Peripheral Nervous System- Definition, Parts, Functions & Disorders

Peripheral Nervous System: Definition, Parts, Functions, Disorders

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:46 PM IST

The Peripheral Nervous System (PNS) connects the central nervous system to the rest of the body, facilitating communication between the brain, spinal cord, and peripheral organs. It consists of nerves and ganglia that control voluntary and involuntary actions. In this article, the definition of the peripheral nervous system (PNS), components of the peripheral nervous system, divisions of the peripheral nervous system, function of the peripheral nervous system, disorders of the peripheral nervous system and diagnostic methods for PNS disorders are discussed. The peripheral Nervous System is a topic of the chapter Neural Control And Coordination in Biology.

This Story also Contains

Definition of the Peripheral Nervous System (PNS)
Components of the Peripheral Nervous System
Divisions of the Peripheral Nervous System
Function of the Peripheral Nervous System
Disorders of the Peripheral Nervous System
Diagnostic Methods for PNS Disorders

Peripheral Nervous System: Definition, Parts, Functions, Disorders

Definition of the Peripheral Nervous System (PNS)

The Peripheral Nervous System is the nervous system that exists outside the brain and spinal cord. It acts as the middle man between the Central Nervous System and the limbs and organs, transmitting impulses necessary for action, voluntary and involuntary, therefore, sustaining the overall functioning of the body.

Components of the Peripheral Nervous System

It comprises nerves, ganglia, and their protective and supporting connective tissue coverings respectively.

Nerves

Groups of axons are wrapped by connective tissue that transmit signals between the CNS and the periphery.

Types Of Nerves

Sensory Nerves: Carry sensory information from receptors to the CNS.
Motor Nerves: Transmit motor commands from the CNS to muscles and glands.
Mixed Nerves: Contain both sensory and motor fibres, thus capable of transmitting signals in both directions.

Ganglia

Sensory (Dorsal Root Ganglia): Contains cell bodies of sensory neurons.
Autonomic Ganglia: Contains cell bodies of neurons in the autonomic nervous system.

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Connective Tissue Coverings

Endoneurium: Surrounds individual nerve fibres (axons).
Perineurium: Encloses bundles of nerve fibres (fascicles).
Epineurium: Encases the entire nerve, providing protection and support.

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Divisions of the Peripheral Nervous System

The PNS is divided into the somatic and autonomic nervous systems, each with distinct functions.

Somatic Nervous System:

Spinal Nerves: 31 pairs that emerge from the spinal cord.
Cranial Nerves: 12 pairs that emerge directly from the brain.
Role in Voluntary Movements: Controls skeletal muscle contractions and mediates conscious movements.

Autonomic Nervous System:

Sympathetic Nervous System: Prepares the body for 'fight or flight' responses.
Parasympathetic Nervous System: Promotes 'rest and digest' activities.
Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

Function of the Peripheral Nervous System

The PNS is essential for reflex actions and the transmission of nerve impulses.

Reflex Arc

Examples of Reflex actions are knee-jerk reflex and withdrawal reflex.

Transmission of Nerve Impulses

Involves the movement of sodium and potassium ions across the neuron membrane, generating an electrical impulse.
Neurotransmitters are released from the axon terminal of one neuron and bind to receptors on the dendrite of another neuron, facilitating signal transmission.

Disorders of the Peripheral Nervous System

Several conditions can affect the PNS, leading to various symptoms and requiring different treatments.

Peripheral Neuropathy

Caused by diabetes, infections, and injuries; symptoms include numbness, tingling, and pain in the affected area.
Treatment Includes medications, physical therapy, and addressing underlying causes.

Guillain-Barré Syndrome

An autoimmune disorder is often triggered by an infection, leading to muscle weakness and paralysis.
Treatment includes Plasmapheresis, immunoglobulin therapy, and supportive care.

Carpal Tunnel Syndrome

Compression of the median nerve, causing pain and numbness in the hand.

