Muscles are a part of the body of us humans and of many animals. Muscles all work in a similar way: they contract and relax. We usually think first of our muscles in the arms and legs. But our heart is also a muscle, and a very special one at that. All or several muscles together are called the musculature.
There are three types of muscles: With the skeletal muscles we move our skeleton, i.e. our bones and thus the whole body. We can control them with the brain. So they do what we want. The connection from the brain to the muscles goes through a lot of nerves. You can think of it as many telephone lines next to each other.
We have a second type of muscle around the intestines, for example. They move the intestine so that the food pulp moves slowly forward. We also have such muscles around the veins because the heart alone is not sufficient as a pump. We cannot control these muscles with our consciousness. The nerves do that independently.
A third type of muscle is the heart muscle. It is similar in many ways to the skeletal muscles, but we cannot control it consciously. The heart muscle works faster when we exert ourselves in sports, for example. But it also sometimes starts to race out of fear or nervousness. The heart muscle cannot get a cramp, no matter how hard and for how long it has to work.