Movement of body parts depends primarily on muscle tissue, which is highly specialized for contraction. The importance of this tissue is emphasized by the fact that almost half the body's mass consists of muscle. There are three types of muscle fibers. Compare and contrast their properties in the table below.

1. Skeletal muscle that is primarily involved in movement of bones (voluntary)
2. Cardiac muscle that enables the heart to beat so that blood can be circulated (involuntary)
3. Smooth muscle (visceral muscle) that provides tone and movement of hollow tubes and organs such as the intestines and uterus (involuntary)

The learning objectives for this unit are:

• Identify the key structural features of the three muscle types (skeletal, cardiac, and smooth) by light and electron microscopy, and explain how each is organized to form a contractile tissue that performs specific types of work.
• Recognize the key features of neuromuscular junctions by electron microscopy and discuss their functional significance.
• Compare and contrast the arrangement of actin and myosin filaments in skeletal, cardiac, and smooth muscle in electron micrographs.
• Compare and contrast the arrangement and functions of transverse tubules, sarcoplasmic reticulum, mitochondria, and contractile filaments in electron micrographs of skeletal and cardiac muscle.
• Identify the structural and functional attributes of connective tissues associated with muscle and the myotendinous junction.  

Let's begin with skeletal or striated muscle.