Bell’s Palsy

Sudden weakness or paralysis of facial muscles, usually temporary.

Diagnostic Methods for PNS Disorders

Accurate diagnosis is crucial for effective treatment of PNS disorders.

Electromyography (EMG)
Nerve Conduction Studies (NCS)
MRI and CT Scans

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Neuron	Nerve Impulse
Diagram of Neuron	Nerve Fibres
Neuron and Nerves	Action Potential

Frequently Asked Questions (FAQs)

1. What is the Peripheral Nervous System?

The PNS is that part of the nervous system which includes nerves and ganglia outside the brain and spinal cord.

2. What are the functions of the Peripheral Nervous System?

The PNS connects the central nervous system with limbs and organs, allowing sensory and motor functions.

3. How is the Somatic Nervous System different from the Autonomic Nervous System?

The somatic nervous system controls, which are voluntary movements, while the autonomic nervous system is in control of involuntary functions.

4. What is peripheral neuropathy and how is it treated?

It is a secondary effect of diabetes, infections and injury; its management involves medications, physical therapy and the treatment of underlying illnesses.

5. What is generally the base of studies of nerve conduction?

Nerve conduction studies quantify the rate of travel and the amplitude of electrical impulses in nerves. The tests are done for diagnostic purposes within the peripheral nervous system.

6. How does the structure of a peripheral nerve differ from a neuron in the CNS?

Peripheral nerves are bundles of axons wrapped in connective tissue, while neurons in the CNS are individual cells. Peripheral nerves have three layers of protective tissue: endoneurium, perineurium, and epineurium. CNS neurons are protected by glial cells and do not have these layers. Additionally, peripheral nerves can regenerate to some extent, while CNS neurons generally cannot.

7. What is the difference between myelinated and unmyelinated nerve fibers in the PNS?

Myelinated nerve fibers are wrapped in a fatty insulating layer called myelin, formed by Schwann cells. This myelin sheath allows for faster signal transmission through saltatory conduction. Unmyelinated fibers lack this insulation and conduct signals more slowly. Generally, myelinated fibers are responsible for rapid, precise signals (like motor control), while unmyelinated fibers often carry slower signals (like some types of pain or temperature sensation).

8. What is a ganglion and what role does it play in the peripheral nervous system?

A ganglion is a cluster of neuron cell bodies located outside the CNS. In the PNS, ganglia act as relay stations for nerve signals. Sensory ganglia contain cell bodies of sensory neurons, while autonomic ganglia contain cell bodies of motor neurons that control involuntary functions. Ganglia help to integrate and process information before it reaches the CNS or effector organs.

9. How do neuromuscular junctions work in the peripheral nervous system?

Neuromuscular junctions are specialized synapses where motor neurons in the PNS connect to skeletal muscle fibers. When an action potential reaches the axon terminal of a motor neuron, it triggers the release of acetylcholine into the synaptic cleft. This neurotransmitter binds to receptors on the muscle fiber, causing ion channels to open. The resulting influx of ions leads to muscle fiber depolarization and ultimately, muscle contraction.

10. What is the difference between preganglionic and postganglionic neurons in the autonomic nervous system?

Preganglionic neurons have cell bodies in the CNS and synapse with postganglionic neurons in autonomic ganglia. They are typically shorter and always use acetylcholine as a neurotransmitter. Postganglionic neurons have cell bodies in the ganglia and extend to target organs. They are usually longer and use different neurotransmitters depending on the division (norepinephrine for sympathetic, acetylcholine for parasympathetic).

11. What is the peripheral nervous system (PNS) and how does it differ from the central nervous system (CNS)?

The peripheral nervous system (PNS) is the part of the nervous system that extends outside the brain and spinal cord. It consists of nerves that connect the CNS to the rest of the body. The main difference is that the CNS (brain and spinal cord) processes and integrates information, while the PNS transmits signals between the CNS and the body's organs, muscles, and sensory receptors.

