Smooth muscle is the simplest of the three kinds of muscle.  It is found where slow, sustained, involuntary contractions are needed such as in the digestive tract, reproductive system and other internal organs. Smooth muscle cells are long and spindle shaped with a single, centrally located nucleus.  Smooth muscle is often arranged in two layers that run perpendicular to one another,  a circular layer whose fibers  appear in cross section as shown on the model above and a longitudinal  layer whose fibers appear like the ends of a cut cable when viewed on-end..