Peripheral Nervous System

The human body is a complex machine, intricately designed and precisely tuned to respond to a myriad of stimuli. At the heart of this intricate design lies the nervous system, a sophisticated network of nerves and cells that transmit signals between different parts of the body. The nervous system is broadly divided into two main components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). This article will focus on the latter, the PNS, which serves as a vital communication bridge between the CNS and the rest of the body. The PNS is like the body’s information highway, transmitting sensory information from the body to the CNS, and motor information from the CNS to our muscles and glands. Understanding the structure and function of the PNS is key to unravelling the mysteries of how we interact with and respond to the world around us.

Table of Contents

Classification of peripheral nervous system

Spinal Nerves: These nerves emerge from the spinal cord.

Cranial Nerves: These nerves emerge from the brain and brainstem.

Somatic Nervous System: This system controls skeletal muscle as well as external sensory organs. It can be further divided into:

Sensory Division: Transmits signals from senses to the CNS.
Motor Division: Transmits signals from the CNS to our muscles.

Autonomic Nervous System (ANS): This is an involuntary part in control of cardiac, smooth and glandular cells. It can be further divided into:

Sympathetic Division: Prepares the body for ‘fight or flight’ response.
Parasympathetic Division: Helps the body to ‘rest and digest’.

Enteric Nervous System: This system surrounds the gastrointestinal tract and can function independently.

Somatic nervous system

The Somatic Nervous System (SoNS), also known as the voluntary nervous system, is a part of the Peripheral Nervous System (PNS). It consists of neurons that are associated with skeletal or striated muscle fibers and influence voluntary movements of the body.

Structure: The SoNS contains both afferent nerves traveling towards the Central Nervous System (CNS) and efferent nerves responsible for sending signals from the CNS towards the rest of the body. It is composed of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Function: The primary function of the SoNS is to connect the CNS with organs and striated muscles in order to enable complex movements and behavior. It also mediates a subset of involuntary muscle responses called reflex arcs.

Reflex Arcs: A reflex arc results in an extremely quick muscle contraction in response to a stimulus, with minimal intervention from the brain. Examples include the immediate withdrawal of a hand on touching a hot stove or a quick change in posture when the foot is placed on a sharp stone.

Referred Pain: For most of your internal organs, your SoNS only helps indicate organ pain using “referred pain.” This is when you feel pain in a specific area, but the pain is actually coming from a problem nearby.

Sensory Division (Afferent)

The sensory division of the SoNS is responsible for carrying sensory information from the body to the CNS.

Function: It processes sensory information that arrives via external stimuli. It processes the senses of hearing, smell, taste, and touch. The sense of touch below your neck uses your somatic nervous system to reach your spinal cord, which then relays signals to your brain.
Pathways: Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nerves.

Motor Division (Efferent)

The motor division of the SoNS is responsible for transmitting signals from the CNS to our muscles, enabling voluntary muscle movements.

Function: The motor division plays a vital role in initiating and controlling the movements of your body. This system is responsible for nearly all voluntary muscle movements.
Neurons: The motor division contains large myelinated axons that release acetylcholine (ACh) at neuromuscular junctions.

Control of Other Organs: For most of your internal organs, your somatic nervous system only helps indicate organ pain using “referred pain.” This is when you feel pain in a specific area, but the pain is actually coming from a problem nearby. Another example of how your somatic nervous system can affect internal organs is controlling your breathing.

Autonomic nervous system

The ANS is a component of the peripheral nervous system that regulates involuntary physiological processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It is always active, even when you’re asleep, and it’s key to your continued survival.

Structure: The ANS contains two anatomically distinct divisions: the sympathetic and parasympathetic nervous systems. It also includes the enteric nervous system.
Regulation: The ANS is regulated by integrated reflexes through the brainstem to the spinal cord and organs. Autonomic functions include control of respiration, cardiac regulation (the cardiac control center), vasomotor activity (the vasomotor center), and certain reflex actions such as coughing, sneezing, swallowing and vomiting.

Sympathetic nervous system

Structure of the SNS

Origin: The origin of the SNS is found within the thoracic and lumbar segments of the spinal cord, also known as the thoracolumbar division (T1 to L2,3).