12. What is axonal transport and why is it important in the peripheral nervous system?

Axonal transport is the movement of cellular components along the axon of a neuron. It occurs in both directions: anterograde (from cell body to axon terminal) and retrograde (from axon terminal to cell body). This process is crucial in the PNS for delivering proteins, lipids, and organelles to the axon terminal, maintaining axon health, and transporting signaling molecules from the periphery back to the cell body. Disruptions in axonal transport can lead to various peripheral neuropathies.

13. How does the peripheral nervous system adapt to injury or damage?

The PNS has a remarkable ability to regenerate after injury, unlike the CNS. When a peripheral nerve is damaged, the distal portion degenerates in a process called Wallerian degeneration. Schwann cells then guide the regrowth of axons from the proximal end. This process is supported by growth factors and can result in functional recovery, although it may be incomplete depending on the severity and location of the injury.

14. What is peripheral neuropathy and what causes it?

Peripheral neuropathy is a condition characterized by damage to peripheral nerves, leading to symptoms like numbness, tingling, and weakness. It can be caused by various factors, including diabetes, vitamin deficiencies, autoimmune diseases, infections, and certain medications. The damage can affect sensory, motor, or autonomic nerves, resulting in a wide range of symptoms depending on which nerves are affected.

15. How does the peripheral nervous system contribute to pain perception?

The PNS contributes to pain perception through specialized sensory receptors called nociceptors. When tissue damage occurs, these receptors are activated and send signals through afferent neurons to the spinal cord and brain. The brain then interprets these signals as pain. The PNS also plays a role in chronic pain conditions through processes like peripheral sensitization, where nociceptors become more sensitive to stimuli.

16. How does the peripheral nervous system contribute to immune function?

The PNS interacts with the immune system in several ways. Sympathetic nerve fibers innervate lymphoid organs and can modulate immune cell function. Certain immune cells have receptors for neurotransmitters released by the PNS, allowing nervous system activity to influence immune responses. Additionally, sensory neurons can detect inflammatory signals and communicate this information to the CNS, which can then adjust autonomic output to modulate inflammation.

17. What is the difference between fast and slow pain pathways in the peripheral nervous system?

Fast pain pathways involve myelinated A-delta fibers that quickly transmit sharp, well-localized pain signals. Slow pain pathways use unmyelinated C fibers, which conduct signals more slowly and are associated with dull, burning, or aching pain that is less precisely localized. These different pathways allow the nervous system to distinguish between immediate, potentially dangerous stimuli and longer-lasting tissue damage.

18. What are the two main divisions of the peripheral nervous system?

The two main divisions of the peripheral nervous system are the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary movements and sensory information, while the autonomic nervous system regulates involuntary functions like heart rate, digestion, and breathing.

19. What is the difference between somatic and visceral sensory neurons in the PNS?

Somatic sensory neurons carry information from the skin, muscles, and joints, providing conscious sensations like touch, pressure, and pain. Visceral sensory neurons, on the other hand, transmit information from internal organs and are often associated with unconscious sensations or reflexes. Visceral sensory information is typically less precise and can be referred to other areas of the body, as in the case of referred pain.

20. What is the role of neurotransmitters in the peripheral nervous system?

Neurotransmitters are chemical messengers that transmit signals across synapses in the PNS. Different parts of the PNS use specific neurotransmitters. For example, acetylcholine is used at neuromuscular junctions and in the parasympathetic nervous system, while norepinephrine is the primary neurotransmitter in the sympathetic nervous system. These chemicals bind to receptors on target cells, initiating various responses depending on the neurotransmitter and receptor type.

21. How does the autonomic nervous system regulate heart rate?

The autonomic nervous system regulates heart rate through its two branches. The sympathetic nervous system increases heart rate by releasing norepinephrine, which binds to beta-adrenergic receptors on the heart. The parasympathetic nervous system decreases heart rate through the vagus nerve, which releases acetylcholine. These two systems work in balance to adjust heart rate based on the body's needs and environmental conditions.

