Understanding Control and Coordination in Living Organisms / The Nervous System / What Is the Nervous System? / The Human Nervous System: Two Major Divisions / Why the Nervous System Matters

Living organisms, from simple tiny creatures to humans, constantly adjust their movements and behaviors in response to the environment. Whether it’s turning toward light, pulling your hand away from a hot stove, or blinking when something comes close to your eye — these actions are all responses to stimuli.

A stimulus is anything in the environment that causes a reaction.

A response is how the organism reacts to it.

Common stimuli include:

• Light

• Sound

• Air and wind

• Heat and cold

• Smell and taste

• Water

• Gravity

Imagine burning your finger and feeling nothing.

Or breaking a bone without experiencing any pain.

It might sound like a superpower, but from a survival standpoint, it would be extremely dangerous. Pain and other sensations are your body’s built-in warning signals. They help you avoid further injury and tell you when something is wrong.

So how does the body detect and respond to these signals?

The answer lies in one of the most remarkable systems ever evolved in living organisms:

The Nervous System

The nervous system is the body’s communication network. It helps you sense your environment, make decisions, coordinate movements, and protect yourself.

It works through special cells called neurons, which act as tiny messengers. Neurons receive information, process it, and send signals through electrical impulses — faster than the speed of a race car.

In simple organisms, the nervous system may just be a basic network of connected cells.

In humans and other complex animals, it becomes a highly developed system with billions of interconnected neurons.

Let’s break it down step by step.

What Is the Nervous System?

The nervous system is a complex network of neurons that carry messages throughout the body. As you move up the evolutionary ladder, the nervous system becomes more sophisticated.

Examples:

• Jellyfish have simple nerve nets.

• Crabs have nerve centers called ganglia.

• Humans and vertebrates have a large, highly developed brain.

In humans, the nervous system allows quick communication between the brain, spinal cord, sense organs, and every other part of the body. It helps us:

• Think

• Learn

• React

• Move

• Feel sensations

It is the master coordinator of all body activities.

The Human Nervous System: Two Major Divisions

The human nervous system is divided into two main parts:

1. Central Nervous System (CNS)

2. Peripheral Nervous System (PNS)

Let’s explore each of them.

1. Central Nervous System (CNS)

The CNS is the core processing unit of the body. It includes:

The Brain

The brain is the command center — the most advanced and complex organ in the human body. It helps you think, remember, understand, make decisions, learn, and even dream.

It is protected by the skull and divided into three major parts:

Forebrain

• Cerebrum

• Thalamus

• Hypothalamus

The forebrain manages reasoning, emotions, memory, and voluntary activities.

Midbrain

Located in the brainstem, it plays a role in vision, hearing, and reflexes.

Hindbrain

• Cerebellum

• Pons

• Medulla

These structures control balance, coordination, and vital functions like breathing and heartbeat.

The Spinal Cord

The spinal cord is a long, cylindrical bundle of nerves. It:

• Connects the brain to the rest of the body

• Coordinates reflex actions

• Transmits nerve impulses to and from the brain

It is protected inside the vertebral column and covered by membranes called meninges.

2. Peripheral Nervous System (PNS)

The PNS includes all the nerves branching out from the brain and spinal cord. It links the CNS to the rest of the body and manages both voluntary and involuntary activities.

Two types of nerve fibers are involved:

Afferent (Sensory) Fibers

Carry information from tissues and organs → to the CNS.

Efferent (Motor) Fibers

Carry instructions from the CNS → to muscles and organs.

The PNS is divided into:

Somatic Nervous System (SNS)

Controls voluntary actions — moving your arm, walking, writing, etc.

Autonomic Nervous System (ANS)

Controls involuntary actions — digestion, heart rate, breathing, sweating, etc.

The ANS has two branches:

• Sympathetic System (fight or flight)

• Parasympathetic System (rest and digest)

Neurons (The Building Blocks of the Nervous System)

A neuron is the basic functional and structural unit of the nervous system.

Parts of a Neuron

• Dendrites: Receive signals

• Cell Body: Contains the nucleus

• Axon: Long fiber that carries impulses

• Myelin Sheath: Insulates the axon and increases speed

• Synapse: A tiny gap where signals pass from one neuron to another

Neurons can transmit signals at astonishing speeds — over 119 m/s (428 km/h).

Nerves (The Body’s Wiring System)

Nerves are long bundles of axons that carry information throughout the body.

There are three main types:

Sensory Nerves

Carry messages from sense organs to the brain.

Motor Nerves

Carry commands from the brain to muscles.

Mixed Nerves

Carry both sensory and motor information.

Cranial Nerves

Originate from the brain and control the head and neck region.

Spinal Nerves

Originate from the spinal cord and serve the rest of the body.

Why the Nervous System Matters

The nervous system allows you to:

• Sense danger

• React instantly

• Move with coordination

• Learn new skills

• Maintain balance

• Interpret the world around you

Without it, survival would be impossible.

FAQ

Q1

What are the two divisions of the nervous system?

The human nervous system controls all activities of the body in a quicker fashion. It can be divided into the central nervous system and peripheral nervous system. The central nervous system includes spinal cord and brain and the peripheral covers the nerves branching from spinal cord and brain.

Q2

What are nerves and neurons?

Nerves are thread-like structures that emerge from the spinal cord and brain. These nerves are actual projections of neurons. A neuron is a basic structural and functional unit of a nervous system that conducts electrochemical signals.

Q3

What are cranial nerves?

The nerves that extend throughout the body on both sides and emerges directly from the brain stem and brain are called cranial nerves. They carry information from the brain to other parts, primarily to the neck and head.

In Summary

• Living organisms respond to their environment using the nervous system.

• The CNS and PNS work together to manage all body activities.

• Neurons transmit signals through electrical impulses.

• The brain and spinal cord control complex functions.

• Nerves connect every part of the body to the CNS.

To explore more diagrams, explanations, or quizzes on the nervous system, visit HN Series — your trusted learning resource for clear and engaging biology content.