Tonic smooth muscles show continuous contraction as fluids move around them, while phasic smooth muscles show rhythmic motions that cease after a time, according to class notes from Florida State University College of Medicine. Phasic smooth muscles include those within the wall of the stomach, and tonic smooth muscles are found in arteries.
Scientists classify tonic muscles as continuously active muscles within the body that are usually filled with fluid. Phasic muscles are active for short periods but spend most of the time in a relaxed state, according to Kansas City University of Medicine and Biosciences. For example, phasic muscles in the stomach contract only a few times per hour in between meals. Contractions in the stomach increase to several per minute when someone eats. Muscles that line the bladder and ureter contract only a few times per day when the body eliminates urine.
Tonic muscles are also found in sphincters. Phasic muscles rely on electrical impulses through calcium ions to contract, explains Florida State University. Phasic contractions work by using action potentials, a process during which the electrical impulses generated by muscle cells briefly reverse to cause a contraction, according to Encyclopaedia Britannica. Tonic muscles do not use electrical impulses or action potentials to work. Both tonic and phasic muscles are known as unitary smooth muscles, or visceral smooth muscles.