22. How does the peripheral nervous system contribute to our sense of balance and spatial orientation?

The PNS contributes to balance and spatial orientation through the vestibular system, part of the inner ear. Specialized sensory receptors in the semicircular canals and otolith organs detect head movement and position relative to gravity. These receptors send signals through the vestibulocochlear nerve (cranial nerve VIII) to the brain, which integrates this information with visual and proprioceptive inputs to maintain balance and spatial awareness.

23. How does the autonomic nervous system maintain homeostasis?

The autonomic nervous system maintains homeostasis by regulating involuntary body functions through its two branches: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems. These systems work antagonistically to control functions like heart rate, blood pressure, digestion, and respiration, adjusting them in response to internal and external stimuli to maintain balance in the body.

24. What is the difference between afferent and efferent neurons in the PNS?

Afferent neurons (also called sensory neurons) carry signals from sensory receptors to the CNS, while efferent neurons (also called motor neurons) carry signals from the CNS to muscles or glands. Afferent neurons transmit information about the environment or body state, while efferent neurons initiate responses or actions.

25. How do reflexes work in the peripheral nervous system?

Reflexes are rapid, automatic responses to stimuli that involve the PNS and spinal cord, bypassing the brain. When a stimulus activates sensory receptors, afferent neurons carry the signal to the spinal cord. Interneurons in the spinal cord then relay the signal to efferent neurons, which trigger a muscle or gland response. This pathway, called a reflex arc, allows for quick reactions to potential dangers or important stimuli.

26. What is the enteric nervous system, and why is it sometimes called the "second brain"?

The enteric nervous system is a complex network of neurons in the walls of the digestive tract, often considered a third division of the autonomic nervous system. It's called the "second brain" because it can operate independently of the CNS, controlling digestive processes, local blood flow, and secretions. It contains about 100 million neurons and can process information and make decisions without input from the brain or spinal cord.

27. What is the role of Schwann cells in the peripheral nervous system?

Schwann cells are specialized glial cells in the PNS that form the myelin sheath around axons. They wrap around the axon in segments, leaving small gaps called nodes of Ranvier. This myelination increases the speed of nerve impulse transmission through saltatory conduction. Schwann cells also play a crucial role in nerve regeneration after injury.

28. How does the peripheral nervous system contribute to circadian rhythms?

While circadian rhythms are primarily controlled by the suprachiasmatic nucleus in the brain, the PNS plays a role in their regulation. The optic nerve, part of the PNS, carries light information from the retina to the suprachiasmatic nucleus, helping to synchronize internal rhythms with the external light-dark cycle. The PNS also helps implement circadian changes in various body functions, such as hormone release and body temperature fluctuations, through autonomic innervation of different organs.

29. How does the peripheral nervous system regulate blood pressure?

The PNS regulates blood pressure through the autonomic nervous system. The sympathetic nervous system can increase blood pressure by constricting blood vessels and increasing heart rate and contractility. The parasympathetic system can decrease blood pressure by slowing heart rate. Additionally, baroreceptors in blood vessels detect changes in blood pressure and send signals to the brain, which then adjusts autonomic output to maintain appropriate pressure.

30. What is the role of the peripheral nervous system in thermoregulation?

The PNS plays a crucial role in thermoregulation by detecting temperature changes and initiating responses. Thermoreceptors in the skin send temperature information to the brain via afferent neurons. The brain then sends signals through efferent autonomic neurons to adjust body temperature. This can involve changing blood flow to the skin, initiating sweating, or triggering shivering, all of which are controlled by the PNS.

31. How does the peripheral nervous system contribute to the fight-or-flight response?

The fight-or-flight response is primarily mediated by the sympathetic division of the autonomic nervous system. When triggered, it increases heart rate, dilates bronchioles, diverts blood flow to muscles, increases sweating, and dilates pupils. These effects are achieved through the release of norepinephrine from sympathetic nerve endings and epinephrine from the adrenal medulla, preparing the body for immediate action in response to perceived threats.

