Episode #8 of the course “How the human body works”
Muscles are movers. If it wasn’t for your muscular system, the signals sent down into your body from your brain wouldn’t do anything. Muscles turn thoughts into actions. Muscles are designed for contracting and relaxing, and muscle movement can be voluntary or involuntary depending on the muscle type. There are three types of muscle: skeletal muscles, smooth muscles, and cardiac muscles.
When people talk about “having muscles,” they are talking about skeletal muscles. We can see and feel skeletal muscles under the skin. These muscles operate in pairs, attached to the body’s bones by connective fibers called tendons. The most well-known tendon connects the calf muscle on the leg to the foot; it’s called the “Achilles’ tendon” after the Greek demigod whose famous weak spot was his ankle.
Skeletal muscles are long, thick bundles of fibers. They work in pairs to contract voluntarily, only when the brain sends a signal to do so. A skeletal muscle exerts a lot of force to pull the bone it is attached to in one direction, and the other muscle in the pair pulls the bone back the other direction. Skeletal muscles must be expandable, and with repetitive motions, they become more adept at pulling the bone.
Smooth muscle lines the walls of internal organs and operates involuntarily. Like skeletal muscle, it contracts with a signal from the brain, but a person does not have to think about it to make it happen. Smooth muscles keep the intestines processing food and the blood circulating without any conscious effort. The third type of muscle, cardiac muscle, is a unique muscle type found in the heart. Cardiac muscle is involuntary, but it exerts a lot of pressure like skeletal muscle. When cardiac muscles do not receive enough oxygen, they suffer an “infarction” and die. This is commonly called a heart attack.
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