  Occur where ligaments, muscles, skin, tendons or bones overlie each other & rub together o Tendon sheath  Essentially elongated bursa that wraps around tendon like bun around a hot dog  Occur only on tendons that are subjected to friction, such as those that travel through joint cavities or are crowded together within narrow canals (as in carpal tunnel of wrist) Types of Synovial Joints Plane • Articular surfaces are flat planes • Short movements in multiple directions (gliding) • Ex. Intercarpal bones of hand, intertarsal bones of hand, articular processes of vertebrae Hinge • Cylindrical end of one bone articulates with trough of another • Uniaxial movement • Ex. Elbow joint, interphalangeal joints Pivot • Rounded end of one bone fits into ring formed by another bone and associated ligament • Uniaxial movement • Ex. Proximal radio-ulnar joint, atloaxial joint Condyloid Joint • Egg shaped articular process fits into oval concavity of another bone • Biaxial movement • Ex. Metacarpophalangeal (knuckle) joint, wrist joint Saddle • Each articular process has concave & convex surface (like a saddle) • Biaxial movement • Ex. First carpometacarpal bone of thumb Ball & Socket Joints • Spherical head of one bone fits into round socket of another • Multiaxial joint • Ex. Shoulder and hip joints Double-Jointed • Do not have duplicated joints • In these individuals, the ligaments and capsule are stretched to allow extraordinary mobility at the joint • In some cases, individuals suffer from collagen deficiencies (marfan syndrome) or bon
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