EARTH PHYSIOLOGY

There are three types of muscles in our bodies - skeletal muscle, cardiac (heart) muscle, and smooth muscle. Although we will be most interested in skeletal muscle in this chapter, let's briefly review the other two so that we can understand the similarities and differences in the muscle types. Let's begin with the differences.

The muscle types differ not only in their function, but also in their appearance (Figure 2). Skeletal muscle has a striped appearance (also called striated). Cardiac muscle is also striated, but the stripes are much less organized and contain branches to allow for extremely rapid communication, and smooth muscle is not striped at all, but instead is composed of obvious, distinct cells with very dark nuclei. The appearance and make up of the different muscle cells are indicative of their function, as we will see. First of all, the heart itself is a muscle - it is called the cardiac muscle. In fact, the heart is the muscle in the body with the greatest endurance. It beats about 100,000 times per day without our conscious control; that is, cardiac muscle contractions are involuntary.

Figure 2. There are three types of muscles in the body. (a) Skeletal muscle has a very organized striated appearance. (b) Cardiac muscle also has a striated appearance but the cells are much less organized. (c) Smooth muscle is not striated but consists of distince cells, each with a very dark and obvious nucleus. There are actually two types of smooth muscle cells, multi-unit (the cells of which can operate independently) and visceral (where the cells operate together as a unit).

Figure 3. Cardiac muscle cells (fibers).

The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle, and neuromuscular muscle fibers that provide the transmission system for rapid conduction of the cardiac excitatory signal throughout the heart (Figure 3). In simple terms, this means that these specialized cardiac muscle fibers keep all of the heart muscles working together in rhythm. Of the three, the ventricular muscle is by far the thickest and most powerful. This makes sense since the ventricles are responsible for providing the powerful thrust to propel blood out of the heart and around the body.

Although there are different regions and thicknesses of muscle in the heart, the entire heart muscle works as a single unit. The specialized structure of the cardiac muscle cells allows for this continuous, rapid, and unified action among all the cardiac cells. The heart must work as if it were one muscle because of the importance of maintaining a constant heart rhythm to keep the blood flow as constant and stable as possible. Think about a cheerleading squad at one of your school sporting events or even a chorus or band ensemble performing at a school assembly. In each case, it is important for the entire group to perform with the same rhythm or else the quality of the performance is decreased. The same goes for the rhythm of the heart (although, unlike a cheerleading, chorus, or band performance, abnormalities in heart rhythm can be dangerous to your health). If the various heart muscle contractions do not occur in rhythm with one another, the quality of the life of an individual can decrease, if not cease altogether In fact, certain dangerous disease states do exist that are characterized by abnormal heart rhythms.

Smooth muscle is another involuntary muscle group. There are two types of smooth muscle: multi-unit smooth muscle and visceral smooth muscle. Multi-unit smooth muscle is composed of cells that can operate independently of one another. Visceral smooth muscle, on the other hand (often called single-unit smooth muscle), is composed of cells that, collectively, function together as a single unit. Some examples of multi-unit smooth muscle found in the body are the muscle in the iris of the eye (which causes the iris to either dilate or constrict), the muscles that cause erection of the hairs when stimulated by the nervous system (for instance, when you are so scared that your hair stands on end), and the muscle of the larger blood vessels (that help to either dilate or constrict certain sections of the blood vessels).

Visceral smooth muscle is found in the walls of most of the hollow organs of the body, especially in the walls of the gut, the intestines, the ureters, the bladder, and the uterus. As you can well imagine, then, each smooth muscle group has a very specialized function distinct from the others: in the uterus it must work to help a woman deliver her baby; in the bladder it must work to help push urine into the urethra, which also contains smooth muscle, to squeeze urine (and sperm in the male) out of the body. Because of smooth muscle, these functions do not depend on gravity to aid in the extraction of their respective biological "contents." The best example to illustrate the nature of visceral smooth muscle is to consider the digestive system including the gut and the intestines.

Figure 4. Circular and longitudinal smooth muscle layers.

Because of the smooth muscle, food requires no help from gravity to make its way through the esophagus, out of the gut, and through the coiled intestinal pathway (in fact, an astronaut's digestive system works very well in a microgravity environment). The power to move the food through the digestive tract is furnished by muscles that stretch the full length of the tract. They form two layers, one running along the tract and the other encircling it in concentric rings. By setting up a churning motion, and by a series of progressive contractions known as peristaltic waves, these twin sets of muscles force food all the way from the esophagus in the throat to the rectum, much as if toothpaste were being squeezed along its tube by some built-in power in the tube walls (Figure 4). The muscles of the esophagus squeeze and relax in concert; these muscles mix the food and propel it along. These peristaltic waves are so powerful that they will move swallowed food even if you stand on your head! The stomach is also lined with smooth muscle. It hangs folded when empty but it can stretch to accommodate more than a quart of food before moving it slowly into the intestines. We have all felt (and even heard) the smooth muscle contractions, or peristaltic waves, coming from our intestines. This occurs as the food is being broken down and digested and just before our bodies signal us to eliminate the "digested food!" The final exit for this digested food is also lined with one of the smooth muscle groups known as a sphincter. So, even though you may have never known the physiological basis for your elimination of solids, each and everyone of us has many years of experience dealing with the practical usage of these muscles! Well, let's change the subject and get on to the main topic for this chapter - skeletal muscle.