Nervous System
1. Functions of the Nervous System
The NERVOUS SYSTEM is the body's CONTROL and COMMUNICATION system.
Main Functions
- Receives information from inside and outside the body (via sense organs).
- Processes and interprets information.
- Sends instructions to muscles and glands (responses).
- Coordinates all voluntary and involuntary actions.
2. The Neuron — Basic Unit
A NEURON (nerve cell) is the structural and functional unit of the nervous system.
Parts of a Neuron
| Part | Function |
|---|---|
| Cell body (Cyton) | Contains NUCLEUS and organelles. Main processing centre. |
| Dendrites | BRANCHED extensions. RECEIVE signals from other neurons or sense organs. |
| Axon | LONG fibre. CARRIES signals AWAY from cell body. |
| Axon terminals | End branches. Release NEUROTRANSMITTERS to pass signal to next neuron. |
| Myelin sheath | INSULATING layer around axon. Speeds up signal transmission. |
Types of Neurons
| Type | Function |
|---|---|
| Sensory neuron | Carries signals from SENSE ORGANS to CENTRAL NERVOUS SYSTEM |
| Motor neuron | Carries signals from CENTRAL NERVOUS SYSTEM to MUSCLES/GLANDS |
| Relay/Interneuron | Connects sensory and motor neurons within the CNS |
3. The Central Nervous System (CNS)
The CNS consists of the BRAIN and SPINAL CORD.
A. The Brain
The brain is protected by the SKULL (cranium) and THREE MEMBRANES called MENINGES.
Cerebrospinal fluid (CSF) cushions the brain.
Main Parts of the Brain
| Part | Location | Functions |
|---|---|---|
| Cerebrum | Largest part (front and top) | THINKING, memory, voluntary movements, speech, senses. Divided into TWO HEMISPHERES. |
| Cerebellum | Below cerebrum (back) | BALANCE, COORDINATION of movements, posture |
| Medulla oblongata | Base of brain (connects to spinal cord) | INVOLUNTARY actions: heartbeat, breathing, blood pressure, digestion |
B. The Spinal Cord
- A long, cylindrical bundle of nerves running inside the VERTEBRAL COLUMN (backbone).
- Connects the BRAIN to the REST of the body.
- Responsible for REFLEX ACTIONS (without involving the brain).
4. Reflex Action
A REFLEX ACTION is a RAPID, AUTOMATIC response to a stimulus that does NOT involve conscious thought.
Reflex Arc Pathway
- Stimulus (e.g., touching a hot object).
- Receptor (skin) detects stimulus.
- Sensory neuron carries signal to SPINAL CORD.
- Relay neuron (in spinal cord) connects to motor neuron.
- Motor neuron carries signal to muscle/gland (effector).
- Response (e.g., hand is pulled away).
Examples of Reflex Actions
- Pulling hand away from a hot object.
- Knee jerk when tapped below kneecap.
- Blinking when something approaches the eye.
- Sneezing when irritants enter the nose.
Reflex vs Voluntary Action
| Feature | Reflex Action | Voluntary Action |
|---|---|---|
| Consciousness | No (automatic) | YES (conscious) |
| Speed | VERY FAST | Slower |
| Brain involvement | NO (spinal cord) | YES (brain) |
| Example | Touching hot object | Picking up a book |
5. Sense Organs
| Sense Organ | Sense | Stimulus |
|---|---|---|
| Eye | Sight | Light |
| Ear | Hearing and balance | Sound |
| Nose | Smell | Chemical substances (in air) |
| Tongue | Taste | Chemical substances (in food) |
| Skin | Touch, pressure, pain, heat, cold | Physical and thermal stimuli |
The Eye (Quick Overview)
- Cornea: Transparent front layer (bends light).
- Iris: COLOURED part. Controls pupil size.
- Pupil: Opening through which light enters.
- Lens: Focuses light on retina.
- Retina: Contains LIGHT-SENSITIVE cells (rods for dim light, cones for colour).
- Optic nerve: Carries signals from retina to brain.
The Ear (Quick Overview)
- Outer ear: Pinna collects sound → ear canal → eardrum vibrates.
- Middle ear: Three small bones (hammer, anvil, stirrup) amplify vibrations.
- Inner ear: Cochlea (hearing) and semicircular canals (balance).
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Parts of neuron and function | 2-3 marks | Very High |
| Brain parts and functions | 3 marks | Very High |
| Reflex action and reflex arc | 3 marks | Very High |
| Sense organs (eye and ear) | 2-3 marks | High |
| CNS vs PNS | 2 marks | Medium |
Common Mistakes
- Writing that the cerebellum controls THINKING (that is the cerebrum — cerebellum controls balance/coordination).
- Confusing sensory and motor neurons (sensory = TO the CNS, motor = FROM the CNS).
- Thinking reflex actions involve the brain (they go through the SPINAL CORD for speed).
- Saying the iris is the 'coloured part of the eye' without mentioning its function (controls pupil size).
Self-Test (5 Questions)
Q1. Which part of the brain controls BALANCE and COORDINATION? (1 mark)
- A) Cerebrum
- B) Cerebellum
- C) Medulla
- D) Spinal cord
Q2. 'What is a reflex action? Give one example.' (2 marks)
Q3. 'Name the three types of neurons in a reflex arc.' (2 marks)
Q4. 'Which part of the brain controls involuntary actions like heartbeat?' (1 mark)
Q5. 'Draw a simplified reflex arc showing stimulus → receptor → sensory neuron → spinal cord → motor neuron → response.' (3 marks)
Answers
A1. B) Cerebellum. A2. A reflex action is a RAPID, AUTOMATIC response to a stimulus without conscious thought. Example: Pulling hand away from a hot object. A3. Sensory neuron, Relay neuron (interneuron), Motor neuron. A4. Medulla oblongata. A5. Diagram showing: stimulus → skin receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → muscle (effector) → response (hand pulls away).
