5.2.2 Peripheral Nervous System

Introduction

The peripheral nervous system (PNS) is the network of nerves that connects the central nervous system (CNS) to the rest of the body. It carries information from the senses to the brain and spinal cord, and from the brain and spinal cord to muscles and glands. This topic explains the structure and divisions of the peripheral nervous system.

Definition

The peripheral nervous system (PNS) is composed of nerves which link the central nervous system with different body parts.

Main Idea

The peripheral nervous system (PNS) is composed of nerves which link central nervous system with different body parts.

• Nerves which arise from the brain are called cranial nerves.
• The nerves which arise from the spinal cord are called spinal nerves.
• There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in the human body.
• Some cranial nerves are sensory, some are motor, and some are mixed.
• While, all spinal nerves are mixed nerves.

The peripheral nervous system can be divided into:

• Somatic nervous system
• Autonomic nervous system

Core Concept

Divisions of the Peripheral Nervous System

Types of Nerves

Detailed Explanation

1. Cranial Nerves

What they are: Nerves which arise from the brain are called cranial nerves.

Number: There are 12 pairs of cranial nerves in the human body.

Types:

• Some cranial nerves are sensory (carry impulses towards the brain).
• Some are motor (carry impulses away from the brain).
• Some are mixed (carry both sensory and motor impulses).

Simple explanation: Cranial nerves come from the brain. They carry information to and from the brain for functions like smell, vision, hearing, and facial movement.

2. Spinal Nerves

What they are: The nerves which arise from the spinal cord are called spinal nerves.

Number: There are 31 pairs of spinal nerves in the human body.

Types:

• All spinal nerves are mixed nerves.
• They carry both sensory and motor impulses.

Simple explanation: Spinal nerves come from the spinal cord. They carry messages between the spinal cord and different parts of the body.

3. Somatic Nervous System

What it is: Generally it consists of the cranial and spinal nerve fibers that connect the CNS to skin and skeletal muscles.

Function:

• Controls voluntary movements.
• Carries sensory information from the skin, muscles, and joints to the CNS.
• Carries motor commands from the CNS to skeletal muscles.

Simple explanation: The somatic nervous system controls voluntary actions like walking, talking, and writing. It connects the CNS to the skin and skeletal muscles.

4. Autonomic Nervous System

What it is: It includes those fibres that connect the CNS to the visceral organs, such as the heart, stomach, intestines, and various glands.

Function:

• It is concerned with the unconscious activities.
• Controls involuntary functions like heartbeat, digestion, and breathing.

Simple explanation: The autonomic nervous system controls involuntary actions. It manages organs like the heart, stomach, and intestines without you thinking about it.

5. Divisions of the Autonomic Nervous System

The autonomic nervous system is divided into:

(a) Sympathetic nervous system:

• It consists of only spinal nerves.
• It functions only in the state of emergency.
• It prepares the body for fight or flight response.
• Increases heart rate, blood pressure, and blood flow to muscles.
• Decreases digestion.

Simple explanation: The sympathetic nervous system prepares your body for emergencies. It helps you fight or run away.

(b) Parasympathetic nervous system:

• It consists of a few cranial and spinal nerves.
• It controls various autonomic nerves during the state of rest.
• In short, it returns the body functions to normal after they have been altered by sympathetic stimulation.

Simple explanation: The parasympathetic nervous system calms your body down after an emergency. It helps you rest and digest.

Summary of the Peripheral Nervous System

Intermediate Level

The peripheral nervous system (PNS) links the CNS to different body parts.

Types of nerves:

Divisions:

Autonomic nervous system divisions:

Key points:

• Cranial nerves arise from the brain.
• Spinal nerves arise from the spinal cord.
• All spinal nerves are mixed.
• Some cranial nerves are sensory, motor, or mixed.
• The somatic nervous system controls voluntary actions.
• The autonomic nervous system controls involuntary actions.
• The sympathetic system prepares the body for emergencies.
• The parasympathetic system returns the body to normal after an emergency.

Important Points

• - The peripheral nervous system (PNS) is composed of nerves which link central nervous system with different body parts.
• - Nerves which arise from brain are called cranial nerves.
• - The nerves which arise from spinal cord are called spinal nerves.
• - There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in the human body.
• - Some cranial nerves are sensory, some are motor, and some are mixed.
• - While, all spinal nerves are mixed nerves.
• - The peripheral nervous system can be divided into somatic and autonomic nervous system.
• - Somatic nervous system generally consists of the cranial and spinal nerve fibers that connect the CNS to skin and skeletal muscles.
• - Autonomic nervous system includes those fibres that connect the CNS to the visceral organs, such as the heart, stomach, intestines, and various glands.
• - It is concerned with the unconscious activities.
• - The autonomic nervous system is divided into sympathetic and parasympathetic nervous system.
• - Sympathetic system consists of only spinal nerves.
• - Sympathetic system functions only in the state of emergency.
• - It prepares the body to fight or flight response.
• - Parasympathetic system consists of few cranial and spinal nerves.
• - Parasympathetic system controls various autonomic nerves during the state of rest.
• - In short, it returns the body functions to normal after they have been altered by sympathetic stimulation.

Remember

• - PNS = cranial nerves + spinal nerves
• - Cranial nerves = 12 pairs, from brain
• - Spinal nerves = 31 pairs, from spinal cord
• - All spinal nerves are mixed
• - Some cranial nerves are sensory, motor, or mixed
• - Somatic nervous system = voluntary movements
• - Autonomic nervous system = involuntary activities
• - Sympathetic = fight or flight
• - Parasympathetic = rest and digest

Common Mistakes

• 1. Confusing cranial and spinal nerves.
• - Cranial nerves = from brain (12 pairs).
• - Spinal nerves = from spinal cord (31 pairs).
• 2. Thinking all cranial nerves are mixed.
• - Some cranial nerves are sensory, some are motor, and some are mixed.
• - All spinal nerves are mixed.
• 3. Mixing up the somatic and autonomic nervous systems.
• - Somatic = voluntary movements.
• - Autonomic = involuntary functions.
• 4. Confusing sympathetic and parasympathetic.
• - Sympathetic = fight or flight.
• - Parasympathetic = rest and digest.
• 5. Thinking the autonomic nervous system is under conscious control.
• - The autonomic nervous system is involuntary.
• 6. Forgetting the number of cranial and spinal nerves.
• - Cranial nerves = 12 pairs.
• - Spinal nerves = 31 pairs.
• 7. Believing the sympathetic system works during rest.
• - The sympathetic system works during emergencies.
• - The parasympathetic system works during rest.

Quick Revision

60-Second Revision

• PNS = cranial nerves + spinal nerves
• Cranial nerves = 12 pairs, from brain
• Spinal nerves = 31 pairs, from spinal cord
• All spinal nerves = mixed
• Some cranial nerves = sensory, motor, or mixed
• Somatic nervous system = voluntary movements
• Autonomic nervous system = involuntary activities
• Sympathetic = fight or flight
• Parasympathetic = rest and digest
• Warning: Do not confuse cranial nerves (from brain) with spinal nerves (from spinal cord). All spinal nerves are mixed.