Examples of involuntary muscles, also known as smooth muscles, are the muscles of the urogenital tract, digestive tract, circulatory system and respiratory tract. Involuntary muscles are not controlled by the organism's will. Their actions are automatic and facilitated by the autonomic nervous system.
The structure of an involuntary muscle cell consists of a long, spindle-shaped cell without striations. The cells may occur as individual cells, in groups or as sheets. Contractions occur either in slow, spontaneous rhythms or as sustained contractions for longer durations.
The involuntary muscle is one of three types of muscles found. The other two types are skeletal and cardiac. Skeletal muscle is a type of voluntary muscle, meaning it is controlled by the organism's will. The skeletal muscle is also referred to as a striated muscle because of the alternating muscle fibers that are light and dark colored. Present in these fibers are specialized protein filaments that are responsible for the contraction of the muscle.
Skeletal muscles are attached to bones of the body through tendons. When skeletal muscles are discussed in combination with the bones of the body, the combined system is called the musculoskeletal system. Over 600 muscles are present in the entire human body.