Nervous System, Somatic

The somatic nervous system (SNS) is a division of the peripheral nervous system (PNS). The SNS controls voluntary activities, such as movement of skeletal muscles and includes both sensory and motor nerves. Sensory nerves convey nerve impulses from the sense organs to the central nervous system (CNS), whereas motor nerves convey nerve impulses from the CNS to skeletal muscles.

All nervous tissue, including that of the SNS, consists of two main cell types: neurons and glial cells. Neurons transmit nerve signals and are surrounded by glial cells, which provide mechanical and physical support as well as electrical insulation between neurons.

A neuron consists of a cell body, the soma, which contains the nucleus and surrounding cytoplasm; several short thread-like projections called dendrites; and one long filament, called the axon. The dendrites receive information from other nearby cells and transmit the signals to the soma, and the axon carries signals away from the neuron. Both axons and dendrites are surrounded by protective coatings called myelin sheaths. The average adult brain contains about 100 billion neurons. Neurons are also the longest cells of the body, a single axon can be several feet long. Two types of neurons are found in the SNS: sensory neurons, which typically have long dendrites and short axons and carry messages from sensory receptors to the CNS, and motor neurons, which have a long axon and short dendrites and transmit signals from the CNS to muscles or glands.

The nervous system of the human body is divided into the CNS, consisting of the spinal cord and brain, and the PNS, consisting of all the nerves that connect the CNS with organs, muscles, blood vessels, and glands. The PNS is subdivided into the SNS and ANS. The ANS is further divided by function into sympathetic and parasympathetic systems.

The SNS consists of sensory and motor nerve divisions. The sensory division, also called the afferent division, contains neurons that receive signals from the tendons, joints, skin, skeletal muscles, eyes, nose, ears, and tongue, and many other tissues and organs. These signals are conveyed to the cranial and spinal nerves. The motor division, also called the efferent division, contains pathways from the brainstem and spinal cord to the lower motor neurons of the cranial and spinal nerves. When these nerves are stimulated, they cause the skeletal muscles to contract. This is called voluntary contraction.

The nerves of the sensory-somatic system are:

All of the spinal nerves are mixed nerves containing both sensory and motor neurons. They consist of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. In spinal nerves, some nerves fibers are ascending, meaning that they carry messages to the brain, whereas others are descending, meaning that they carry messages from the brain.

Sensory input to the nervous system occurs through the senses of vision, taste, smell, touch, and hearing, also called the special senses. Additional input is provided by the somatic senses of pain, temperature, and pressure. This sensory input uses sensors, which are also called sensory receptors. The major sensory receptors are:

The overall role of the nervous system is to act as an internal communications system that allows the body to react to environmental changes and to perform all activities required to maintain life. The PNS is the message carrier between the CNS and the rest of the body, and it cannot function with an impaired SNS. Thus, the role of the SNS in human health is crucial.

The major function of the SNS is the voluntary control of the muscle system of the body and the processing of sensory information to the CNS. All conscious knowledge of the external world and all the motor activity performed by the body to respond to it operates through the SNS.

The SNS is the portion of the PNS where, together with motor neurons and the brain, an individual consciously decides to move a muscle and actually makes the muscle movement occur. This portion of the nervous system is considered “voluntary” in that individuals determine that they will enable whatever body movement needs to occur, such as getting out of bed, brushing their teeth, or sitting on the sofa. Although most of these types of movements occur in a fraction of a second and appear to be done without forethought, the SNS enables the necessary connections between the brain and spinal nerves, allowing skeletal muscles to perform these everyday movements.

The SNS then, is the portion of the nervous system that enables individuals to participate in exercise. This system is responsible for motor (efferent) fibers to communicate between the brain and the muscles that are used during a workout. Motor neurons between the brain and muscles work in concert to allow muscle contraction when performing an exercise routine. Motor neurons connect to multiple muscle fibers throughout the body.

The SNS is directly responsible for enabling the body's control of, and response to, exercise. Skeletal muscles need signals or “instruction” from motor neurons of the SNS to contract.

When exercising, the SNS “excites” a muscle or muscle group into action. As individuals incorporate fitness into their daily routines, their muscles increase in size, resulting in a body that is toned and strong. Resistance training is especially helpful in this regard; however, both aerobic and anaerobic exercise have overall health benefits associated with more energy, stamina, strength, and endurance.

As noted previously, the SNS is also responsible for control of all five senses, and as such, plays a major role in how individuals relate to food. All five senses come into play during eating. While sitting down to a meal, people visualize and smell their food, and interpret the sound and feel of food when chewing and tasting the meal. The SNS is inherently associated with sensory perception, that is, how one perceives food, and it shapes one's nutritional habits. Through the SNS the eyes, tongue, and even muscles of the jaw are part of the process involved with food intake, resulting in the sensation of enjoying the overall eating experience.

Monique Laberge, PhD
Revised by Laura Jean Cataldo, RN, EdD