I. There are three fundamental kinds of muscle cells in the human body: cardiac muscle, which constitutes the main subject of this chapter; skeletal muscle (the kind that moves your arms on command); and smooth muscle (the kind that moves involuntarily, but consistently, allowing, say, liquid in the intestines to keep moving). All three muscles use variants of the actin/myosin system, along with a smattering of other proteins, for contractility.
