In this module, we consider the effectors of the body that govern voluntary and involuntary movement. These effectors are specialized cells called muscle which are capable of generating force (tension). Muscle cells are classified as one of three types: skeletal, smooth, and cardiac. Although all three types generate tension, each is specialized for a given function. Skeletal muscle governs voluntary movement of the limbs and is critical for expansion of the lung during breathing. Smooth and cardiac muscle are contractile cells found in the walls of blood vessels and the heart, respectively. We will return to the basic principles that govern these cells types when we consider the respiratory and cardiovascular systems. The things to do this week are to watch the 4 videos, to answer the in-video questions, to read the notes, and to complete two problem sets (skeletal muscle and smooth & cardiac muscle). It will be most effective if you follow the sequence of videos. The notes provide a more detailed summary of each topic. We encourage you to find which resource (videos and/or notes) works best for you and to try the problems sets. The problem sets are not graded. Both your understanding and retention will increase with application of the new learned information.