Peripheral Nervous System: Definition, Parts, Functions, Disorders

What Is 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 neurons are:

• 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.

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.

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 patellar 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

Peripheral Nervous System NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• PNS and its types (SNS & ANS)

• Disorders of PNS

Practice Questions for NEET

Q1. Which among the following is not part of PNS ?

1. Cranial nerves

2. Brain

3. Spinal nerves

4. Both 1 and 2

Correct answer: 2) Brain

Explanation:

The PNS comprises all the nerves of the body associated with the CNS (brain and spinal cord). The nerve fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres

The afferent nerve fibres transmit impulses from tissues/organs to the CNS and the afferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.

Hence, the correct answer is option 2) Brain.

Q2. Myelinated nerve fibres are found in

1. Spinal nerves

2. Cranial nerves

3. Autonomic nervous system

4. Both a and b

Correct answer: 4) Both a and b

Explanation:

There are two types of axons, namely, myelinated and unmyelinated. The myelinated nerve fibers are enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibers are found in spinal and cranial nerves. Unmyelinated nerve fiber is enclosed by a Schwann cell that does not form a myelin sheath around the axon and is commonly found in autonomous and somatic neural systems.

Hence, the correct answer is option 4) Both a and b.

Q3. Bundle of cell bodies in the peripheral nervous system is called

1. Tract

2. Ganglion

3. Stimulus

4. Nerve termination

Correct answer: 2) Ganglion

Explanation:

A ganglion is considered a structure found within the peripheral nervous system (PNS) that is comprised of a cluster of neuronal cell bodies and may or may not be enclosed in a connective tissue capsule. Ganglia serve as relays for communication of neurons, integration of signals, as well as modulation of responses. They are mainly related to sensory neurons such as the dorsal root ganglia, and other autonomic functions including sympathetic and parasympathetic ganglia. In contrast to the central nervous system where similar aggregates are referred to as nuclei, ganglia are aggregates situated outside the brain and the spinal cord.

Hence, the correct answer is option 2) Ganglion.

Also Read:

• MCQs on Human Nervous System

• Mechanism of Vision

• Autonomic Nervous System

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