so in this video we're going to look at the general structure of a sovial joint and this is the third structural classification of a joint we talked about fibrous cartilaginous now sovial so the sovial joint is actually a special joint because it's where bones are separated by uh you know actually a fluid filled joint cavity and this joint cavity which is full of fluid is sovial fluid but that's aligned and sort of supported and encased by a synovial membrane now it's important to have this fluid joint cavity because all sovial joints are diarthrotic or freely movable so when you think about the most movable joints of your body like your shoulder your hip your knee your elbow your interial or like joints between your fingers those are all synovial joints which are considered diarthrosis are freely movable now uh includes pretty much all the joints of your limbs and uh the characteristics of synovial joints are interesting we'll talk about those coming up soon and we also have specialized sovial tissues like Bersa and tendon SE that are associated with tissues that also move a lot now uh in terms of their stability they're influenced by three major factors that we'll talk about coming up soon but remember sovial joints are highly movable which means they're also less stable remember there's a trade-off between mobility and stability if you have a very movable joint it's often the least stable and we'll talk about that as well so in terms of the general structure of a sovial joint uh for one they're going to have this articular cartilage because they're going to include long bones and these long bones are going to have highing cartilage at the places where the joints occur or articulations and the purpose of this articular cartilage is actually to prevent um sort of crushing of the ends of the bones and so that actually allows for some cushioning between those bones and that way there's a nice sort of Highland cushion between those bones at the Joint now we also find that sovial joints have a joint cavity also called the sovial cavity and it's full of synovial fluid and this actual cavity is pretty small but it's full of fluid and it's a potential space and that's actually filled with this coval fluid which is a lubricant now cenovio fluid is mostly water in some hyaluronic acid and it's going to be kind of viscous which gives it a nice lubrication uh characteristic now uh articular cartilage or articular capsule rather is the joint capsule which actually surrounds and encases this synovial cavity now it's two layers thick we have an external fibrous layer and an internal sovial layer the fibrous layer is what's outside of the joint and it really surrounds and encases this synovial joint to hold it together nicely and it's made of a dense irregular connective tissue which is tough now the inner layer of this articular or joint capsule is the synovial membrane now the sovial membrane is what lines the inner surfaces of the sovial joint and this is made of a loose connected tissue which actually makes sovial fluid and it secretes this by basically filtering blood plasma now sovial fluid is this viscous slippery filtrat of plasma and hyaluronic acid it lubricates and nourishes the articular cartilage and what's interesting is that it nourishes articular cartilage we talked about our cartilage is avascular and now what's cool then is that for the cartilage you find within sovial joints every time you compress that synovial fluid by like you know bearing weight on the joint uh which what's really neat then is that the coval fluid gets forced into the cartilage to nourish it and then when you take compression off of that joint and it relaxes that noal fluid can actually rush out of the cartilage back into the joint cavity it's this sort of tidal movement of sovial fluid that uh allows for nourishment of the cartilage now we also find fosic cells that can remove microbes and debris if that occurs because we need this to be a lubricant now there's different types of reinforcing ligaments for sovial joints as well we have capsular ligaments which are part of the fibrous layer of the articular capsule we have extra capsular ligaments which are found outside of their articular capsule and they basically just support the joint from the outside and then intracapsular ligaments which are actually found within the joint itself so an example of an intracapsular ligament would be like the anterior cruciate ligament of your knee because you actually find this within the joint cavity itself which means it's going to be surrounded by uh sovial fluid and covered with sovial membrane so uh we also find that synovial tissues have nerves and blood vessels so if you damage sovial tissue it's gonna it's gonna be painful but it also is involved with monitoring your joint position and stretch which we'll come back to in the nervous system chapter now we have lots of capillary beds within synovial tissue that way they're able to nourish the synovial membrane and you actually filter that blood plasma or the fluid of your blood to make synovial fluid so if you look at this picture here this is showing a typical synovial joint and imagine this could be like the articular surfaces of uh let's say like between your finger bones now if you look here we see that they have a long bone and another long bone and they're cut in cross-section so what you're looking at is basically compact bone that lines the outside got some spongy bone inside we have this epipal lime which used to be the growth plate and we talked about at the epiphysis at the very end of the epiphysis we have articular cartilage now the articular cartilage here is actually going to protrude into the sovial cavity or joint cavity and this allows for some cushioning between the joints now the actual joint cavity itself is surrounded by this articular capsule or joint capsule and this actually would go all the way around the joint that's what's going to hold this synovial fluid in place so there's two layers of this articular capsule or joint capsule we have the fibrous layer which is a dense irregular connected tissue and then a sovial layer which is actually made of sovial membrane which is a connective tissue membrane that looks like epithelium but basically what it does is it takes blood plasma and it filters that across to make sovial fluid which you find filling the joint cavity now sovial fluid is kind of a viscous slippery solution which doesn't have a lot of protein and it doesn't have a lot of cells there's mostly just water electrolytes and uh hyaluronic acid uh within this solution now uh what we find then too are some supporting ligaments like extra capsular ligament here we know this is an extracapsular ligament because you find this outside of the joint and if there were ligaments inside of the joint cavity we actually would call those intracapsular ligaments but they're not shown here so uh in terms of other structural features of coval joints we can have fatty pads which are actually going to help for to provide some cushioning between uh joint spaces and then the articular discs are also called minisi which are basically little rings of fibrocartilage that