Components: The SNS pathway can be divided into three components:

Preganglionic Neurons: Neurons of the intermediolateral column of the spinal cord, found within the levels T1-T12 and L1-L3.
Sympathetic Ganglia: Includes the sympathetic trunk (paravertebral ganglia) and prevertebral (splanchnic) ganglia.
Postganglionic Neurons: The axons of the ganglionic neurons that leave the ganglia in the form of grey rami communicants which join the rami of the spinal nerves.

Functions of the SNS

Fight or Flight Responses: The SNS is essential for preparing the body for emergency response in endangering situations, also known as the “fight-or-flight” response. This includes an increase in heart rate, bronchial dilation, increase in cardiac output, and dilation of pupils. Blood circulation is preferentially targeted towards skeletal muscle, with a reduction in blood flow towards non-essential organs.
Regulating Body Temperature: The SNS regulates body temperature, both by mobilizing fat reserves to enhance heat production and by changing blood flow to the skin.
Homeostasis: The SNS can maintain homeostasis through actions like sweating to dissipate heat, or by altering cardiac output based on position and activity level.

Parasympathetic nervous system

Structure of the PNS

Origin: The PNS is also known as the craniosacral division of the Autonomic Nervous System (ANS), as its central nervous system components are located within the brain and the sacral portion of the spinal cord.
Components: The PNS consists of many pathways that connect its craniosacral components with the peripheral tissues1. Each parasympathetic pathway consists of two neurons, the presynaptic (preganglionic) and postsynaptic (postganglionic) neurons, which are connected by the axons of the presynaptic neurons.
Neurons: The presynaptic neurons of the PNS are located within the medulla oblongata and sacral spinal cord1. They give off long axons (presynaptic fibers) that leave the CNS and travel towards the postsynaptic neurons1. Once they reach them, the presynaptic fibers synapse with the bodies of the postsynaptic neurons.

Sympathetic nervous system vs parasympathetic nervous system

Aspect	Sympathetic Nervous System	Parasympathetic Nervous System
Origin	Thoracolumbar division (T1 to L2,3) of the spinal cord	Craniosacral division of the CNS (brain and sacral spinal cord)
Function	Prepares the body for ‘fight or flight’ response. Regulates body temperature. Maintains homeostasis	Often referred to as the ‘rest and digest’ system. Slows down heart rate, decreases blood pressure, and promotes digestion
Effect on Heart Rate	Increases heart rate	Decreases heart rate
Effect on Pupils	Dilates pupils	Constricts pupils
Effect on Digestion	Slows down digestion	Speeds up digestion
Effect on Respiration	Increases respiration rate	Decreases respiration rate
Effect on Blood Pressure	Increases blood pressure	Decreases blood pressure

Functions of the PNS

Rest and Digest: The PNS is often referred to as the ‘rest and digest’ system as it slows down heart rate, decreases blood pressure, and promotes digestion. It’s responsible for conserving energy, relaxing muscles, and enhancing processes like digestion, salivation, and urination.
Eyes: The PNS constricts your pupils to limit how much light enters your eyes. It also makes changes that can help improve your close-up vision, and causes tear production in your eyes.
Nose and Mouth: The PNS makes glands in your mouth produce saliva, and glands in your nose produce mucus. This can be helpful with digestion and breathing during times of rest.
Lungs: The PNS tightens airway muscles and ultimately reduces the amount of work your lungs do during times of rest.
Heart: The PNS lowers your heart rate and the pumping force of your heart.
Digestive Tract: The PNS increases your rate of digestion and diverts energy to help you digest food3. It also tells your pancreas to make and release insulin, helping your body break down sugars into a form your cells can use.
Waste Removal: The PNS relaxes the muscles that help you control when you pee (urinate) or poop (defecate).
Reproductive System: The PNS manages some of your body’s sexual functions, including feeling aroused (erections in people with a penis and secreting fluids that provide lubrication during sex in people with a vagina).

Enteric nervous system

Structure of the ENS

Location: The ENS is located in the gastrointestinal tract, extending from the esophagus to the rectum.
Components: The ENS consists of a large, mesh-like network of neurons that governs the function of the gastrointestinal tract. It contains somewhere between 200 and 600 million neurons.
Neurons: There are three different types of neurons in the ENS: efferent (motor), afferent (sensory), and interneurons.
Ganglia: The neurons of the ENS are grouped together in thousands of clusters called ganglia that are largely contained in two main networks: the myenteric plexus and submucosal plexus.

