Peristalsis is a type of intestinal motility, characterized by radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut.
A time-space diagram of a peristaltic wave after a water swallow. High-pressure values are red, zero pressure is blue-green. The ridge in the upper part of the picture is the high pressure of the upper esophageal sphincter which only opens for a short time to let water pass.
A simplified image showing Earthworm movement via peristalsis
Smooth (soft) muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations. It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.
Smooth muscle shown in the tunica media in the walls of arteries and veins
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
A series of axon-like swellings, called varicosities from autonomic neurons, loosely form motor units through the smooth muscle.
