Created by CK-12 Foundation/Adapted by Christine Miller

One Piano, Four Hands

Did you ever see two people play the same piano? How do they coordinate all the movements of their own fingers — let alone synchronize them with those of their partner? The peripheral nervous system plays an important part in this challenge.

The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside of the central nervous system (CNS). The main function of the PNS is to connect the CNS to the rest of the organism. It serves as a communication relay, going back and forth between the CNS and muscles, organs, and glands throughout the body.

The PNS is mostly made up of cable-like bundles of axons called nerves, as well as clusters of neuronal cell bodies called ganglia (singular, ganglion). Nerves are generally classified as sensory, motor, or mixed nerves based on the direction in which they carry nerve impulses.

• Sensory nerves transmit information from sensory receptors in the body to the CNS. Sensory nerves are also called afferent nerves. You can see an example in the figure below.
• Motor nerves transmit information from the CNS to muscles, organs, and glands. Motor nerves are also called efferent nerves. You can see one in the figure below.
• Mixed nerves contain both sensory and motor neurons, so they can transmit information in both directions. They have both afferent and efferent functions.

The PNS is divided into two major systems, called the autonomic nervous system and the somatic nervous system. In the diagram below, the autonomic system is shown on the left, and the somatic system on the right. Both systems of the PNS interact with the CNS and include sensory and motor neurons, but they use different circuits of nerves and ganglia.

Somatic Nervous System

The somatic nervous system primarily senses the external environment and controls voluntary activities about which decisions and commands come from the cerebral cortex of the brain. When you feel too warm, for example, you decide to turn on the air conditioner. As you walk across the room to the thermostat, you are using your somatic nervous system. In general, the somatic nervous system is responsible for all of your conscious perceptions of the outside world, as well as all of the voluntary motor activities you perform in response. Whether it’s playing a piano, driving a car, or playing basketball, you can thank your somatic nervous system for making it possible.

Somatic sensory and motor information is transmitted through 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves are in the head and neck and connect directly to the brain. Sensory components of cranial nerves transmit information about smells, tastes, light, sounds, and body position. Motor components of cranial nerves control skeletal muscles of the face, tongue, eyeballs, throat, head, and shoulders. Motor components of cranial nerves also control the salivary glands and swallowing. Four of the 12 cranial nerves participate in both sensory and motor functions as mixed nerves, having both sensory and motor neurons.

Spinal nerves emanate from the spinal column between vertebrae. All of the spinal nerves are mixed nerves, containing both sensory and motor neurons. The areas of skin innervated by the 31 pairs of spinal nerves are shown in the figure below. These include sensory nerves in the skin that sense pressure, temperature, vibrations, and pain. Other sensory nerves are in the muscles, and they sense stretching and tension. Spinal nerves also include motor nerves that stimulate skeletal muscles to contract, allowing for voluntary body movements.

Autonomic Nervous System

The autonomic nervous system primarily senses the internal environment and controls involuntary activities. It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. In general, the autonomic nervous system is responsible for all the activities that go on inside your body without your conscious awareness or voluntary participation.

Structurally, the autonomic nervous system consists of sensory and motor nerves that run between the CNS (especially the hypothalamus in the brain), internal organs (such as the heart, lungs, and digestive organs), and glands (such as the pancreas and sweat glands). Sensory neurons in the autonomic system detect internal body conditions and send messages to the brain. Motor nerves in the autonomic system affect appropriate responses by controlling contractions of smooth or cardiac muscle, or glandular tissue. For example, when sensory nerves of the autonomic system detect a rise in body temperature, motor nerves signal smooth muscles in blood vessels near the body surface to undergo vasodilation, and the sweat glands in the skin to secrete more sweat to cool the body.

The autonomic nervous system, in turn, has three subdivisions: the sympathetic division, parasympathetic division, and enteric division. The first two subdivisions of the autonomic system are summarized in the figure below. Both affect the same organs and glands, but they generally do so in opposite ways.

• The sympathetic division controls the fight-or-flight response. Changes occur in organs and glands throughout the body that prepare the body to fight or flee in response to a perceived danger. For example, the heart rate speeds up, air passages in the lungs become wider, more blood flows to the skeletal muscles, and the digestive system temporarily shuts down.
• The parasympathetic division returns the body to normal after the fight-or-flight response has occurred. For example, it slows down the heart rate, narrows air passages in the lungs, reduces blood flow to the skeletal muscles, and stimulates the digestive system to start working again. The parasympathetic division also maintains internal homeostasis of the body at other times.
• The enteric division is made up of nerve fibres that supply the organs of the digestive system. This division allows for the local control of many digestive functions.

Disorders of the Peripheral Nervous System

Unlike the CNS — which is protected by bones, meninges, and cerebrospinal fluid — the PNS has no such protections. The PNS also has no blood-brain barrier to protect it from toxins and pathogens in the blood. Therefore, the PNS is more subject to injury and disease than is the CNS. Causes of nerve injury include diabetes, infectious diseases (such as shingles), and poisoning by toxins (such as heavy metals). PNS disorders often have symptoms like loss of feeling, tingling, burning sensations, or muscle weakness. If a traumatic injury results in a nerve being transected (cut all the way through), it may regenerate, but this is a very slow process and may take many months.

