MUSCULAR-SKELETAL SYSTEM

The skeletal system is organized into what is called axial and apeducular. Our axial skeleton, down the main axis of our body: skull, the cranial bones, facial bones, vertebral column, the ribs, the thorasic cage, and the sternum, which is where the ribs meet in the front. Attached to the axial skeleton is the appendages, upper appendages and lower appendages.

The skull, we mentioned there are 8 bones making up the cranium, and then 12 bone making up the face. Then we have the vertebral column, and we said the vertebral column is regionalized, cervical, thorasic, lumbar, safal, and coxagial. If we take out a single vertebrae and look at it, most vertebrae will have the same basic architecture: a body, sometimes called a centrum, boney processes extending from the posterior part of the vertebrae, called the vertebral arch, and the arch has a part that attaches called the pedical, then it has a lamina on the back. Most vertebrae have what is called a spinus process, if you palpate the midline of your back you will feel bumps, those are the spinus processes, and maybe vertebrae have little lateral projections called transverse processes. Most vertebrae have a body, an arch, transverse processes, and a spinus process. when we talked about the spinal cord, what fit inside of the arch? The spinal cord itself. This opening inside is called the vertebral feramen, and the spinal cord sits inside, surrounds the spinal cord and protects it. These other boney out propings are primarily to allow muscles to attach.

You have 26 vertebrae that make up this segment. In the cervical region, the individual vertebrae are articulating with each other, meaning forming a joint, and there is a small pad of connective tissue in between each one. This is the intervetebral disc made out fibrocartilage. You have 7 cervical, 12 thorasic, 5 lumbar, and the 5 sacral are all fused in one single bone called the sacrum, and then you have 4 coxagial that are all fused together into a single point. When we count vertebrae, we have 7+12+5+1+1 = 26 vertebrae. In between the two neighboring vertebrae is a little hole out through the side, and the spinal nerve comes out through the side, this is called the intervertebral ferangi. That is the basic architecture of the vertebral column.

Other axial components include the thorasic cage. If we look top down on the vertebral column, the ribs extend around the side. The head of the rib is actually articulating with the vertebrae, and the side of the rib comes along and follows the transverse process of the vertebral column, and then attaches in the front to the sternum. There are 12 thorasic vertebrae, there is 1 rib per each vertebrae. The sternum down the front of the body is comprised of three different bones that are all fused together. The manubrium, the sternal body, and the zifloid. The ribs come around and attach either to the menubrium or the sterna body. There are three kinds of ribs, sometimes called true ribs, false ribs, and floating ribs, but the correct normenclature is vertrbrosternal, vertebrocostal, and vertebro. All ribs attach to the vertebral column, the question is if the ribs attach individually to the sternum or if they attach indirectly via a complicated cartilage, or do they not attach at all. Of your 12 ribs, 2 of them do not attach at all, called the floating ribs. Some of your ribs have their own individual cartilages, called the costal cartilages. The first 7 vertebrae each have their own cartilage, and ribs 8, 9, and 10 do not, they all attach to the costal cartilage of 6 and 7. 8, 9, and 10 are sometimes called the false ribs because they do not actually attach to the sternum, they attach to the cartilages. Ribs 11 and 12 do not attach at all in the front. There are 12 ribs categorized based on their attachments. All of them attach to the vertebral column; some of them attach via their own piece of cartilage, called the

verbrosternal ribs; some have a common piece of cartilage, called the vertebrocondral ribs; the ones that do not attach in the front are called vertebro ribs.

That accounts for the axial skeleton. Everything else is apendicular. The first thing we need are attachments for our appendicular skeleton, some way to fix the appendages to the axial skeleton. The things that do that are called gurtals. The term gurtal; a gurtal is something that is attached in the front and in the back. For us, only our pelvis is a true gurtal. Our pectoral gurdal is not really a gurdal because it is not attached in the back. For us, our pectoral gurdal consists of two bones, our scapula and our clavical. The scapula is free floating in the back and the clavical articulates with the scapula. In the front, your clavical articulates with your menubria, which is the top of your sternum. The clavical stops you from moving your scapula too far to the midline. You have two bones that make up the pectoral gurdal on each side, your scapula and your clavical. Your scapula is also the place where the humerus attaches. The pectoral gurdal attaches the upper appendage to the axial skeleton, though it is not a true gurdal because it is not fused on the back, but the bones are held together with muscles.

As for your pelvic gurdal, the coxical bones are attached to the sacrum. The coxical bones fuse in the front and are attached in the back. Your pelvis looks like it is one structure, but the sacrum is part of the axial skeleton, and the coxal bones themselves, there are three bones on each side, which include the iliac, the ishium, and the pubic bone. There are 6 total coxal bones, and the pelvic gurdal is a true gurdal, the bones are not mobile.

Let us now look at the appendages that attach to these bones. They are all set up in three segments for both the upper and lower appendage. A proximal segment, a middle segment, and a distal segment. In the upper appendage, there is one bone in the proximal segment, the humerus. The two bones in the middle segment are the ulna and the radius. In your hand, you have 2 rows of 4 carpuls, 5 metacarpuls, and 14 phalynx, three in each finger except your thumb. In the upper appendage, we have 32 bones on each side in the upper appendage.

