The human nervous system is divided into two primary sections: the central nervous system and the peripheral nervous system. The central nervous system, or CNS, consists of the brain and spinal cord. The peripheral nervous system, or PNS, includes all the nerves that are not part of the CNS. The PNS can be further divided into the sensory division, which transmits sensory information; and the motor division, which sends impulses from the CNS to the muscles. The motor division can be further separated into the somatic nervous system and the autonomic nervous system.
General Function of the Somatic Nervous System
The somatic nervous system regulates the movements of muscles under conscious control, such as the skeletal muscles. Any voluntary muscle movement, therefore, is controlled by the somatic nervous system. Once you decide upon an action, your brain sends a signal to your somatic nervous system to perform the movement. Examples of voluntary muscle movement include simple activities like giving a thumbs-up or complex motions like running or swimming.
The Somatic Nervous System and Reflexes
Sometimes the somatic nervous system reacts to a stimulus so quickly that it appears to bypass conscious thought and voluntary movement. This reaction is called a reflex, and it usually occurs in response to perceived danger or pain. In a reflex motion, a sensory nerve sends a signal to your spinal cord, which in turn activates your motor neurons to deal with the threat. Your brain only gets involved after the threat has been avoided.
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General Function of the Autonomic Nervous System
The autonomic nervous system regulates activities that are not under conscious control. These involuntary functions include the regulation of blood flow, sweating, digestion and many others. The autonomic nervous system works closely with the somatic nervous system. For example, you use your somatic nervous system to engage in voluntary physical activity, like running. Once you start running, your autonomic nervous system kicks in to speed up your heart rate and breathing, increase blood flow to your muscles and fire up your sweat glands.
Sympathetic vs. Parasympathetic Nervous System
The autonomic nervous system can be further subdivided into the sympathetic and parasympathetic nervous systems. These opposing systems balance each other out. The sympathetic nervous system acts to stimulate the autonomic nervous system, such as by increasing your heart rate in response to a fright or physical activity. The parasympathetic nervous system acts to calm down your heart rate once the danger or activity has passed. Although it takes practice, the parasympathetic nervous system can be brought somewhat under conscious control through breathing exercises, such as those taught in yoga, in martial arts and to police and military.