Seeing how your muscular system moves you

Functions of the muscular system
Supporting your structure
Moving you
Poised positioning
Maintaining body temperature
Pushing things around inside
Cardiac muscle
Diaphragm
Digestive smooth muscle
Controlling release

Muscle tissues always have work to do. They pull things up and push things down and around. They move things inside you and outside you. And they move you, too, of course. They work together with all the other systems in your body, but more than any other organ system, the muscular system specializes in movement: of food and air into and out of your body; of the circulation of your blood within your body; of different parts of your body relative to one another, as when you change position; and of your body through space — what you normally think of as "movement".

All muscle tissue is strong. Most is enduring, some of it astoundingly so. Its cells are crowded with mitochondria, thousands of little factories constantly turning out molecules of ATP, a refined fuel. The muscle cells use the fuel to manufacture strong and flexible proteins, with which they build and repair themselves and do work.

All the work that muscle tissues do is done by the coordinated contraction and release of millions of sarcomeres, tiny structures within the muscle cells. Muscle activity accounts for most of the body’s energy consumption.

In this chapter, we give you an overview of some of the things muscles do and how they do it, and then we name the muscles. At the end of the chapter, we list some common muscle ailments, one of which you’ve probably experienced.

Functions of the muscular system

This section focuses mainly on the functions of the skeletal muscles, the muscles that move your bones. The skeletal muscles comprise a substantial portion of your body mass, and most of what you eat goes to fuel their metabolism. In this section, you’ll find out something about what they do with all that energy.

Supporting your structure

Muscles are attached to bones on the inside of your body and skin on the outside, with various types of connective tissue between the layers. Thus, they hold your body together. Along with your skin and your skeleton, your muscles shield your internal organs from injury from impact or penetration.

Moreover, and don’t take this personally, but your body is heavy. As it does with everything else, gravity pulls your weight downward (toward the planet’s center). But gravity doesn’t only pull on the soles of your feet — it pulls on all your weight. If gravity had its way, you’d be lying on the floor right now. Your muscles pull your weight up ("oppose" the pull of gravity) and hold you upright. Gravity is a relentless cosmic force, and eventually, gravity will win. But while you’re still fighting, your muscles need fuel and rest.

Moving you

Contracting and releasing a muscle moves the bone it’s attached to relative to the rest of the body. The movement of the bone, in turn, moves all the tissue attached to it through space, as when you raise your arm. Certain combinations of these types of movements move the entire body through space, as when you walk, run, swim, skate, or dance.

Muscle contraction is responsible for little movements, too, like blinking your eyes, dilating your pupils, and smiling.

Poised positioning

A very close interaction outside of your conscious control between some muscle cells and the nervous system keeps you not just upright, but in balance. Nerve impulses throughout the muscular system cause muscles to contract or relax to oppose gravity in a more subtle way when, say, you’re shifting your weight from one side to the other as you step. This interaction is called muscle tone, and it’s what is enabling you to hold your head up right now.

When you step down a steep incline in rough terrain, your muscle tone brings your abs and your back muscles into action in a different way than when you step across your living room rug. The mechanisms of muscle tone may move your arms up and away from your body to counterbalance the pull of gravity with an accuracy and precision you could never calculate cognitively. Below your conscious level, the mechanisms of muscle tone are active every minute of every day, even when you’re asleep.

Muscle tone relies on muscle spindles — specialized muscle cells that are wrapped with nerve fibers. (See the "Skeletal muscle" section later in this chapter for information about spindle fibers.) The central nervous system stays in contact with the muscles through the muscle spindles. (Turn to Chapter 7 for the structures of the central nervous system.) Spindles send messages about your body position through the spinal cord to the brain; to initiate the fine adjustments, the brain sends signals through the spinal cord and nerves to the muscle spindles about which muscles to contract and which to release.

Maintaining body temperature

Muscle contributes to homeostasis (see Chapter 2) by generating heat to balance the loss of heat from the body surface. Muscle contraction uses energy from the breakdown of ATP and generates heat as a byproduct. Shivering is a series of muscle contractions that generate extra heat to keep your temperature up in cold situations. If the heat generated by the muscles raises body temperature too high, other thermoregulatory processes, such as sweating, are activated.

Pushing things around inside

The other two types of muscle tissues, smooth muscle and cardiac muscle, have their own important functions, discussed in other chapters. Here, we give you an overview of some of the muscles that keep things moving within your body, all without any thought from you.

Cardiac muscle

The cardiac muscle that makes up the walls of the heart contracts rhythmically, pumping the blood into the arteries. It hits with so much force that the artery walls briefly stretch — detectable as your pulse. However, there must be enough force from the blood pushing on the walls by the time it reaches the capillaries to push out the plasma. This is why blood pressure is so important. Smooth muscle in the walls of the arteries help control this by contracting to constrict the vessel or relaxing to dilate it. Damage to this layer is one cause of arteriosclerosis (hardening of the arteries), which takes away the arteries’ subtle control of blood pressure. See Chapter 9 for more about the heart and blood vessels.

Diaphragm

The diaphragm is a skeletal muscle whose contraction and release forces air in and out of the lungs. Chapter 10 covers the respiratory system in all its breathtaking glory.

Digestive smooth muscle

The organs of the digestive tract have walls containing smooth muscle that contracts in pulsating waves, pushing ingested material along. Think of this muscular lining as a conveyor belt on a disassembly line. Refer to Chapter 11 for details.

Controlling release

Sphincter muscles are essentially valves: rings of smooth muscle that are fully contracted in their resting state, holding some material in one place, and then relaxing only briefly to allow the material to move through. You find sphincters at various places in the digestive system (refer to Chapter 11), from the very beginning to the very end, and in other parts of the body as well.

Most sphincters aren’t under conscious control. Two of them — the urinary sphincter, which holds urine in the bladder, and the anal sphincter, which holds feces in the colon — come under conscious control usually at around 2 years of age. This control allows the release of these bodily wastes under culturally appropriate circumstances. Its acquisition is considered a milestone in infant development.

By the way, human males have much stronger urinary sphincter muscles than do females, meaning that they can retain about twice as much urine in the bladder (up to 800mL or 1.69 pints) for twice as long. Kindly keep that in mind, fellas, when you’re on a road trip with girls.