Two other diseases of the PNS are Guillain-Barre syndrome and Charcot-Marie-Tooth disease.

• Guillain-Barre syndrome is a rare disease in which the immune system attacks nerves of the PNS, leading to muscle weakness and even paralysis. The exact cause of Guillain-Barre syndrome is unknown, but it often occurs after a viral or bacterial infection. There is no known cure for the syndrome, but most people eventually make a full recovery. Recovery can be slow, however, lasting anywhere from several weeks to several years.
• Charcot-Marie-Tooth disease is a hereditary disorder of the nerves, and one of the most common inherited neurological disorders. It affects predominantly the nerves in the feet and legs, and often in the hands and arms, as well. The disease is characterized by loss of muscle tissue and sense of touch. It is presently incurable.

The autonomic nervous system is considered to be involuntary because it doesn’t require conscious input. However, it is possible to exert some voluntary control over it. People who practice yoga or other so-called mind-body techniques, for example, can reduce their heart rate and certain other autonomic functions. Slowing down these otherwise involuntary responses is a good way to relieve stress and reduce the wear-and-tear that stress can place on the body. Such techniques may also be useful for controlling post-traumatic stress disorder and chronic pain. Three types of integrative practices for these purposes are breathing exercises, body-based tension modulation exercises, and mindfulness techniques.

Breathing exercises can help control the rapid, shallow breathing that often occurs when you are anxious or under stress. These exercises can be learned quickly, and they provide immediate feelings of relief. Specific breathing exercises include paced breath, diaphragmatic breathing, and Breathe2Relax or Chill Zone on MindShift™ CBT, which are downloadable breathing practice mobile applications, or “Apps”. Try syncing your breathing with Eric Klassen’s “Triangle breathing, 1 minute” video:

Triangle breathing, 1 minute, Erin Klassen, 2015.

Body-based tension modulation exercises include yoga postures (also known as “asanas”) and tension manipulation exercises. The latter include the Trauma/Tension Release Exercise (TRE) and the Trauma Resiliency Model (TRM). Watch this video for a brief — but informative — introduction to the TRE program:

TRE® : Tension and Trauma Releasing Exercises, an Introduction with Jessica Schaffer, Jessica Schaffer Nervous System RESET, 2015.

Mindfulness techniques have been shown to reduce symptoms of depression, as well as those of anxiety and stress. They have also been shown to be useful for pain management and performance enhancement. Specific mindfulness programs include Mindfulness Based Stress Reduction (MBSR) and Mindfulness Mind-Fitness Training (MMFT). You can learn more about MBSR by watching the video below.

Mindfulness-Based Stress Reduction (UMass Medical School, Center for Mindfulness), Palouse Mindfulness, 2017.

Attributions

Figure 8.6.1

Kid’s piant duet by PJMixer on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.

Figure 8.6.2

Nervous_system_diagram by ¤~Persian Poet Gal  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.6.3

Afferent_and_efferent_neurons_en.svg by Helixitta on Wikimedia Commons is used under a CC BY-SA 4.0   (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 8.6.4

Autonomic and Somatic Nervous System by Christinelmiller on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 8.6.5

Dermatoms.svg by Ralf Stephan (mailto:ralf@ark.in-berlin.de) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 8.6.6

The_Autonomic_Nervous_System by Geo-Science-International on Wikimedia Commons is used and adapted by Christine Miller under a CC0 1.0 Universal
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

References

Erin Klassen. (2015, December 15). Triangle breathing, 1 minute. YouTube. https://www.youtube.com/watch?v=u9Q8D6n-3qw&feature=youtu.be

Jessica Schaffer Nervous System RESET. (2015, January 15). TRE® : Tension and trauma releasing exercises, an Introduction with Jessica Schaffer. YouTube. https://www.youtube.com/watch?v=67R974D8swM&feature=youtu.be

Mayo Clinic Staff. (n.d.). Charcot-Marie-Tooth disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/charcot-marie-tooth-disease/symptoms-causes/syc-20350517

Mayo Clinic Staff. (n.d.). Diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/diabetes/symptoms-causes/syc-20371444

Mayo Clinic Staff. (n.d.). Guillain-Barre syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/guillain-barre-syndrome/symptoms-causes/syc-20362793

Mayo Clinic Staff. (n.d.). Shingles [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/shingles/symptoms-causes/syc-20353054

Mayo Clinic Staff. (n.d.). Stroke [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/stroke/symptoms-causes/syc-20350113

Palouse Mindfulness. (2017, March 25).  Mindfulness-based stress reduction (UMass Medical School, Center for Mindfulness), YouTube. https://www.youtube.com/watch?v=0TA7P-iCCcY&feature=youtu.be

Plethrons, (2015, March 23). Phantom limbs explained. YouTube. https://www.youtube.com/watch?v=ySIDMU2cy0Y&feature=youtu.be

Reactions. (2015, December 1). Why do hot peppers cause pain? YouTube. https://www.youtube.com/watch?v=73yo5nJne6c&feature=youtu.be