32. How does the peripheral nervous system contribute to proprioception?

Proprioception, the sense of body position and movement, relies heavily on the PNS. Specialized sensory receptors called proprioceptors are located in muscles, tendons, and joints. These receptors detect changes in muscle length, tension, and joint position. The information is then transmitted via afferent neurons to the CNS, allowing for awareness of body position and coordination of movements.

33. What is the role of the peripheral nervous system in digestion?

The PNS plays a crucial role in digestion through both the autonomic and enteric nervous systems. The parasympathetic system stimulates digestive processes, increasing gut motility and secretions. The sympathetic system generally inhibits digestion. The enteric nervous system, often considered part of the PNS, can independently control many aspects of digestion, including peristalsis, secretion, and local blood flow in the gut.

34. How does the peripheral nervous system contribute to the pupillary light reflex?

The pupillary light reflex involves both the PNS and CNS. When light enters the eye, photoreceptors stimulate sensory neurons that carry signals to the brain via the optic nerve. The brain then sends signals through parasympathetic fibers of the oculomotor nerve (cranial nerve III) to the circular muscles of the iris, causing pupil constriction. This reflex helps regulate the amount of light entering the eye.

35. How does the peripheral nervous system regulate breathing?

While breathing is primarily controlled by the respiratory center in the brainstem, the PNS plays a crucial role in its regulation. Sensory receptors in the lungs and blood vessels detect changes in oxygen and carbon dioxide levels, sending this information to the brain via afferent neurons. The brain then adjusts breathing rate and depth through efferent signals to the diaphragm and intercostal muscles. The vagus nerve, part of the parasympathetic system, can also influence breathing patterns.

36. What is the role of the peripheral nervous system in the stress response?

The PNS, particularly the sympathetic nervous system, is integral to the stress response. When stress is perceived, the sympathetic system activates the "fight-or-flight" response, releasing norepinephrine and stimulating the adrenal glands to release epinephrine. This leads to increased heart rate, blood pressure, and alertness. The PNS also carries sensory information about stressors to the brain, which can further modulate the stress response.

37. What is the difference between the sympathetic and parasympathetic nervous systems in terms of target organ effects?

The sympathetic and parasympathetic systems often have opposing effects on target organs. For example, the sympathetic system increases heart rate and blood pressure, while the parasympathetic system decreases them. The sympathetic system dilates pupils and bronchioles, while the parasympathetic system constricts them. In the digestive system, the sympathetic system generally inhibits digestion, while the parasympathetic system promotes it. These opposing actions allow for fine-tuned regulation of body functions.

38. How does the peripheral nervous system contribute to the sensation of taste?

Taste sensation involves specialized receptors called taste buds, which are innervated by cranial nerves VII, IX, and X of the PNS. When taste molecules bind to receptors on taste bud cells, they trigger the release of neurotransmitters that stimulate these sensory nerve fibers. The nerve fibers then carry taste information to the brain, where it is processed and interpreted as different flavors.

39. What is the role of the peripheral nervous system in wound healing?

The PNS contributes to wound healing in several ways. Sensory neurons detect tissue damage and initiate the inflammatory response. The sympathetic nervous system regulates blood flow to the wound site. Neuropeptides released by sensory and autonomic nerve endings can influence various aspects of wound healing, including inflammation, cell proliferation, and tissue remodeling. Additionally, proper innervation is crucial for the function of regenerated tissue.

40. How does the peripheral nervous system regulate blood glucose levels?

The PNS, particularly the autonomic nervous system, plays a role in regulating blood glucose levels. The sympathetic system can increase blood glucose by stimulating the liver to release glucose and inhibiting insulin secretion from the pancreas. The parasympathetic system, via the vagus nerve, can stimulate insulin secretion, lowering blood glucose. Additionally, sensory neurons in the pancreas and gut can detect glucose levels and send this information to the brain, which then adjusts autonomic output accordingly.