The human body has three different kinds of muscle: smooth, skeletal and cardiac. Smooth muscle is the involuntary, non striated muscle that is found in your digestive tract, blood vessels, lymph system, bladder, respiratory system, uterus, skin – almost any part of the body you can think of that requires movement of some type that is automatically regulated without your conscious control. For example, smooth muscle lines your digestive tract and is responsible for moving the partially digested food through the small and large intestine by peristalsis, the systematic contraction and relaxation of muscle fibers that produces a squeezing effect. Peristalsis is what happens when you swallow food as well. Smooth muscle cells contain only one nucleus; whereas skeletal muscle cells contain multiple nuclei.
Skeletal muscle is under our conscious control and is involved in every movement the human body
makes; be it walking, talking, chewing, riding a bike, and so on. Skeletal muscle fibers are structurally different from smooth muscle in that they are striated – under a microscope, dark lines appear along their length. Each space between two striations in a skeletal muscle cell is called a sarcomere and is made up of actin and myosin proteins. When a muscle contracts or relaxes, actin and myosin fibers slide against one another becoming closer together or further apart. Thus the striations appear (if we could see this in action under a microscope) to come closer together or further apart.
Individual smooth muscle cells are fusiform in shape and flatten out in their relaxed state and fatten up in their contracted state. Individually, these cells have more elasticity than skeletal muscle cells.
Cardiac muscle cells are striated, involuntary cells found in the heart. They differ from skeletal and smooth muscle in many ways, but two are key: cardiac muscle fibers appear branched under the microscope, but still have a striated appearance and the chemical mechanisms that activate their contraction or relaxation are different from those of skeletal or smooth muscle.