Stimulus

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

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

A common wrong answer is to call the action the stimulus. That is incorrect because the action happens after the change has been detected.

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

A stimulus is any change in the surroundings or inside the body that can be detected and can produce a response. Rule: stimulus is the change detected first, and response is the action that follows. 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.

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. Rule: stimulus is the change detected first, and response is the action that follows. A student feels a sharp pinprick. The pinprick is the stimulus. Pain receptors detect it, the signal reaches the nervous system, and the hand is pulled away as the response. Questions often ask students to identify stimulus and response in everyday situations, or to arrange the sequence correctly. A common wrong answer is to call the action the stimulus. That is incorrect because the action happens after the change has been detected.