Joints: Crash Course A&P #20
I am so glad that you guys are not five years old! If you were all in, like, kindergarten, the only way I could teach you about your skeletal system would be to sing you that stupid song. You know, “The toe bone’s connected to the foot bone, the foot bone’s connected to the ankle bone...the ankle bone’s connected to the leg bone.” Like apparently there’s one leg bone. For the purposes of teaching college-level anatomy and physiology, let’s just say that that song is … reductive. Insufficient. For that song to be at all useful for remembering all 206 bones that make up your skeletal structure, it would have to begin with something like: “The distal phalanx is connected to the middle phalanx …” and then end with something like: “the frontal bone meets the parietal bone along the coronal suture.” And it would probably take like two and a half hours to sing the whole thing. But there is way more to know about your skeletal system than just the names of the bones. In addition to bones, there’s cartilage and fibrous connective tissue that allows them to work together.
And maybe more importantly, if you’re going to gonna talk about the physiology of your skeleton, you have to talk about joints. Joints are the meeting places between two or more bones -- and even though it might not sound mathematically possible, you actually have more joints in your body than you have bones. In a lot of places -- like your hands and feet -- each individual bone is part of more than one or two joints. And then there’s the matter of what all that bone, cartilage, connective tissue -- and the joints that they form -- actually work together to do. And that is … move. Body movements happen when muscles contract across joints, moving one bone toward another. And studying the different types of movements your body is capable of, is not only pretty fun, it’s also one of the best practical ways to understand your bones and joints. If you ask me, all of that is infinitely more interesting than memorizing what goes where and pretending like a “backbone” is really an anatomical structure.
So. Aren’t you glad you’re a grown-up? Dont worry, it’s not anyone you knew. It’s plastic, and it’s a little bit smaller than the average adult skeleton. But -- other than having me stand in front of a giant x-ray machine for 10 straight minutes -- I can’t think of a better way to walk you through the major anatomical structures of your skeleton.
Now, if you managed to retain our introduction to human bones last week, you’ll recall that anatomists typically divide the skeleton into two major parts: the axial and the appendicular. The axial structure includes all 80 midline bones that form your skull, vertebral column, and thoracic cage. Starting at the top here, your skull’s eight cranial and fourteen facial bones form your body’s most complex bony structure, coming together to do some pretty great things, like protect your brain, help you see, smell, hear, eat, and you know, have a face. The 33 irregular bones in your vertebral column start with your atlas vertebra -- the one that holds up your skull, and is named for the Greek god Atlas, who held the world on his shoulders as a punishment. I’m not saying you’ve got a huge head or anything. I’m just saying that’s what it’s called. The vertebrae run down from your skull to your pelvis, providing the central support for your upper body and completing the enormously important job of protecting your spinal cord -- the main communication line between nearly all of your body, and your brain.
Most of your vital organs are protected by the 12 pairs of ribs and dagger-like sternum that together comprise your thoracic cage, which also provides attachment points for your back, chest, shoulder, and neck muscles. Now, the appendicular skeleton includes your upper and lower limb appendages and pectoral and pelvic girdles that attach to the axial skeleton at the shoulder and the thigh. And even though your arms and legs clearly serve different functions -- unless you spend a lot of time walking around on your hands or picking stuff up with your feet -- both sets of limbs share a similar set-up. They’re each composed of three major segments -- a common arrangement we see in loads of animals all the way back to fish, which anatomists refer to as “one bone, two bones, lots of bones and digits.” We can’t all be Dr. Seuss. And that’s just about enough anatomy for this plastic skeleton to pay for itself. I’d rather focus on the physiology of the skeleton, and in order to do that, we’ve got to talk about joints. As is often the case in anatomy, we classify joints both by what they’re made of, and by what they do.
Because form follows function, we can't really talk about one without talking about the other. So, the structural classification of your bones is all about what kind of material binds those bones together -- like, is it a fibrous tissue or cartilage, or a special fluid-filled joint cavity. But the functional classification focuses on how much that joint can move. So, for example, you have joints in your body that don’t move at all.
These include, say, the joints between the bones that make up your cranium. These non-moving joints are called synarthroses. But you also have joints that move only slightly, like the spot where your two pubic bones meet to form your pelvis. That joint exists mainly to absorb shock from walking and running, but it also has proven very handy during things like childbirth. These partly-moving joints are called amphiarthroses.
