---
title: "What counts as a stimulus?"
subtitle: "Year 9 Biology · Coordination and control · Lesson 1"
---

# What counts as a stimulus?

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

> **Key question:** You pull your hand off a hot pan before you feel the pain. After lunch, your blood sugar is brought back down without you noticing at all. Both are responses to a stimulus.


:::teacher-note

**In one sentence:** Students identify stimuli as internal or external changes and trace them through the stimulus–response pathway (AC9S9U01, node 1, a hinge node).

**Students should be able to say:** I can explain that a stimulus is a change inside or outside the body. I can name the parts of the stimulus–response pathway and what each does. I can trace an internal and an external stimulus through the pathway.

**Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

:::


## Engage


:::keep-together

**Which of these is a stimulus?**

☐ Only things that hurt

☐ Only things outside you: light, sound, touch, heat

☐ Any change your body detects, outside or inside

☐ Only things you notice

My prediction, and why:

{{lines-3}}

:::


:::teacher-note

**What happens:** Any change your body detects, outside or inside: blood sugar rising and body water falling are stimuli too, though you don't notice them.

:::


## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: the pathway


:::keep-together

### Inside or outside

**Internal or external stimulus?**

Groups: Internal, External

- A loud bang: {{field-15}}
- Blood sugar rising: {{field-15}}
- The smell of toast: {{field-15}}
- Body temperature falling: {{field-15}}
- Too little water in the blood: {{field-15}}
- Cold wind on your skin: {{field-15}}

:::


:::teacher-note

**Answer:** A loud bang → External; Blood sugar rising → Internal; The smell of toast → External; Body temperature falling → Internal; Too little water in the blood → Internal; Cold wind on your skin → External

:::


:::keep-together

### The parts

**Match each part of the pathway to what it does.**

A. A muscle or gland that carries out the response

B. Processes the information and decides

C. A change inside or outside the body

D. What the body does

E. Detects the change

{{columns 60% 40%}}

{{rows 1}}

| Part | Letter |
|---|---|
| `Stimulus` | |
| `Receptor` | |
| `Command centre` | |
| `Effector` | |
| `Response` | |

:::


:::teacher-note

**Answer:** Stimulus → C. A change inside or outside the body; Receptor → E. Detects the change; Command centre → B. Processes the information and decides; Effector → A. A muscle or gland that carries out the response; Response → D. What the body does

:::


:::keep-together

### Hot pan

**In the hot pan pathway, which is the effector?**

☐ The skin

☐ The spinal cord

☐ The heat

☐ The arm muscles

:::


:::teacher-note

**Answer:** The arm muscles

:::


:::keep-together

### Smell receptors

**The smell of toast is the stimulus. Which is the receptor, and what does it do?**

☐ Smell receptors in the nose: they detect chemicals from the toast and send impulses to the brain

☐ The brain: it smells the toast

☐ The toast: it gives off the smell

☐ The salivary glands: they make your mouth water

:::


:::teacher-note

**Answer:** Smell receptors in the nose: they detect chemicals from the toast and send impulses to the brain

:::


:::typical

## Elaborate: a new pathway


:::keep-together

**A runner's muscles use oxygen fast, so carbon dioxide in the blood rises. Breathing speeds up. Sort each into its part of the pathway.**

Groups: Stimulus, Receptor, Command centre, Effector, Response

- Carbon dioxide in the blood rises: {{field-15}}
- Chemical receptors in blood vessels and the brain: {{field-15}}
- Brain stem: {{field-15}}
- Diaphragm and rib muscles: {{field-15}}
- Faster, deeper breathing: {{field-15}}

:::


:::teacher-note

**Answer:** Carbon dioxide in the blood rises → Stimulus; Chemical receptors in blood vessels and the brain → Receptor; Brain stem → Command centre; Diaphragm and rib muscles → Effector; Faster, deeper breathing → Response

:::

:::


:::wider

## Elaborate: plants respond too


:::keep-together

**A seedling on a windowsill bends towards the light. Which part of the pathway is missing compared with your body?**

☐ Nothing is different

☐ There are no nerves: the command is a plant hormone (auxin) carried in the plant, not an impulse

☐ There is no stimulus

☐ There is no response

:::


:::teacher-note

**Answer:** There are no nerves: the command is a plant hormone (auxin) carried in the plant, not an impulse

Plants still detect a stimulus and respond, but they coordinate with hormones only.

:::

:::


## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- A **stimulus** is any change the body detects, **external** (light, heat, sound) or **internal** (blood sugar, temperature, water).
- **Stimulus → receptor → command centre → effector → response.**
- Receptors detect; the brain, spinal cord or a gland decides; muscles and glands are the effectors.

## Exit ticket

:::keep-together

**Give one internal stimulus and name its receptor and effector.**

{{lines-3}}

:::


:::teacher-note

**Answer:** For example: body temperature rising; temperature receptors in the brain; sweat glands.

:::