Update: I thought it might be interesting to discuss some topics that I teach in my exercise physiology classes. Learning more about how your body works may help you appreciate and take care of it better. Today I will discuss muscles.
All voluntary movements, whether as complex as a golf swing or as simple as scratching your nose, require the contraction of skeletal muscles. These act on the bony levers of the body, and the movement we desire occurs. Contraction of skeletal muscle requires a signal from the motor areas of the brain, which tells the muscles to contract, and the availability of energy in the form of a chemical called adenosine triphosphate, or ATP. Exercise physiology, in its simplest sense, is the study of the various factors that trigger or support muscular contractions.There are two basic types of muscles in the body: smooth and striated. Smooth muscle is located in the walls of blood vessels and most internal organs and is not under voluntary control. Striated muscles received this name because you can see a repeating pattern of light and dark stripes when you look at them with a microscope.
There are two types of striated muscles: skeletal and cardiac. There are major differences between cardiac and skeletal muscle. For instance, skeletal muscle will not contract unless a signal comes from the nervous system. Cardiac muscle is "autorhythmic" and will contract even when a heart is transplanted into another person with no nerve hookups at all.
A skeletal muscle is also "all-or-none." That is, when a skeletal muscle is signaled to contract, it contracts fully, with as much force as it can generate. To lift a heavier load, our bodies simply contract more muscle cells (fibers) to meet the challenge.
Since cardiac muscles all contract each time the heart beats, there is a need for these fibers to vary the strength of their contraction. This is done by increasing or decreasing the amount of calcium available to the heart. If we begin to exercise, the body secretes noradrenalin from the sympathetic nervous system onto receptors in the heart, and more calcium goes into the muscles to cause a more forceful contraction. Of course, a more forceful contraction pumps more blood and therefore provides more oxygen to the cells to support the work that we are doing.
The basic unit of any muscle is the muscle cell or fiber. Skeletal muscle cells are long, cylindrical structures that contain numerous nuclei. Muscle cells vary in length from only a few centimeters to more than 30 centimeters, depending on where they are located and what they do.
Each muscle cell is enclosed by a cell membrane known as the sarcolemma. This membrane is extremely important to the cell's function and determines what gets into and out of the cell. It also carries a small electrical charge (about minus 75 millivolts) that is necessary for muscle contraction.
There is connective tissue surrounding the cell membrane and groups of fibers in the muscle that extend to the end of the muscle, where it attaches to the bone (as tendons). This membrane ties the muscle cells together so that the contractile force from any muscle cell or groups of cells will cause the bony levers to work.
More about muscles next week.