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Question Question 2
The diagram shows a reflex arc. Study it and answer the following questions.
- Name the parts numbered a to d.
- Using the letters of alphabet shown in the diagram, indicate the direction in which the impulse enters and leaves part c. X to Y or Y to X?
- What is reflex action?
- With the help of a flow chart, show a simple reflex action.
- The parts numbered a to d are:
a → sensory neurons
b → motor neurons
c → association neurons
d → cell body of motor neurons - The impulse enters through X and leaves through Y.
- An automatic, spontaneous and mechanical response to a stimulus; controlled by the spinal cord without the involvement of the brain is called a reflex action.
- Below flow chart shows a simple reflex action of immediate withdrawal of hand on touching a hot object:
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ICSE Class VIII | Academic Year 2026-2027
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Biology | Chapter 9: Nervous SystemWeb Content
Nervous System – Interactive Study Guide
Quick Overview
The nervous system controls and coordinates all body activities through electrical impulses. It consists of the CNS (brain + spinal cord) and PNS (nerves connecting CNS to body).
Neuron – Quick Structure
Parts & Functions
- Cell body: Nucleus, metabolic centre
- Dendrites: Receive impulses (towards cell body)
- Axon: Carries impulses away (from cell body)
- Myelin sheath: Insulation, speeds up impulse
- Synaptic knob: Transmits to next neuron
Brain Parts – Quick Reference
| Part | Function | Memory Aid |
|---|---|---|
| Cerebrum | Thinking, memory, senses, voluntary actions | BRainpower |
| Cerebellum | Balance, coordination | BaLLance |
| Medulla | Involuntary actions (breathing, heartbeat) | Must Do |
Reflex Action – Step by Step
Reflex Arc
Stimulus → Receptor (skin) → Sensory neuron → Spinal cord (relay neuron) → Motor neuron → Effector (muscle) → Response (hand pulls away)
Controlled by spinal cord (NOT brain) for faster response.
Voluntary vs Involuntary vs Reflex
| Voluntary | Involuntary | Reflex | |
|---|---|---|---|
| Control | Conscious | Unconscious | Unconscious |
| Centre | Cerebrum | Medulla | Spinal cord |
| Speed | Slow | Moderate | Very fast |
| Example | Writing | Breathing | Knee-jerk |
Self-Test Questions
Test Yourself
- Draw and label a neuron.
- Why are reflex actions controlled by the spinal cord and not the brain?
- Name the three parts of the brain and state one function of each.
- What is the difference between sensory and motor neurons?
- Draw a reflex arc and label all parts.