in this video we're going to examine sinovial joints so these are joints where the bones are separated by a fluid filled joint cavity so this is sort of a defining feature of sinovial joints all sinovial joints are going to be diotic meaning they're going to be freely movable so these are the types of joints that you usually find between Limbs and so we can characterize these C joints based on the range of motion so plane hinge pivot condar saddle and ball and socket we'll specifically talk about these range of motions and the movements Allowed by joints in the next video but in this video we want to focus on the anatomy of a joint what are the components that make up a sinovial joint so when we look at the distinguishing featur of sinovial joints first and foremost wherever the bones will articulate that is where the bones meet you're going to have a layer of highin cartilage shown here in blue and now I'm kind of marking it with my red pen so the ends of the bones where the articulation is going to occur this is where you have Highland cartilage so this highland cartilage isn't very thick it's about 1 mm in thickness but it's preventing that bone on bone so the bones aren't slamming against each other so you're not crushing the ends of the bone now in between this highland cartilage you're going to have a joint or a sinovial cavity so this cavity is going to be a relatively small space but it is filled with fluid and we'll get to that fluid in a moment so when we look at this joint cavity it has a capsule so the capsule is going to have two layers so in this particular figure here in green that's our articular or joint capsule the outer region which I'm kind of highlighting right now it's green in the image but I'm kind of highlighting here in a dark blue that outer region is made up of dense irregular connective tissue so it's so it's a very fibrous layer while the internal surface which I'm kind of showing just with an arrow pointing towards the cavity those cells that's going to be part of the inner sinovial membrane so this is going to consist of very loose connective tissue and the cells within this connective tissue are resp responsible for the production of sinovial fluid so sinovial fluid is actually derived from plasma so you have some vascular here some blood vessels here and from the blood you extract fluid okay so filtrate of the plasma and the inner sinovial membrane also adds hyaluronic acid so that makes the sinovial fluid is sort of this very viscous substance um think of it sort of as a lubricating agent now in addition in the sinovial fluid you do have a few fosic cells to help remove any kind of debris or microbial pathogens that might be in that area so how does the sinovial fluid work well think of the fluid region as sort of like a wet sponge when you squeeze the wet sponge the fluid can kind of seep out and then if you let that sponge go the fluid can kind of kind of seat back into the sponge and that's sort of the mechanism that allows you to lubricate these articulations we actually call this mechanism weeping lubrication because obviously the capsule is a self-contained structure the fluid is not going into the spaces of the body so let's look at other distinguishing features of sinovial joints so sinovial joints are going to be further characterized by a number of ligaments that help to reinforce the structure now on the fibrous layer part of the joint cavity some sometimes this can be extended so shown here sort of in white and I'm kind of drawing like the little circles with my navy blue pen that is what we call the capsular reinforcing ligament so it's just a thickened part of the fibrous layer but you can have ligaments that are outside of the capsule or deep to the capsule that provides some limited structural support since the covi fluid does come from plasma you're going to have blood vessels in and around the sinovial joints as well as nerve fibers so these nerve fibers are going to be key to help monitor joint position and detecting stretch as well as to detect any kind of pain now for some joints you might have an additional sort of fatty layer we call these fatty pads and that's going going to be found between the Fus layer and the sinovial membrane so again we're going to look over here so you might have some region here where you have extra cushioning going to be very important in several types of joints and then lastly you will have articular disc or manisi which you're uh you're going to find in knee joints for example so this is a fibrocartilage layer this is a fibrocartilage layer that's going to separate the articulating surfaces especially important where the two bone Ends don't really mesh up or match up very well so this fibral cartilage helps to stabilize the joint and additionally it helps to reduce the wear and tear that's occurring between the two moving bones so that's the overall structure of sinovial joints but there are some structures that are associated with sinovial joints that are technically not a part of the sinovial joint so two of these structures one is the bers so the Bersa or bers are basically just sacks that are filled with sinovial uh fluid so these sacks are going to be lined by sinovial membrane so no different from the articular cavities uh and they're going to be found in areas where you have a lot of friction so basically interactions between ligaments and the bone muscles and the bone skin in the bone tendons in the bone uh anytime things are really rubbing against each other you're going to have a bursa so here we have an example of the right shoulder joint you have the subacromial Bersa that's where the acromion of the scapula is going to interact with the head of the humorus so you have this extra cushioned layer that helps to mitigate friction as you um move so another structure associated with sinovial joints are the tendon sheets so the tendon sheets are basically just modified bird and they wrap around tendons to help to minimize or dissipate friction so here we have an example of the tendon from the bicep muscle the bicep brachi muscle and it is completely enclosed sort of like a hot dog is enclosed by a bun by the tendon sheath so again look at the the coloration right you have this fibrous out layer you have this sinovial membrane that is secreting the sinovial fluid and you're wrapping it around the tendon so that tendon which ordinarily comes around the bone during motion that would cause a lot of friction and so that tendon sheath helps to mitigate some of that friction all right so the last topic I want to tell tou on are some of the stabilizing features that are associated with sinovial joints now we've already talked about sometimes bones fitting onto bones so the articulating surfaces and you would think that that would play a major role in stability and to some extent that's not incorrect when we look at ball and socket joints where the head of the bone is in a deep pocket yes that adds stabilization but most joints are not going to be ball and socket joints uh so the standard articulation where the bones meet that is going to play a very minor role ligaments you would think ligaments the number of ligaments should have a significant stabilizing effect and again the more ligaments you have the more stable you are however keep in mind that ligaments if you stretch them they kind of stay in a stretched state so they're very similar to sort of the uh stretching of Taffy right it's going to take a long time for it to recoil back to its original shape so if you are placing an undue amount of stress on a joint then those ligaments are going to overstretch and that's not going to help stabilize that joint so the most important stabilizing fact factor is actually the muscle tendons that cross the joint so they are going to remain taut because of muscle tone or muscle tonis muscles are inserted onto bones we'll discuss this in great de to great detail later on so muscles are going to connect two bones two or more bones and they are constantly stretched they are constantly taut you might contract the muscle so it's becoming more taut but even in a relaxed muscle there's a limited amount of u totness in that muscle so muscle tendons then are going to be very important in reinforcing um relatively fragile joints like the shoulder joint the knee joint and especially the arches of the feet