Question 4
What is reflex action? Explain the reflex arc with the help of an example.
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.
The path that an impulse takes in a reflex action is called a reflex arc. The reflex arc involves three neurons — sensory neurons, association neurons and motor neurons.
- If we touch a hot object, a stimulus triggers an impulse in a receptor on the skin.
- The impulse then travels from the receptor through a sensory neuron to the spinal cord.
- In the spinal cord, it passes through an association neuron to a motor neuron and finally reaches the muscles.
- The muscles of the effector organ contract moving it away from the hot object.
Below flow chart shows a simple reflex action of immediate withdrawal of hand on touching a hot object:
Nervous System – Interactive Study Guide
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
- 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
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
- 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.