help to fit between the ends of the bones and they help stabilize The Joint they also help reduce wear and tear so you're going to find a lot of articular discs between you know uh joints that are going to Bear a lot of weight so an example of where you would find a meniscus or articular disc is actually in the knee so in the knee joint we actually have the lateral and medial minisi which are basically just little rings of fibrocartilage that help to cushion within that joint itself now uh we also find that there's Bersa and tendon sheets now these are really interesting examples of where you can find synovial membrane outside of joints and instead Bersa and tendon sheets really support other skeletal tissues so think of a Bersa is like a bag of synovial fluid that lubricates like you know like a ball bearing and it's not strictly part of synovial joint but it's usually closely associated so an example of where you'd find like a Bersa would be like uh surrounding muscles or between a tendon in skin or uh between two bones uh near a joint so uh this actually provides some additional lubrication uh in tissues that surround joints now we also find that there are tendon sheets which are elongated Bersa that WRA completely around tendons and you find this around tendons that are subjected to a lot of friction like especially the wrist you're going to find a lot of tendon sheets to lubricate those wrist tendons so this is Show an example of a Bersa here in the uh basically in the shoulder joint now this Bersa here is structured a lot like a water balloon and it basically will roll around uh within that joint uh to basically help to lubricate the uh you know AC chromian of your scapula and the head of the humorus here that way you don't get extra friction between these tissues that are that are around the joint so Bersa helped to support joints and by providing additional areas of lubrication now we also find Tenon sheath so you might you might find then that like near this joint you could have things like the tendon of like the long head of Tri of biceps brachi but because this tendon moves a lot you can find a tendon sheath which is basically a sovial membrane that surrounds the tendon that way the tendon can rub back and forth but it's going to be lubricated uh by the sinovial membrane so you don't have a lot of friction around these tenons that move quite a bit so if you look at the the structure of a Bersa remember it's like a bag of synovium and it will roll with the joint so when the head of your humorus rolls when you're move your arm around uh your shoulder joint around rather this basically this Bursa is going to roll around and help to lubricate between these tissues that way you don't get friction rub so there's three factors that influence the stability of synovial joints and this is important to consider because we said that synovial joints were all diarthrotic now if they're diarthrotic that means that they are freely movable but we said that highly movable joints are often the least stable so to talk about uh synovial joint stability refers to you know well how do we just keep this joint together well some important factors would include first of all the shape of the articular surface like what is the shape of the bones in terms of uh joint stability you find that for the most part this has a pretty minor role but with like ball and socket joints this is going to be where you have a ball that sits in a basically a socket shaped bone and this is going to provide some ility here now we also have supporting ligaments and the more ligaments the stronger the joints going to be now these ligaments also have kind of a limited role as well because they can only do so much to keep those joints together and if you remember we have different types of ligaments we have capsular extra capsular and intracapsular ligaments and then muscle tone is the most important uh component here now muscle tone is different in physiology than maybe if you learned you know like from popular culture uh muscle tone is actually uh basically like a a resting amount of muscle tension that your muscles can produce and so your muscles always are in a state of contraction even if you think that you're they're relaxed and so this muscle tone helps to basically Pro produce this steady state of contraction and tension on tendons and tendons are taut as they cross joints which basically helps keep those bones in normal position if you lose muscle tone joints can subluxate or become partially dislocated so muscle tone is an important factor to basically help keep these joints in normal position so this table actually goes into some of the different types of classifications of joints throughout the body so if you look here we have remember like the the cranial sutures which are a fibrous joint this is actually going to be a synarthrosis because they're immovable we talked about how the atlanto occipital joint of your skull is a synovial joint and it's also diarthrotic so it allows for a lot of movement here at this neck joint uh we talked about how the inter vertebral discs of your vertebrae all along your spine are a type of cartilaginous joint which are going to be a symphasis because they're made of fibrocartilage and they're also going to be slightly movable because they're amphiarthrotic uh the um the types of joints where your ribs meet the vertebrae um are also going to be synovial joints and because that they're going to be dionic so allows for some movement of your ribs uh we find that like places like the sternoclavicular joint here like where the stern clavicle meat is also a synovial joint and it's also freely movable which makes sense because if uh if your clavicle is part of the pectoral girdle that supports your upper penage if you touch your clavicle and move your arm around you can feel your clavicle moving quite a bit against the sternum here and then we have our sternocostal joint where basically the ribs meet the sternum this is also a cartilaginous joint but because it's made of Highland cartilage we consider this a synchondrosis where uh basically the bones are united by uh Highland cartilage now this is a synarthrotic joint so these bones don't move a lot against each other but the cartilage can expand a little bit now other places where you find joints throughout the body when you start talking about appendages really these are going to be uh sovial joints right so when you look at look at an appendage they're sovial joints because they move quite a bit and uh again appendages are sovial joints because they move quite a bit now we also talked about the pubic symphysis here which is also a cartilaginous joint it's a fioc card which makes it a symphasis joint and it's going to be slightly movable that way there's some cushioning between your ocx when you walk and we talked about how the distal tibial fibular articulation here is a type of fibrous joint called a syndesmosis because this is going to be uh a a ligament and ligaments are technically syndesmosis which is longer connected tissue fibers that allow for a little bit of movement but in this example it's a synarthrosis which is basically going to be immovable