Functions of the ENS

Digestive Control: The ENS controls the digestive system, connecting through the central nervous system (CNS) and sympathetic nervous system. It controls things like muscle movements in your intestines, the release of digestive enzymes, and the absorption of nutrients.
Autonomous Functioning: The ENS is capable of operating independently of the brain and spinal cord, but it also can carry out some of its functions in the digestive process without communicating with the brain.
Neurotransmitters: The neurons of the ENS control the motor functions of the system, in addition to the secretion of gastrointestinal enzymes. These neurons communicate through many neurotransmitters similar to the CNS, including acetylcholine, dopamine, and serotonin.

Spinal nerves

Origin of Spinal Nerves

Roots: Each spinal nerve originates from two roots: an anterior (ventral) root and a posterior (dorsal) root.
Location: Spinal nerves exit the spinal column through intervertebral foramina, which are openings between adjacent vertebrae.
Components: Each spinal nerve is formed by the combination of nerve root fibers from its dorsal and ventral roots. The dorsal root is the afferent sensory root and carries sensory information to the brain. The ventral root is the efferent motor root and carries motor information from the brain.

Functions of Spinal Nerves

Mixed Nerves: Spinal nerves are mixed nerves that carry both motor and sensory information between the spinal cord and various parts of the body.
Motor Information: Spinal nerves transmit motor commands from the Central Nervous System (CNS) to the muscles of the periphery.
Sensory Information: Spinal nerves receive sensory information from the periphery and pass them to the CNS.
Autonomic Functions: Spinal nerves also carry autonomic signals that control involuntary functions such as heart rate, digestion, and glandular activity.

Cranial nerves

Origin of Cranial Nerves

Location: Cranial nerves originate from the brain and brainstem. They emerge from the central nervous system above the level of the first vertebra of the vertebral column.
Components: There are 12 pairs of cranial nerves which are part of the peripheral nervous system. Their names are an indication of some of their anatomical or functional features, and their numbers (Roman numerals) indicate the sequential order in which they emerge from the brain.
Types: Cranial nerves I and II are nerves of the cerebrum, nerves III to XII are nerves of the brainstem (XI partly emerging from spinal cord)1.

Functions of Cranial Nerves

Sensory, Motor, and Mixed Nerves: Of the 10 brainstem nerves, 1 (VIII) is a purely sensory nerve, 5 (III,IV,VI,XI and XII) are primarily motor nerves and 4 (V,VII,IX and X) are mixed nerves, i.e., containing both sensory and motor fibers.
Special Senses: The cranial nerves are generally concerned with the specialized (special) senses of smell, taste, vision, hearing, and balance, and with the general senses.
Motor Activities: They are also involved with the specialized motor activities of eye movement, chewing and swallowing, breathing, speaking, and facial expression.

Here are some examples of cranial nerves and their functions:

Cranial nerve I (Olfactory nerve): Perception of smell or olfaction.
Cranial nerve II (Optic nerve): Perception of vision, adjusts lens and constricts pupil helping in the reflex of focusing.
Cranial nerve III (Oculomotor nerve): Movements of eyeball, elevation of upper eyelid, constriction of pupil, accommodation of the lens (focusing by the lens).
Cranial nerve V (Trigeminal nerve): Sensory nerve of the face, oral and nasal regions, and motor nerve of the chewing muscles.
Cranial nerve VII (Facial nerve): Controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity.
Cranial nerve X (Vagus nerve): Supplies motor parasympathetic fibers to all the organs except the adrenal glands, from the neck down to the second segment of the transverse colon.

Conclusion

In conclusion, the Peripheral Nervous System (PNS) is a vast network of nerves that plays a crucial role in connecting the Central Nervous System (CNS) to the rest of the body. It is composed of spinal nerves, cranial nerves, and the autonomic nervous system, which further includes the sympathetic, parasympathetic, and enteric nervous systems. Each of these components has a unique structure and function, working in harmony to transmit sensory and motor information, regulate involuntary physiological processes, and maintain homeostasis. Understanding the intricacies of the PNS not only provides insights into how our bodies respond to various stimuli but also sheds light on the complexities of human physiology. As we continue to explore the depths of the PNS, we open doors to potential advancements in medical science, paving the way for improved diagnostic and therapeutic strategies.