Peripheral Nervous System

Definition, Parts and Function

By Olivia Guy-Evans, published April 23, 2021

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The nervous system of the body is split into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and the spinal cord components. 

The PNS is all the nerves that branch out from the CNS components and extend to other parts of the body – to the sense organs, muscles, and glands. The PNS connects the CNS to the rest of the body. 

The primary function of the peripheral nervous system is to connect the brain and spinal cord to the rest of the body and the external environment. The peripheral nervous system transmits information to and from the CNS.

This is accomplished through nerves that carry information from sensory receptors in the eyes, ears, skin, nose and tongue, as well as stretch receptors and nociceptors in muscles, glands and other internal organs.

The sense organs are able to detect changes in the environment and relay information through the sensory nerves to the CNS. The brain can then send signals through the nerves to the muscles, resulting in the muscles to move in response. 

Therefore, there is always a stream of incoming and outgoing information between the PNS, CNS, and the body through the form of nerve impulses.

The main functions of the PNS are voluntary movements such as chewing food, walking, and facial expressions. The PNS also regulates autonomic functions such as breathing, heart rate, and digesting – the unconscious bodily behaviors. 

The PNS is thus especially important for humans to survive. Unlike the CNS which is protected by the skull and the vertebrae of the spine, the nerves, and cells of the PNS are not enclosed by bones. This makes the PNS more susceptible to damage by trauma.

Parts of the PNS

The PNS can be divided into two components: the somatic nervous system and the autonomic nervous system.

The somatic nervous system (SNS) and the autonomic nervous system (ANS) are both part of the peripheral nervous system. The SNS controls voluntary actions such as walking. The ANS is responsible for the control of vital functions such as heart beat and breathing. It is also involved in the acute stress response where it works with the endocrine system to prepare the body to fight or flight.

Somatic Nervous System

The somatic nervous system is associated with activities traditionally thought of as conscious or voluntary. 

 The somatic nervous system is key for carrying messages throughout the body, in order to initiate and control movement. This system processes sensory information from external stimuli (e.g. through hearing, sight, and touch) as well as motor information, which then carries signals to and from the CNS. 

This is so it can interpret sensory information and control voluntary movements.

The somatic nervous system is involved in the relay of sensory and motor information to and from the CNS; therefore, it consists of motor neurons and sensory neurons.

• Sensory neurons – also known as afferent neurons, they send information to the CNS from the internal organs (glands, muscle, and skin) or from external stimuli (e.g. through sound, heat, touch and light). For instance, when touching a rough surface, this information will be transmitted through the sensory neurons to the brain.
• Motor neurons – also known as efferent neurons, they transmit signals from the CNS to skeletal and muscle to either directly or indirectly control muscle movements. Motor neurons allow us to act in response to external stimuli. For instance, during tasting a food, the sensory neurons will send this information to the brain. The brain will then transmit signals through the motor neurons to encourage the mouth, jaw, and teeth to continue eating the food.

As well as regulating voluntary movements, the somatic nervous system is also responsible for reflexes. This is an involuntary muscle response being controlled by a reflex arc, which is a neural pathway.

 For instance, when touching a very hot surface, the sensory neuron activity will be skipped and instead the brain will send almost instantaneous motor signals to move the hand away from the surface quickly.

Autonomic Nervous System

The autonomic nervous system is responsible for coordinating involuntary behaviors such as heart rate, breathing, and digestion. This system allows these important functions to take place without conscious thought, so they work automatically.

The autonomic nervous system is further divided into two components:

The two systems have complementary functions, operating in tandem to maintain the body’s homeostasis (a state of equilibrium).

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Nerves of the PNS

The peripheral nervous system is made up of thick bundles of axons, called nerves, carrying messages back and forth between the CNS and the muscles, organs, and senses in the periphery of the body (i.e., everything outside the CNS).

