Peripheral nervous system functions, parts and diseases

The peripheral nervous system (SNP) is the division of the nervous system that is represented by all the nerves that are not part of the brain or the spinal cord, that is, they do not belong to the central nervous system.

Our nervous system, as well as that of most vertebrate animals, is divided into two: the central nervous system and the peripheral nervous system, both closely related to each other..

The central nervous system is the division that includes the brain and spinal cord, which represent the center of operations of the human body. Meanwhile, the peripheral nervous system represents all the nerves that connect the organs with the central nervous system..

In terms of its function, the peripheral nervous system connects the central nervous system to the skin, organs and extremities. It is the means by which the brain and spinal cord receive and send information to other parts of the body.

Article index

• 1 Functions of the peripheral nervous system
  • 1.1 The somatic nervous system
  • 1.2 The autonomic nervous system
• 2 Parts of the peripheral nervous system
  • 2.1 Nerves
  • 2.2 Lymph nodes
  • 2.3 Nerve plexuses
• 3 Diseases of the PNS
• 4 References

Functions of the peripheral nervous system

The functions of the peripheral nervous system are carried out by each of its three divisions:

The somatic nervous system

It is responsible for sending sensory information from the body to the brain, using afferent fibers. For example, when touching something cold, the stimulus reaches the central nervous system, processing the information and giving the sensation of cold. 

The somatic nervous system is also responsible for transmitting messages from the brain to the muscles, using efferent fibers. We can say that this system is the one by which we control our voluntary movements, such as walking, running, smiling, talking, among others..

This system has two main types of neurons:

• The motor neurons, which inform skeletal muscles when to contract or relax to perform voluntary movements.
• The sensory neurons, that are responsible for transmitting the information of what we see, hear, smell, touch and taste to the brain, to "elaborate" the sensations. In addition, they are responsible for informing the central nervous system of the homeostatic state of the other organs..

The autonomic nervous system

The word "autonomous" or "autonomic" is used to refer to the fact that this part of the system "governs itself", since we do not have control over the functions of our body directed by this part of the peripheral nervous system..

The autonomic nervous system is in charge of all the "automatic" processes of our body, that is, of those that we do 24 hours a day, which are vital, but of which we do not have conscious control, such as:

• The breathing.
• Maintaining the heart rate.
• The digestion.
• Maintain internal temperature.
• Maintain homeostasis of internal organs.

The autonomic nervous system is further subdivided into the sympathetic and parasympathetic nervous systems:

• The sympathetic nervous system it is the one that helps us prepare for emergency situations such as lack of water, physical damage or extreme temperatures.
• The parasympathetic nervous system It is the one that is responsible for maintaining body functions with a minimum expenditure of energy.

Parts of the peripheral nervous system

The peripheral nervous system is basically made up of nerves, ganglia and nerve plexuses that are not part of the central nervous system (brain and spinal cord):

Nerves

Nerves are bundles of nerve fibers and these in turn are clusters or groups of axons of many neurons. We could say that the nerves represent the "electrical wiring" of our body, which connects our organs with the power source, located in the central nervous system..

The "wires" or nerves in the peripheral nervous system that carry messages to and from the brain can be:

• The cranial nerves, that connect the brain directly to structures in the head such as the eyes, ears, and face. We have 12 cranial nerves that connect our brain with the organs and muscles of our head, that is, the eyes, ears, nose, mouth, etc..

• The spinal nerves, that emerge from the spinal cord and connect with the rest of the organs. In our body there are 31 pairs of these, each one connected to a region of the body; are groups of sensory and motor fibers that derive from the spinal cord.

• The autonomic nerves, They include motor nerves that are connected to the smooth muscles of the blood vessels and the digestive system, as well as to the heart muscle. As their name suggests, they are part of the autonomous division, that is, of which we are not aware.

These nerves are very important transporters or conductors, which can carry sensory or motor information.

Ganglia

Ganglia are groups of neuronal cell bodies that form a kind of spherical-looking organs, which are covered or contained by a capsule..

Nerve plexuses

In addition to nerves, the peripheral nervous system is made up of nerve plexuses, which are networks of spinal nerves. There are 4 and they are known as the cervical plexus, the branchial plexus, the lumbar plexus and the sacral plexus..

Nerve plexuses are networks of nerve fibers that are made up of different interlocking nerve branches that usually derive from the spinal cord..

PNS diseases

The peripheral nervous system is more susceptible to different types of conditions, since it is not protected by bones like the central nervous system is, which is contained in the bony cavities that we call the "skull" and "spinal column.".

Many of these peripheral nervous system conditions are known as peripheral neuropathies, and they are the result of damage to the peripheral nerves..

Symptoms vary greatly depending on the type of peripheral nerve that is affected, but are generally characterized by numbness, severe pain, extreme sensitivity, weakness in the extremities and other areas of the body..

They can also present as lack of coordination and paralysis (somatic nervous system), digestive problems, bladder and heart pressure (when it affects the peripheral autonomic nervous system).

The main causes of peripheral neuropathies are:

• Trauma
• Infections
• Metabolic problems
• Hereditary conditions or disorders
• Exposure to toxins
• Autoimmune diseases
• Tumors
• Diabetes
• Bone marrow conditions

References

1. Fox, S. I. (2002). Human physiology. McGraw-Hill.
2. Ganong, W. F. (1995). Review of medical physiology. Mcgraw-hill.
3. Lentz, T., & Erulkar, S. (n.d.). Encyclopaedia Britannica Kids. Retrieved July 25, 2020, taken from kids.britannica.com
4. Mayo Clinic Staff. (2019). Mayo Clinic. Retrieved July 22, 2020, taken from mayoclinic.org
5. Pleasure DE. Examples of Peripheral Nervous System-Specific Diseases. In: Siegel GJ, Agranoff BW, Albers RW, et al., Editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999. Taken from ncbi.nlm.nih.gov
6. Society for Neuroscience. (2002). Brain facts: A primer on the brain and nervous system. Society for Neuroscience.