The Fundamentals of Muscle Physiology
By the basic mechanical process of contraction, the muscles of our body can accomplish a variety of functions both simple and complex. There are three types of muscle, each having unique physiological functions: smooth, cardiac, and skeletal. Smooth muscle is responsible for some of the body's automatic or involuntary muscle movements such as movement of food through the digestive tract. Cardiac muscle is found only in the walls of the heart, producing the constant pumping action required to circulate blood. The discussion here will concentrate on skeletal muscle. Skeletal muscle is the largest organ in our bodies and comprises about 40% to 50% of an individual's body weight. As the name implies, this type of muscle is integrated with the skeleton and together they form the musculoskeletal system. Basically, muscle and bone form a lever system and by shortening of the muscle, movement of the bone occurs. The movement can be large as in the movements of our legs in walking or it can be as small as the click of a mouse. We will see later in this discussion that different muscles can have different characteristics depending on the type of movement. Common to all muscles, however, is the contraction or shortening of the muscle. The beginning of this discussion will focus on this process.
 To understand the mechanisms by which muscles contract, we first need to recognize the structural properties of muscle. Muscle tissue consists of groups of individual muscle cells called fascicles(Fig. 1). Muscle cells, also known as muscle fibers, are relatively long and often extend the length of the muscle. Myofibrils are cylinder shaped structures within the muscle fiber that contain the contractile "machinery". They make up about 80% of the volume of whole muscle. The alternating A bands (dark) and I bands (light) in myofibrils results in the striated appearance of skeletal muscle when viewed under a microscope. (Striations are also seen in cardiac muscle and both skeletal and cardiac muscle are categorized as striated muscle.)
 This banding pattern in myofibrils is due to an organized collection of thick and thin filaments (Fig. 2).The area in this structure where the thick and thin filaments overlap is where we see the dark A bands of the myofibril. The light I bands contain only thin filaments. The thin filaments are brought together at the vertically oriented Z line. The area between the Z lines is called a sarcomere. The sarcomere is the basic contractile unit otherwise known as the functional unit of muscle. In other words it is the smallest structure in muscle that shortens or contracts.
 Figure 3 shows a different view of this structure. Each thick filament is surrounded at each end by six thin filaments. The sarcomere shortens as the thin filaments slide across the thick filaments. This sliding-filament mechanism leads to muscle contraction. For an animation of this process, click on Fig. 3. |
