Muscle tissue, one of the four primary tissue types, consists chiefly of muscle cells that are highly specialized for contraction. Three types of muscle tissue exist: (1) skeletal muscle, (2) cardiac muscle, and (3) smooth muscle.  Without these muscle tissues, introduced in Chapter 4, nothing in the body would move, and no body movement could occur. Skeletal muscle tissue moves the body by pulling on bones of the skeleton, making it possible for us to walk, dance, bite into an apple, or play the ukulele. Cardiac muscle tissue pushes blood through the circulatory system. Smooth muscle tissue pushes fluids and solids along the digestive tract, regulates the diameters of small arteries, and performs a variety of other functions.

This chapter deals primarily with the structure and function of skeletal muscle tissue, in preparation for our discussion of the muscular system (Chapter 11). This chapter will also provide an overview of the differences among skeletal, cardiac, and smooth muscle tissues.

Skeletal muscle tissue forms skeletal muscles, organs that also contain connective tissues, nerves, and blood vessels. Each cell in skeletal muscle tissue is a single muscle fiber. Skeletal muscles are directly or indirectly attached to the bones of the skeleton. Our skeletal muscles perform the following five functions:

1. Produce skeletal movement. Skeletal muscle contractions pull on tendons and move the bones of the skeleton. The effects range from simple motions such as extending the arm or breathing to the highly coordinated movements of swimming, skiing, or typing.
2. Maintain posture and body position. Tension in our skeletal muscles maintains body posture--for example, holding your head in position when you read a book or balancing your body weight above your feet when you walk. Without constant muscular activity, we could not sit upright or stand.
3. Support soft tissues. The abdominal wall and the floor of the pelvic cavity consist of layers of skeletal muscle. These muscles support the weight of visceral organs and shield internal tissues from injury.
4. Guard entrances and exits. The openings of the digestive and urinary tracts are encircled by skeletal muscles. These muscles provide voluntary control over swallowing, defecation, and urination.
5. Maintain body temperature. Muscle contractions require energy; whenever energy is used in the body, some of it is converted to heat. The heat released by working muscles keeps our body temperature in the range required for normal functioning.

We will begin our discussion with the gross anatomy of a typical skeletal muscle. We will then consider at the microscopic level the structural features that make contractions possible.