The Human Body: Muscular System (2nd Ed, 1980)

Some 400 muscles make up nearly half the weight of your body and form your muscular system. The system that's responsible for every external and internal movement your body makes. Muscles not only help you move from place to place and do things, they help you breathe. They move blood and food through your body. They do things you can see and things you can't see. But these different activities aren't all done by the same kinds of muscles. For instance, the muscles that move your body are skeletal muscles. Skeletal muscles work with the bones of your skeleton. A skeletal muscle is attached by a tendon to one end of a bone. The muscle's other end is attached across a joint to another but more movable bone. When the muscle contracts, that bone moves. Muscles only do work when they contract. So, to move the bone back, another muscle is needed. Skeletal muscles always work together like this, in opposing pairs or larger groups. In your arm, the biceps is the muscle that raises your lower arm, and the opposing triceps straightens it out. One muscle at the back of your jaw opens your mouth, but opposing it are two larger muscles that close your jaw. Chewing needs more force than just opening your mouth does. So, all over your body, opposing groups and pairs of skeletal muscles are at work. The word muscle is from the Latin word for mouse, and each of your muscles has been given a Latin name that helps to explain something about what the muscle does. For example, take one of the muscles in your arm, the extensor carpi radialis brevis. Brevis means it's short. Radialis refers to the radius, one bone it's attached to. Carpi refers to the carpals, the wrist bones at the other end, and extensor means its work is to extend or straighten out the wrist. Right above it, another muscle attached to the same bones, does the same job, but it's longer than the brevis. Muscles can contract because of the special structure of muscle tissue. A skeletal muscle is a bundle of long fibers. Even under a microscope, separate cells can't be seen, but each fiber has many nuclei. The dark and light bands give this kind of muscle its name. This is striated muscle. Each fiber is made up of smaller threads called fibrils, whose thick and thin filaments cause the striped appearance of the muscle and also cause its movement. The action of the filaments is a sliding motion as parts of one filament attach themselves to parts of the other and pull, something like this. This motion contracts the fibrils. They contract the bundles of muscle fibers and you bend your arm or move some other part of the body. Muscles whose fibers are arranged parallel to the direction of their pull can shorten the most when they contract. When fibers are arranged at angles to each other, a muscle has a shorter but more powerful contraction. Some muscles fan out and form a broad base of support for a strong pull at a single point. Others curve around like cables to twist or rotate your arms and legs. Now, the kind of muscle that moves your bones, skeletal muscle, is also referred to in a different way. Because you can control it, it's also called voluntary muscle. It responds to conscious signals from your brain. Nerve signals travel from your brain toward the muscle you want to move. When the signal reaches the nerve end plate that lies alongside the muscle fiber, it causes a change in the nerve cell's outer membrane. The change allows a chemical transmitter to cross the space between the nerve and the muscle. Another electrical signal starts and the filaments move. The muscle contracts. Other nerve fibers carry signals back to your brain and keep you informed of the activity of the muscle and the bone it moved. Now, while conscious decisions by your brain cause skeletal muscles to contract, there are There are other muscles not attached to your skeleton that you have no conscious control over. These muscles, like those that move blood or food through your body, are controlled by automatic nerve signals. This kind of muscle is different in structure from a striated skeletal muscle. This involuntary muscle is a smooth muscle. Smooth muscles have clearly defined cells with individual nuclei and don't appear striped under microscope. These smooth muscles contract slowly and rhythmically. In the elastic walls of your blood vessels, smooth muscles wind around the vessel in one layer and are laid along the vessel's length in a second layer. By contracting and relaxing, these muscles help your heart push blood through your body. Smooth muscles in your esophagus are arranged in the same way. When you swallow, the contractions of its smooth muscles squeeze the food along. Food is moved down your esophagus into your stomach. When the muscular churning stomach has done its job, a ring-shaped muscle called a sphincter opens and the food is pushed along through your digestive tract by the smooth muscles of your intestines. Your smooth muscles are capable of small delicate movements, too. The smooth involuntary muscles that form the irises of your eyes respond to light. They open your pupils wider to let in more light when the light is dim and close them to let in less light when it's bright. Now, there is one more kind of muscle, cardiac muscle. Under the microscope, it looks something like striated muscle, but its fibers branch and join and form an irregular network instead of a bundle. And normally, cardiac muscle isn't consciously controlled. In that way, it's more like smooth muscle. In your body, cardiac muscle is found in only one organ, your heart. A region of specialized muscle tissue called the pacemaker starts the rhythmic contraction of cardiac muscle that squeezes blood out of your heart and into your arteries. In your blood are complex substances from your food that contain the energy for all muscular contractions. This energy is in the chemical bonds that hold together the atoms and molecules of a simple sugar, glucose. As enzymes break apart the glucose molecules in the muscle cells, the energy is transferred to the bonds of another molecule, commonly called ATP, adenosine triphosphate. Then, when nerves signal muscle fibers to contract, molecules of ATP break apart and energy is released. The formation of most ATP uses oxygen and produces carbon dioxide, water, and heat in the muscle fibers and cells. These products are carried away by the blood. When you're active, the increasing carbon dioxide in the blood signals the lungs to bring in more oxygen and the heart to beat faster. But as you continue to use your muscles, they may use up oxygen faster than the lungs and heart can deliver it. Without enough oxygen, glucose breaks down and forms ATP through a different series of reactions, and the product of these reactions, lactic acid, begins accumulating in your muscles. Lactic acid is what makes your muscles feel tired. But rest allows the blood to bring in enough oxygen to reverse the chemical reactions that produce this acid. So, your heart and lungs work together to maintain the action of your muscles and your muscular system. It's a system that begins with electrical signals, sometimes consciously sent by your brain, sometimes not, but always carried by your nerves. It's a system composed of three different kinds of contracting muscles. Cardiac, smooth, striated. Three kinds, contracting and relaxing. They push blood through your arteries and veins, and food through your digestive system. They give shape to your body and move your bones.