Understanding muscle tone
Action potentials usually maintain muscle tone in skeletal muscles, but muscle cells are affected by other influences that can affect their contraction and thus the tone of the muscle.
Smooth muscle occurs in the walls of hollow organs, most notably the blood vessels and the GI tract, and in a few other places.  Contraction of smooth muscle is generally involuntary and can be stimulated by the APs from the autonomic nervous system, but often for the slow maintenance of muscle tone contraction is controlled by hormones, other external molecules, or external pressure.  The mechanisms that cause this contraction generally involve the phosphorylation of myosin light chains. (1) (2)