Introduction to the somatic and autonomic nervous systems

Whether you’re playing volleyball with friends or just watching TV, your nervous system is always operating, making sure that the right organs function at the right time. The nervous system is structurally divided into two regions, called the central nervous system, or CNS, and the peripheral nervous system, or PNS. The peripheral nervous system can be further subdivided functionally into the somatic nervous system, and the autonomic nervous system.

Okay, let’s start with the somatic nervous system, which describes a set of nerve fibers that control voluntary actions and convey sensory information from the skin, skeletal muscles and joints. Somatic sensory fibers allow transmission of sensory information such as touch, pain, temperature, and proprioception. For example, somatic sensory fibers inform our CNS that our cup of coffee is too hot. Then there are somatic motor fibers, which only innervate skeletal muscle and control voluntary actions of the body, like putting the coffee cup back on the table until it cools down.

On the flip side, the autonomic nervous system controls the involuntary activities within the body. This system consists of visceral motor fibers that carry motor signals to smooth muscle, such as that found in the intestinal walls that allow for peristalsis to occur, as well as cardiac muscle, and glandular tissue.

We also have visceral sensory fibers, which are not typically defined as part of the autonomic nervous system, but they act in conjunction with the visceral motor fibers of the autonomic nervous system to control visceral function. Visceral sensory fibers travel with the visceral motor fibers carrying sensory information from the viscera back to the CNS, where visceral motor fibers will act in response to this sensory information. For example, they provide information about things such as the amount of oxygen in your blood, your arterial blood pressure, and the level of distention of your stomach after that big meal! This visceral sensory information is continuously regulating the activity of the visceral motor neurons of the autonomic nervous system - even while you sit here and watch this osmosis video!