In the lower appendage, we have a proximal segment that has the femur, our thigh, and from our knee to our foot is our leg, and then we have a specialized bone covering our joint capsule called our patella. In the leg, we have our tibia and fibula. The fibula is near the outside, tibia near the midline. Then we our tarsles, metatarsles, and phalyx. If this is your foot, the weight of your tibia sits down on your tarsles and the weight is distributed either to the back. There are 7 tarsles (lower ankle) and 5 metatarsles and 14 phalynx just like your hand. In your lower appendage, you have three coxal bones and a femus, patella, tibia, fibula, and 26 bones in your foot; the total in the lower appendage on each side is 33 bone.

Now let us look at the basic anatomy of a muscle. Most muscles attach to skeletal components, at two connection points, an origin and an insertion. Those are differentiated from each other based on which one is attached to the moveable bone. Normally the origin is fixed in place while the bone to which the muscle is attached is being moved, and the point of attachment on the moveable bone is called the insertion. Let us take our muscle and section it and look at the internal organization of the muscle. The muscle is broken down into functional units called fascicles, and all of the fascicles are surrounded by connective tissue that attaches them all together. The fascicles are made up of individual muscle cells. What I have here are hundreds of individual muscle cells that are grouped together. The muscle cells are bound together by connective tissue; the individual muscle cells are sometimes called muscle fibers. Each individual muscle cell is surrounded by its own layer of connective tissue. So we have muscle cells bound together to form fascicles; fascicles group together to form a muscle. The individual cells have connective tissue wrapped around them called endomesium; the fascicles have

connective tissue that surrounds them called perimecium; the whole muscle together is surrounded by epimecium. All of muscle function is a molecular process, something that happens inside of the muscle cells themselves. The force generated within the muscle cell is transferred to the connective tissue around the muscle cell, then transferred out to the connective tissue around the fascicle, then transferred out to the connective tissue around all of the fascicles, and then transferred to the connective tissue that attaches to the bones. If we zoom in on one individual muscle fiber, inside of the muscle cell are hundreds and thousands molecular groupings called myofibrils. The myofibrils are comprised of actin and myosin. When we look at the myofibrils of a muscle we will see a banding pattern, and the bands consist of different regions. The functional unit of a myofibril is a sarcomere. A sarcomere is comprised of a couple different components: it has a z disc on each end, and attached to the z disc are actin molecules. Spanning in between the actin molecules are myosin molecules. The banding pattern results from the way light passing through these proteins is bent, so there are distinct regions that you can see: an H zone, an A band and an I band. In a sarcomere, there is H, A and I. the A band is by definition any place where there is myosin, including where it overlaps with the actin. An I band is any place where there is only actin. The H zone is where there is only myosin. A sarcomere is the molecule machine, and what it will do will pull the z disc closer to each other. The myosin molecule will attach to the actin and systematically pull it toward the middle, called the M line. Eventually the actin and the myosin will become as fully overlapped as possible, and in doing so, since H is where there is only myosin, when it is not overlapping with actin, what happens as the amount of overlap increases? H zone becomes smaller. The I band, where there is only actin, as the decree of overlap increases, the I band also gets smaller. So the sarcomere shortens, and there are hundreds of them for each myofibril, and hundreds of myofibrils in a muscle. Eventually the collective molecular force from each sarcomere shortening at a molecule level will be successively transferred to the higher connective tissue, and ultimately to the bone.

The next thing to look at is what happens at a molecular level. There are two very complex protein molecules with many different components, we will only learn the most simplictic aspects of each. The actin is a protein that consists of two interwoven proteins. The protein has on it different regions that allow it to attach to myosin, called the actin binding sites. In order to regulate the activity of the muscle, we need some type of regulatory protein. Overlaying the binding sites is a regulatory protein called tropomyosin. There is one tropomyosin associated with each one of the actin filaments, and the tropomyosin can cover the binding sites. To make the tropomyosin to move out of the way, there is another molecule called troponin. An actin is not a single thing, it consists of individual components woven together, the actin molecule itself plus each regulatory component, the tropomyosin and the troponin. When the muscle is in a relaxed state, the actin binding sites are covered by tropomyosin. In order for you to move, troponin will push the tropomyosin out of the way, revealing the actin binding sites, and allowing actin to interact with myosin.

Let us talk about myosin and its components. Myosin is the machine, actin is what is being acted upon, it does no molecule work but is worked upon by the myosin. The myosin is also made of multiple components, including a rod-like region and a head region. The head part of the myosin molecule does the work. For example, a rachet is a kind of tool with a leaver attached to a socket head that gets attached to various mechanical appliances, and when you want to do mechanical work, you move the leaver into position, either clockwise to tighten or counter clockwise to loosen. The first thing we have to do is put it into a high energy state, and then we can apply force to do work. At a biological level, we are doing molecular work, and work

requires energy. And work of motion is kinetic energy, work of position is potential energy. The myosin molecule has to have two states, one where it has the potential to do work, which is its high energy state, and one where it is actually doing kinetic work, and that is achieved by the head. The head acts like a rachet, the head has to be in a high energy state, and then that energy has to be transferred to the actin and as the molecule returns to a low energy state, it drags the actin with it.