Back To CourseBiology 101: Intro to Biology
24 chapters | 226 lessons
People have skeletal systems made up of over 200 bones. Actually, the exact number in adults is 206 if you were wondering. In this lesson, we'll talk about how these bones and other connective tissues are organized into a skeletal system to give the body its form and protect vital organs. We'll also talk about the muscular system and how the skeletal and muscular systems work together to allow a wide range of movements and physical capabilities in humans.
Humans have an endoskeleton, which is located inside the body. Most of the human skeleton is made of bone, which you may recall is composed of mineralized calcium phosphate that is secreted into the extracellular matrix by bone cells called osteocytes. The human skeleton also contains quite a lot of cartilage too. Bones are hard and inflexible, but cartilage is rubbery and flexible, which makes it very useful in parts of the skeleton where movements require flexibility of the skeleton or padding is needed between bones that would otherwise rub against each other. Ligaments made of collagen make flexible connections between bones that keep the bones in their proper places while allowing a wide range of motion.
The most basic function of the skeletal system is to support and provide structure for the body. Without our skeletal systems, we wouldn't be able to hold our shape, and we definitely would not be able to stand upright. A second major function of the skeletal system is to provide protection for vital organs like the brain, heart and lungs, which are protected by the skull and rib cage.
The last basic function of the skeletal system is to facilitate movement. Part of this function is a direct result of providing support for the body, but beyond support, bones also function as levers that the muscles can manipulate to run, jump, lift, grab, or basically move in almost any way you can think of.
In addition to acting as levers, the positioning of bones and the way that they are attached to each other and to muscles define the kinds of movements that the body can make.
So now that we've talked about the skeletal system and its functions, let's talk about the muscular system and how it contributes to the form and movement of the human body. Muscle tissue is one of the four major tissue types found in the human body, and all muscle cells function in the same basic way; they contract or get shorter and pull whatever they are attached to closer together.
Muscles only do work when they contract, they do not actively push things further apart when they stretch back out. Instead, muscle cells are said to relax when they stretch back out because it is a passive action that does not result in any force being applied by the cell. In contrast, contraction requires energy and results in a force being exerted by the cell.
Some muscles in our bodies are under our conscious control and allow us to control our movements. These muscles are said to be voluntary muscle or muscle that can be contracted consciously by an animal. In contrast, the body also contains muscle that is not under our conscious control. This involuntary muscle is muscle that cannot be consciously contracted by the animal. There are three types of muscle cells, two that are involuntary and one that is voluntary. These three muscle types are cardiac muscle, smooth muscle and skeletal muscle.
Cardiac muscle cells are involuntary muscle cells that make up the heart. Cardiac muscle is only found in the heart and a key difference between cardiac muscle and other muscle types is that cardiac muscle cells pass electrical signals to each other. In this way, when one cardiac muscle cell gets a signal to contract, it passes that signal along to all its neighbors. The electrical signal is quickly passed to all of the muscle cells of the heart chamber and they contract in unison.
Smooth muscle cells are involuntary muscle cells that are found throughout the body, except for the heart.
Unlike cardiac and skeletal muscle, smooth muscle does not have visible stripes, or striations, when viewed under a microscope. This is actually the reason why it's called smooth muscle. Smooth muscle is found in many places throughout the body like blood vessel walls, the stomach, the small intestine, the large intestine and the bladder.
Skeletal muscle cells are voluntary striated muscle cells that control body movements. These are the muscles that people are generally most familiar with because many of them are visible and we actively control them all the time in our every day lives An interesting feature of skeletal muscle is that each skeletal muscle cell contains lots of nuclei. This is because each mature skeletal muscle cell is actually formed from lots of immature muscle cells that fuse together to form one large cell.
So I mentioned earlier that bones can function as levers that the muscles manipulate to move in almost any way you can think of. I also mentioned that the positioning of bones and the way that they are attached to each other and to muscles define the kinds of movements that the body can make. For example, let's look at a few movements that your arms can make.
Hold your hand out in front of you, palm up. Go ahead we'll leave the screen blank while you do this. Now turn your palm down. Did you see how easily you flipped your hand over by twisting just your forearm? Now try twisting your upper arm the same way. Maybe you can twist it some, but not nearly to the extent that you can twist your forearm.
Okay, we're back to showing visuals again. The reason why you can twist your forearm so much easier than your upper arm is because you have two long bones in your forearm, the radius and ulna, which can be twisted around each other and allow you to twist your forearm. In contrast, you have only one long bone in your upper arm, so there is no second bone to twist around. This limits the amount that you can twist your upper arm.
Now let's look at how the contraction and relaxation of different muscles can control movement of your arm at the elbow. Bending of the arm at the elbow is controlled by two opposing muscles - the biceps muscle and the triceps muscle.
The biceps muscle is this muscle here in the arm. One end of the biceps muscle is attached by tendons to the top of the shoulder blade, underneath the shoulder muscle here. The other end of the biceps muscle is attached by tendons to the inner side of the radius and ulna here.
Opposing the biceps muscle is the triceps muscle, which is this muscle right here. The triceps muscle is attached by tendons on one side to the shoulder blade just below the arm, right about here. And on the other side, tendons attach the triceps to the backside of the ulna right about here near the elbow. When the triceps muscle is relaxed and the biceps muscle contracts, it pulls the radius and ulna closer to the upper arm, and the arm bends at the elbow. In contrast, when the biceps muscle relaxes and the triceps muscle contracts, the ulna is pulled back away from the upper arm, and the arm straightens. If both muscles contract at the same time, the two forces cancel each other out and the muscles flex without moving the arm.
Other pairs of opposing muscles are found all over the human body and each muscle controls a different type of movement, which we can combine into more complex movements like walking or typing on a computer.
So let's review. Humans have an endoskeleton, which is located inside the body. The human skeleton is composed mostly of bone and cartilage. Bones are hard and inflexible, but cartilage is rubbery and flexible, which makes it very useful in parts of the skeleton where movements require flexibility of the skeleton or padding is needed between bones that would otherwise rub against each other. Three of the primary functions of the skeleton are to support and provide structure for the body, to provide protection for vital organs and to facilitate movement.
We have three types of muscle in our bodies: cardiac muscle, which is involuntary muscle of the heart; smooth muscle, which is involuntary muscle in the rest of the body; and skeletal muscle, which is voluntary muscle that controls body movement. All three of these muscle types function in basically the same way; they contract or get shorter and pull whatever they are attached to closer together.
Muscles only do work when they contract. They do not actively push things further apart when they stretch back out and are said to be relaxed. By using tendons to attach to bones in specific places, muscles form opposing pairs of muscles all over the human body. These muscles use the bones of the skeleton as lever arms and control various specialized movements, which we can combine into more complex movements like walking or typing on a computer.
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Back To CourseBiology 101: Intro to Biology
24 chapters | 226 lessons