Somatic Nervous System

Definition, Function and Examples

By Olivia Guy-Evans, published May 03, 2021

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The somatic nervous system (SNS) plays an important role in initiating and controlling nearly all voluntary movements of the body. The SNS is a branch of the peripheral nervous system, along with the autonomic system (ANS), although they function in different ways.

Whilst the ANS regulates automatic behaviors, such as breathing and heart rate, those which do not require conscious thought, the SNS regulates mostly conscious movements of the body.

The main function of the SNS is to transmit signals between the body’s muscles and the brain and the central nervous system (brain and spinal cord) to control voluntary movement and reflexes.

It can do this through processing the sensory information which arrives through external stimuli via the senses (through sight, taste, touch, smell, and hearing).

The role of the somatic nerves is to also react to the environment through somatosensory function and motor function. For instance, when humans or animals are experiencing coldness, they can move to a warmer place for survival.

Being able to control the internal environment is called homeostasis, which needs to be in balance for survival. The SNS and the ANS work together to regulate bodily function and provide reactions to external stimuli (Fukudo, 2012).

The SNS consists of neurons (nerve cells) which are located in either the brain stem or spinal cord. They are extremely long in length as they do not synapse until they reach their termination point at the skeletal muscle (Rea, 2014).

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Functions

The main function of the SNS is to control all voluntary movement. There are receptors in the skin, sense organs (eyes, mouth, nose, and ears), and skeletal muscles which are able to detect changes in the environment, such as temperature, light, or texture.

Once environmental changes have been detected, impulses are created within the sensory neurons, which then carry signals to the

spinal nerves within the spinal cord. These signals will then travel up the spinal cord to the brain.

The brain will then integrate this sensory information and will determine an appropriate response. This response is then transmitted back down the spinal cord, reaching motor neurons.

The impulses will then be carried through the motor neurons, out of the spinal cord and continue to the nerves of the skeletal muscles, causing them to contract if needed.

Essentially, there are two pathways involved in the SNS. The afferent pathway will carry sensory information from sensory organs to the CNS. Whereas the efferent pathway will carry motor information from the CNS to the muscles to regulate motor functions.

As well as controlling all voluntary muscular systems of the body, the SNS also processes reflex arcs. Reflex arcs are neural pathways which produce involuntary movements, typically in response to stimuli perceived as imminent danger.

This occurs when sensory neurons sense something within the environment and carry this signal directly to the spinal cord, but this is not transmitted to the brain.

The spinal cord will instead transmit signals through the motor neurons to the muscles in order to trigger a reflex movement. This way, the muscles move without any input from the brain, to generate a response that is so fast that it is completed almost automatically.

An example of a reflex arc being used would be when moving a hand away after touching a hot surface. Another instance is the ‘knee jerk’ reaction.

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Parts of the SNS

Parts of the somatic nervous system The SNS consists of two major types of neurons; sensory neurons and motor neurons. These neurons function to transmit signals throughout the body.

Neurons contain an axon, which is the longest part of the cell, enabling signals to be transmitted through them.

There are also dendrites at the neuron endings, which allows multiple signals to branch out in order to pass onto the next neuron.

Sensory neurons within the SNS, also known as afferent neurons, carry sensory information from the body to the central nervous system (CNS).

Sensory neurons send information to the central nervous system from internal organs (glands, muscle, and skin) or external stimuli from the senses, such as information about the texture of an object.

Motor neurons within the SNS, also known as efferent neurons, carry motor information from the CNS to muscle fibers throughout the body.

These types of neurons transmit signals from the spinal cord and brainstem to skeletal and smooth muscle to either directly or indirectly control muscle movements.

The contraction of muscle is caused by the activation of motor neurons that supply the skeletal muscle. The primary neurotransmitter (chemical messenger) which is transmitted through the neurons is acetylcholine.

Whilst acetylcholine has an inhibitory effect in the ANS, within the SNS it is excitatory. The electrical signals from the motor neurons are converted to chemical messages at the neuromuscular junction, which is where acetylcholine is released by the motor neuron (Cuevas, 2015).

Cranial nerves play a vital role in carrying information to and from the brain. These nerves allow sensory information to transmit from the organs of the brain (ears, eyes, nose, and mouth), as well as transmitting motor information from the brain to these organs.

For instance, when eating food, the brain will convey motor messages through the nerves to move the mouth in order to chew and swallow.

Ten types of the cranial nerves originate from the brain steam and primarily control the voluntary movement and structures of the head.

Similarly, spinal nerves send somatosensory information into the spinal cord, then they send motor information back out of the spinal cord. The spinal nerves carry signals from receptors around the body to the spinal cord.

These signals are then transmitted to the brain for processing. These nerves help to control the function and movement for the rest of the body below the head and neck (Akinrodoya & Lui, 2020).

Damage to the SNS

As the SNS is responsible for receiving sensory information and motor movements, symptoms associated with damage to this system are experiencing numbness, muscle weakness, and pain.

Diabetes is the most common cause of neuropathy in the PNS, but damage can also be the result of autoimmune diseases, infections, and trauma. Damage to the nerves through injury can affect the functions of the afferent and efferent pathways of the SNS.

A number of diseases involved in sensory and motor control arise from either an issue with the CNS, PNS, or the muscle itself. Due to the range of functions covered by the SNS, these diseases can either be localized to a specific region of the body, or it can be widespread and generalized.

Some conditions could be the result of issues with the axons of the neurons (axonal neuropathy) or could be the result of issues with the myelin sheath (demyelinating neuropathy) which is the protective layer of the neurons (Akinrodoya & Lui, 2020).

Motor neuron disease is the result of the death of neurons, making it a neurodegenerative disease. Eventually, the muscles of people with this disease will waste away and will result in a loss of function.

Multiple sclerosis is an autoimmune disease which causes the destruction of peripheral nerves, resulting in a variety of sensory and motor problems.

Whilst these diseases are not always preventable, there are some lifestyle changes which can aid in stopping the preventable symptoms of SNS damage/weakening.

This can include sticking to a healthy diet, in order to maintain a healthy weight, not smoking, avoiding alcohol, regular exercise, and correcting vitamin deficiencies can also help to limit the chance of developing issues.

References

Akinrodoye MA, Lui F. Neuroanatomy, somatic nervous system. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Updated April 2, 2020.

Cuevas, J. (2015). The Somatic Nervous System. Reference Module in Biomedical Sciences. Elsevier.

Dorland, W. A. N. (2011). Dorland's Illustrated Medical Dictionary E-Book. Elsevier Health Sciences.

Fukudo, S. (2012). Hypothalamic-pituitary-adrenal axis in gastrointestinal physiology. In Physiology of the gastrointestinal tract (pp. 791-816). Elsevier Inc..

Rea, P. (2014). Introduction to the nervous system. Clinical Anatomy of the Cranial Nerves; Rea, P., Ed.; Academic Press: Cambridge, MA, USA.