According to Biology-Online, involuntary muscles are muscles which contract without conscious control, and typically found inside internal organs, excluding the heart. The heart, as noted by TeachPE.com, contains cardiac muscle. Cardiac muscle is excluded from the definition of involuntary muscle because it is highly specialized and resembles skeletal muscles, the voluntary muscles which attach to bone, more than it does the involuntary muscles found within other major organs.
TeachPE.com adds that involuntary muscle found in other organs is sometimes called "smooth muscle." Involuntary muscles are found in the stomach, esophagus and lungs, as well as in the walls of blood vessels. While both cardiac and skeletal muscle contain pronounced stripes, involuntary muscle does not. Uic.edu states that involuntary muscle contracts much more slowly than does skeletal muscle. University of Illinois at Chicago says smooth muscle is classified based on the make-up and alignment of the fibers within the muscle; in single-unit smooth muscle, fibers run parallel to each other and contain connections between one other, while in multi-unit smooth muscle, there are no connections.
Smooth muscle is controlled by the autonomic nervous system in contrast to skeletal muscles, which receive signals from the somatic nervous system. As Medline Plus explains, the autonomic nervous system is specialized to handle involuntary actions, including breathing, swallowing, blood pressure and heartbeat.