How the Body Works : Control of Involuntary Muscle

Cardiac muscle and smooth muscle function involuntarily-beyond our conscious control. There is smooth muscle in organs of the digestive, respiratory, circulatory and urinogenital systems, while cardiac muscle is found exclusively in the heart. These muscles are controlled by the autonomic nervous system, which is composed of two opposing divisions, the sympathetic, shown in red, and the parasympathetic, shown in blue. Sympathetic nerves leaving the spinal cord lead first to nearby nerve chains before connecting the body organs, where they stimulate muscle activity. In the parasympathetic division, nerves pass from the spinal cord directly to the organs and have the opposite effect-decreasing muscle activity. This dual mechanism provides a method for maintaining the activity of the muscles within controlled limits. The sympathetic system is dominant in situations requiring rapid action. When, for example, danger threatens there is an increase in heart rate, respiration rate, blood pressure and sweat gland activity. The parasympathetic system is dominant while the body is resting-usually during sleep, when the heart rate is slower and respiration is deeper and more regular.