Classification of the Peripheral Nervous System (PNS)
Introduction
The human nervous system is responsible for controlling and coordinating different activities of the body. It receives information from the surroundings, processes it and produces appropriate responses.
The nervous system is divided into two major parts:
Central Nervous System (CNS): Consists of the brain and spinal cord.
Peripheral Nervous System (PNS): Consists of nerves and ganglia located outside the brain and spinal cord.
The peripheral nervous system acts as a communication link between the central nervous system and the rest of the body. It carries sensory information towards the CNS and transmits motor commands from the CNS to muscles and glands.
The PNS is mainly classified structurally and functionally.
[IMG 1 — Horizontal flowchart showing the nervous system divided into CNS (brain and spinal cord) and PNS (cranial nerves, spinal nerves and ganglia).]
1. Structural Classification of the Peripheral Nervous System
Structural classification is based on the physical components of the peripheral nervous system.
The PNS mainly consists of the following structures:
A. Cranial Nerves
Cranial nerves arise directly from the brain and mainly supply structures of the head and neck. Some also extend to internal organs.
Humans have 12 pairs of cranial nerves.
These nerves are involved in functions such as smell, vision, hearing, facial movements and swallowing.
Example: The optic nerve carries visual information from the eyes towards the brain.
B. Spinal Nerves
Spinal nerves arise from the spinal cord and connect it with different parts of the body.
Humans have 31 pairs of spinal nerves, classified as:
8 pairs of cervical nerves
12 pairs of thoracic nerves
5 pairs of lumbar nerves
5 pairs of sacral nerves
1 pair of coccygeal nerves
They carry sensory information from the body towards the spinal cord and motor commands from the spinal cord to muscles and other structures.
C. Ganglia
Ganglia are collections of nerve cell bodies located outside the central nervous system.
They act as relay points for nerve signals.
For example, sensory ganglia contain the cell bodies of sensory neurons that carry information towards the spinal cord.
2. Functional Classification of the Peripheral Nervous System
Functionally, the PNS is divided into two major divisions:
Sensory (afferent) division
Motor (efferent) division
A. Sensory or Afferent Division
The sensory division carries information from sensory receptors towards the central nervous system.
Sensory receptors detect changes such as temperature, pressure, pain and body position.
Examples:
Touching a hot object produces a temperature and pain sensation.
Sensory receptors in the skin detect pressure.
Receptors in muscles and joints provide information about body position.
The information is carried to the brain or spinal cord, where it can be processed.
B. Motor or Efferent Division
The motor division carries commands from the CNS to effectors such as muscles and glands.
It is responsible for producing responses after sensory information has been processed.
The motor division is further divided into:
Somatic nervous system
Autonomic nervous system
[IMG 2 — Horizontal flowchart showing PNS divided into sensory (afferent) and motor (efferent) divisions, with the motor division further divided into somatic and autonomic nervous systems.]
3. Somatic Nervous System
The somatic nervous system (SNS) controls voluntary movements and carries sensory information from the skin, skeletal muscles and joints.
It mainly controls skeletal muscles, which are responsible for activities such as walking, writing, lifting objects and moving the limbs.
Functions
Controls voluntary skeletal muscle movements.
Carries sensory information from the skin and musculoskeletal system.
Helps maintain posture and coordinate body movements.
Participates in certain involuntary reflex actions.
Example: When you decide to pick up a glass of water, the somatic nervous system controls the skeletal muscles of your arm and hand.
Important: Although the somatic nervous system is mainly associated with voluntary movement, it also participates in reflex actions that occur without conscious decision-making.
4. Autonomic Nervous System
The autonomic nervous system (ANS) controls many involuntary activities of the body.
It regulates the functions of internal organs, smooth muscles, cardiac muscles and glands.
These activities generally occur automatically without conscious control.
Functions
Regulates heart rate.
Controls the diameter of blood vessels.
Regulates digestive activities.
Controls pupil size.
Influences glandular secretions.
Helps maintain internal body conditions.
The autonomic nervous system is divided into three major components:
Sympathetic nervous system
Parasympathetic nervous system
Enteric nervous system
A. Sympathetic Nervous System
The sympathetic nervous system prepares the body to respond to stressful or emergency situations. This response is commonly called the fight-or-flight response.
During sympathetic activation:
Heart rate increases.
Pupils dilate.
Airways widen.
Digestive activity generally decreases.
Blood flow is redistributed towards tissues needed for physical activity.
Example: When a person suddenly faces danger, the sympathetic nervous system increases heart rate and prepares the body for quick action.
B. Parasympathetic Nervous System
The parasympathetic nervous system supports normal body maintenance, digestion and energy conservation. It is commonly called the rest-and-digest system.
During parasympathetic activation:
Heart rate generally decreases.
Digestive activity increases.
Intestinal movements increase.
Salivary and digestive secretions are stimulated.
Pupils generally constrict.
Example: After eating a meal, the parasympathetic nervous system helps stimulate digestion and intestinal activity.
C. Enteric Nervous System
The enteric nervous system is a network of neurons present in the wall of the gastrointestinal tract.
It controls many digestive activities, including:
Movement of food through the digestive tract.
Intestinal muscle contractions.
Regulation of digestive secretions.
Coordination of local digestive reflexes.
It can coordinate many digestive functions independently, although it also communicates with the sympathetic and parasympathetic nervous systems.
[IMG 3 — Horizontal comparison infographic showing sympathetic (fight or flight), parasympathetic (rest and digest) and enteric (digestive control) divisions with their major functions.]
5. Difference Between Somatic and Autonomic Nervous Systems
Feature | Somatic Nervous System | Autonomic Nervous System |
|---|---|---|
Main control | Mainly voluntary | Involuntary |
Main effectors | Skeletal muscles | Smooth muscles, cardiac muscle and glands |
Main function | Controls body movements | Regulates internal organ activities |
Example | Walking or writing | Heart rate and digestion |
Motor pathway | Generally one motor neuron from CNS to skeletal muscle | Generally a two-neuron pathway involving a ganglion |
6. Difference Between Sympathetic and Parasympathetic Nervous Systems
Activity | Sympathetic | Parasympathetic |
|---|---|---|
Heart rate | Increases | Generally decreases |
Pupil | Dilates | Constricts |
Airways | Dilates | Generally constricts |
Digestion | Generally decreases | Increases |
Main role | Fight or flight | Rest and digest |
Remember: Both divisions are important. They work together to maintain normal body functions rather than acting as completely separate systems.
7. Clinical and Pharmaceutical Importance
Understanding the peripheral nervous system is important in pharmacy because several medicines act on its nerves and receptors.
For example:
Atropine blocks muscarinic acetylcholine receptors and reduces certain parasympathetic effects, such as salivary secretions.
Salbutamol stimulates beta-2 adrenergic receptors and helps relax bronchial smooth muscle, producing bronchodilation.
Neostigmine inhibits acetylcholinesterase, increasing the availability of acetylcholine at cholinergic synapses.
These examples provide a foundation for understanding autonomic pharmacology, which is studied in greater detail in later pharmacy subjects.
Key Takeaways
The peripheral nervous system connects the CNS with the rest of the body.
Structurally, it includes cranial nerves, spinal nerves and ganglia.
Functionally, it is divided into sensory (afferent) and motor (efferent) divisions.
The somatic nervous system mainly controls skeletal muscle movements.
The autonomic nervous system regulates involuntary activities of internal organs.
The autonomic nervous system includes sympathetic, parasympathetic and enteric divisions.
Sympathetic activity is associated with fight or flight, while parasympathetic activity supports rest and digestion.