Finally, there are the diarthroses, which are fully movable, like your classic knee and elbow joints. They’re mostly found in your limbs. But if you eventually find yourself in the business of examining and treating people’s joints -- and there are a lot of doctors who do -- then it’ll be especially useful to know the structural classifications. Structurally speaking, you’ve got fibrous, cartilaginous, and synovial joints. Fibrous joints connect bones with dense fibrous connective tissue, and are mostly immovable -- like those sutures between your skull bones. As you might guess, your cartilaginous joints unite bones using cartilage, and are similar to their fibrous brethren in that they don’t move very much, and they lack a joint cavity. But the family of joints that make all the sports, and the entire art of breakdancing, and yoga possible, are the freely movable synovial joints.
Most joints in your body fall into this category. Although they do make use of cartilage and fibrous connective tissues like ligaments, they’re different in that the bones they join are separated by a fluid-filled joint cavity. The cool thing about these cavities is that they contain a bit of viscous, egg-white-like synovial fluid that acts like grease on a hinge. Without that lubricant, just running down the street could cause enough friction not only to wear out your joint surfaces, but actually overheat your joints enough to essentially cook the surrounding tissue and leave your legs smoking like a desperate Looney Tunes character. These freely moveable synovial joints come in six different configurations that together allow pretty much any movement your body can make, from a subtle head nod to a vigorous jumping jack. All of which you will find in action at your typical dance party. Seriously, though, dance parties are an excellent place to study skeletal physiology.
I mean let’s just take this from my perspective: I’m at my high school prom. I’m wearing a tux that is really uncomfortable and I’m feeling kind of shy, standing on the edge of the crowd. Maybe I want to test the waters out a little bit with like a little hand dancing. Just the hand going. Waving my hand from side to side -- it’s a type of gliding movement, one that occurs when one flat bone surface glides over another. All these motions use gliding, or plane joints, like the one between the distal ends of your radius and ulna, and the carpal bones of your wrist. But say after a while I feel like these gliding movements just aren’t enough to fully express myself. I have to enlist the more versatile angular movements.
These are the types of motion that either increase or decrease the angle between two bones across any plane in the body, like if I bob my head, or kick my leg up, or raise the roof. If people still do that. If that bending motion decreases the angle of the joint and brings those bones together, like when I bend my arm together, that movement is called flexion. When I bend it back, I’m increasing that angle, and it’s called extension.
And if I continue that motion beyond my normal anatomical positioning, and it’s somewhat dangerous, that is called hyperextension. These motions enlist the hinge joints. Similarly, my oval-shaped condylar knuckle joints allow my fingers to bend and ball up for a good fist pump. Now I’m getting super into it. I might like raise my arms up over my head. Well, the act of raising my arms away from my body is called abduction, while moving them back down toward the body is called adduction.
And this here, like, lasso movement? That’s circumduction -- a combination of all five of those movements that allow the forearm to move in a circle while the elbow joint stays relatively stable. Which really makes you wanna give credit to those cowboys. Now if you want to get really serious about that lassoing and expand it enough to include the entire arm, the move eventually will morph from an angular movement to a rotational one -- a motion that turns the bone around its axis, in this case, the humerus.
Hip and shoulder joints use a ball-and-socket design that allow rotational movement, but the more flexible a joint is, the more unstable and fragile it is -- which is one reason why you see so many dislocated shoulders and hip replacements. And it’s worth pointing out that some movements are just special and unique and weird, and only occur in a few select joints -- especially in your hands, feet, and jaw. For example, your ability to touch your thumb to your fingertips or give a thumbs up is thanks to opposition movement, facilitated by your saddle joint, which makes your thumbs opposable. The pivot joint between your radius and ulna allow you to rotate your palm forward or anteriorly in a motion called supination, and turning it backward or posteriorly is called pronation. So that’s what I like to think is the more grown-up approach to teaching you about your skeleton. It did not involve any singing, but yes, a little dancing. And it was worth it, because you learned about the basic structure of your skeletal system, including the arrangements of both the axial and the appendicular skeleton.
And we went over the structural and functional classifications of your joints, along with the major types of movement that they make possible. Special thanks to our Headmaster of Learning Thomas Frank for his support for Crash Course and free education. This episode was co-sponsored by Greg Avarbuch, Faht-Mah ikh-Bahl, and Ricky D. Shields. Thank you to all of our Patreon patrons who help make Crash Course possible through their monthly contributions. If you like Crash Course and want to help us make great new videos like this one, you can check out patreon.com/crashcourse Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr.
Brandon Jackson. Our director is Nicholas Jenkins, the editor and script supervisor is Nicole Sweeney, our sound designer is Michael Aranda, and the graphics team is Thought Café.
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