师：同学们大家好，上一节课我们学习了肌肉的第一课时。大家还记得学了什么吗？通过学习，我们知道了肌肉的分布及作用，还了解了肌肉是如何工作的，并且能利用所学知识进行自我保健。上节课结束前，老师留了个思考题：当我们睡觉时，身体哪些部位的肌肉仍在工作？同学们，课后有去了解吗？接下来，我们一起看一则视频，了解更多有关肌肉的知识。
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师：Each time you take a step, 200 muscles work in unison to lift your foot, propel it forward and set it down. It's just one of the many thousands of tasks performed by the muscular system. This network of over 650 muscles covers the body and is the reason we can blink, smile, run, jump and stand upright. It's even responsible for the heart's dependable thump.
First, what exactly is the muscular system? It's made up of three main muscle types. Skeletal muscle, which attaches the tendons to our bones. Cardiac muscle, which is only found in the heart. And smooth muscle, which lines the blood vessels and certain organs like the intestine and uterus. Skeletal muscles form the bulk of the muscular system, make up about 30 to 40% of the body's weight and generate most of its motion. Some muscles are familiar to us, like the pectorals and the biceps. Others may be less so, like the buccinator, a muscle that attaches your cheek to your teeth, or the body's tiniest skeletal muscle, a 1 - millimeter long tissue fragment called the stapedius that's nestled deep inside the ear.
Unlike the skeletal muscles, the body's cardiac and smooth muscles are managed by the autonomic nervous system, beyond our direct control. That makes your heart thump roughly 3 billion times over the course of your life, supplying the body with blood and oxygen. Autonomic control also contracts and relaxes smooth muscle in a rhythmic cycle that pumps blood through the smooth internal walls of blood vessels, enables the intestine to constrict and push food through the digestive system and allows the uterus to contract when a person is giving birth.