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# What counts as a stimulus?

Year 9 Biology · Coordination and control

<!-- Speaker note: Students identify stimuli as internal or external changes and trace them through the stimulus–response pathway (AC9S9U01, node 1, a hinge node). About 20 minutes (Engage and Explore 7 min · Explain 8 min · Evaluate 3 min · Typical adds 2 min, Wider about 2 min). -->

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## Today

**We are learning to** a stimulus is any change the body detects, outside it or inside it. Every response follows the same pathway: stimulus, receptor, command centre, effector, response.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a reaction-time practical (the ruler drop) or the knee-jerk reflex. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconception is that stimuli are only external. Three of the five board stimuli are internal on purpose. Board notes: each pathway is simplified to one receptor, one command centre and one effector; the blood sugar and water pathways mix glands and hormones into the boxes, which the next lessons separate. -->

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## Why it matters

> 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.

<!-- Speaker note: Earlier you met body systems. This lesson: how the body detects change and responds. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'only things outside you'. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-9%2Fstimuli%2Fstep-2&code=widget%3A+coordination%0Amode%3A+pathway%0Aneed%3A+4%0Astimulus%3A+Touching+a+hot+pan+%7C+external+%7C+Heat+and+pain+receptors+in+the+skin+%7C+Spinal+cord+%7C+Arm+muscles+%7C+You+pull+your+hand+away%0Astimulus%3A+Bright+light+in+your+eyes+%7C+external+%7C+Light+receptors+in+the+retina+%7C+Brain+%7C+Iris+muscles+%7C+Your+pupils+get+smaller%0Astimulus%3A+Blood+sugar+rises+after+lunch+%7C+internal+%7C+Cells+in+the+pancreas+%7C+Pancreas+%7C+Pancreas+releases+insulin%3B+liver+and+muscle+cells+take+up+sugar+%7C+Blood+sugar+falls+back+to+normal%0Astimulus%3A+Body+getting+too+hot+%7C+internal+%7C+Temperature+receptors+in+the+brain+and+skin+%7C+Hypothalamus+%28brain%29+%7C+Sweat+glands+and+skin+blood+vessels+%7C+You+sweat+and+your+skin+flushes%0Astimulus%3A+Too+little+water+in+the+blood+%7C+internal+%7C+Receptors+in+the+brain+%7C+Hypothalamus+%28brain%29+%7C+Pituitary+gland+releases+ADH%3B+kidneys+keep+water+%7C+You+make+less%2C+darker+urine+and+feel+thirsty%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-9/stimuli/?full=step-2#step-2)

<!-- Speaker note: Click a stimulus to send it through the pathway. Notice which ones are **internal** and which are **external**. -->

<!-- Speaker note: Teacher: ask which stimuli they would never notice happening. -->

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## What happened?

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

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Inside or outside

**Internal or external stimulus?**

Groups: Internal · External

A loud bang · Blood sugar rising · The smell of toast · Body temperature falling · Too little water in the blood · Cold wind on your skin

* 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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the sort is the checkpoint; 'cold wind' prompts the external change / internal effect distinction. -->

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## The parts

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

| | |
|---|---|
| `Stimulus` | A. A muscle or gland that carries out the response |
| `Receptor` | B. Processes the information and decides |
| `Command centre` | C. A change inside or outside the body |
| `Effector` | D. What the body does |
| `Response` | E. Detects the change |

* 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

<!-- Speaker note:  -->

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## Hot pan

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

- The skin
- The spinal cord
- The heat
- The arm muscles

* Answer: The arm muscles

<!-- Speaker note:  -->

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## 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

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

<!-- Speaker note:  -->

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## Elaborate: a new pathway

**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 · Chemical receptors in blood vessels and the brain · Brain stem · Diaphragm and rib muscles · Faster, deeper breathing

* 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

<!-- Speaker note: What this step is for: Applying the pathway to an unseen example shows the pattern has transferred. -->

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## Elaborate: plants respond too

**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

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

<!-- Speaker note: Plants still detect a stimulus and respond, but they coordinate with hormones only. What this step is for: Plants detect and respond to stimuli without nerves, which separates the pathway idea from the nervous system. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

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

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

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: building pathways yourself and why the command centre matters (node 2), then nerves versus hormones (node 4) and feedback (node 5). In class: ruler-drop reaction times; knee-jerk reflex; pupil response with a torch. -->