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Stimulus

A stimulus is any change in the surroundings or inside the body that can be detected and can produce a response.

In one line: Stimulus is the signal, response is the reply.

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Understand it

  • A stimulus may be light, sound, heat, touch, pain, gravity, water, or even a chemical change.
  • The body or plant senses that change and then reacts in a suitable way.
  • In humans, a stimulus is usually detected by a receptor, and in plants it is detected by cells that respond by growth or movement.
  • The important idea is simple: stimulus first, response after that.

Example

When you touch a hot vessel, the heat acts as a stimulus and your hand moves away quickly. In a plant, sunlight falling from one side acts as a stimulus and the shoot bends towards it.

Rule to remember

Rule: stimulus is the change detected first, and response is the action that follows.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A student feels a sharp pinprick.

  1. 1The pinprick is the stimulus.
  2. 2Pain receptors detect it, the signal reaches the nervous system, and the hand is pulled away as the response.

Method you can reuse

  1. 1Read the situation carefully.
  2. 2Find the change that starts the action.
  3. 3Name that change as stimulus.
  4. 4Identify the action taken by the body or plant.
  5. 5Name that action as response.

Visual tip

A simple flow line can be shown as stimulus → receptor → nervous system or control centre → response.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

If a person hears a loud sound and instantly turns the head, what is the stimulus and what is the response?

Think first, then show answer
The loud sound is the stimulus because it is the change that is detected. Turning the head is the response because it is the action taken after sensing the sound.

Common wrong answer

A common wrong answer is to call the action the stimulus.

Why it loses marks: The action happens after the change has been detected.

Don't mix this up

Many students think stimulus and response mean the same thing. They do not. Stimulus is the change that is noticed, while response is the action taken after noticing it.

Exam tip

In answers, always write the change first, then the body part that detects it, and finally the response. This keeps the sequence correct.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions often ask students to identify stimulus and response in everyday situations, or to arrange the sequence correctly.
  1. 1

    Enough for 1 mark

    A stimulus is any change in the surroundings or inside the body that can be detected and can produce a response.

  2. 2

    Add for 2 marks

    A stimulus may be light, sound, heat, touch, pain, gravity, water, or even a chemical change.

  3. 3

    Add for 3 marks

    The body or plant senses that change and then reacts in a suitable way.

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