The nerve cells (or neurons) are the information processing units of the brain that are responsible for sending, receiving, and transmitting signals throughout the body. The neurons are essentially the cells which make up the central nervous system and the peripheral nervous system. 

 The structure of neurons allows it to receive and carry messages effectively to other neurons and throughout the body. It contains a soma (cell body), which is the core of the neuron, responsible for ensuring that everything is functioning efficiently. 

The axon is a tail-like structure of the neuron which functions in carrying signals away from the soma to the end of the neuron, for the purpose of transmitting signals to other neurons. 

The dendrites of the neurons are tree-root shaped, located at the end of the neuron. Their function is to receive and pass on information through their synapses to other neurons nearby. 

 The nerves are essentially bundles of thousands of individual axons all wrapped in a protective membrane in the PNS. Messages are sent along each of the axons to the peripheral organs of the body and send back information to the CNS. 

Each axon within the nerve is an extension of an individual neuron. Within the PNS, there are some nerves that are attached to the spinal cord (spinal nerves) and others which are attached directly to the brain (cranial nerves).

Spinal nerves

Spinal nerves are relatively large nerves that serve all of the body below the neck, conveying sensory and motor information from the body and carrying messages to the muscles and glands. 

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 also transmit motor signals from the brain to the body’s muscles and glands, so the brain’s directions can be carried out quickly.

The PNS is made up of 31 pairs of spinal nerves, which radiate out from the spinal cord, operating different locations of the body:

• 8 cervical nerves, which serve the chest, head, neck, shoulders, arms, and hands (called C1-C8).
• 12 thoracic nerves, which serve the back, abdominal muscles, and intercostal muscles (called T1 – T12).
• 5 lumbar nerves, which serve the lower abdomen, thighs, and legs (called L1-L5).
• 5 sacral nerves, which serve the legs, feet, and genital areas (called S1-S5).
• 1 coccygeal pair of nerves (called Co1).

Each spinal nerve exits the spinal cord by traveling through openings at the right and left sides of the vertebrae of the spine. These nerves divide and subdivide to form a network connecting the spinal cord to every part of the body. 

The longest nerve in the human body, the sciatic nerve (from the lumbar region) networks off until it reaches the tips of the toes, measuring up to a meter or more in length.

Cranial nerves

The cranial nerves primarily coordinate the muscle and sense receptors of the head and neck. These nerves are different to the spinal nerves as they do not enter through the spinal cord to reach the brain, instead, they are directly linked to the brain. 

Cranial nerves allow sensory information to transmit from the organs of the brain (ears, eyes, nose, and mouth), as well as conveying motor information from the brain to these organs. 

For instance, when eating food, the brain will transmit motor messages through the nerves to move the mouth in order to chew and swallow. When tasting the food, the sensory information gets sent back to the brain through a set of cranial nerves.

There are 12 pairs of cranial nerves attached to the brain:

• Olfactory nerves, which are sensory nerves, related to the sense of smell.
• Optic nerves, which are sensory nerves, related to the sense of sight.
• Oculomotor, trochlear, and abducens nerves, which are motor nerves, responsible for regulating voluntary eye movements.
• Vestibulocochlear nerves, which are sensory nerves, related to the sense of hearing, linked with sound, orientation, and balance.
• Glossopharyngeal and hypoglossal nerves, which are both sensory and motor nerves, responsible for tongue muscle movements and sense of taste.
• Vagus nerves, which are both sensory and motor nerves, responsible movements of the lower head, throat, neck, chest, and abdomen, as well as autonomic functions such as breathing and heartrate.
• Spinal accessory nerves, which are both sensory and motor nerves, responsible for the muscle and movements of the head, neck, and shoulders.
• Facial nerves, which are both sensory and motor nerves, related to the taste buds and movements of the face (facial expressions).
• Trigeminal nerves, which are both sensory and motor nerves, which carry signals from the eyes, teeth, and face, as well as carrying impulses from the lower jaw and muscles involved with chewing.

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