---
title: "Who decides?"
subtitle: "Year 9 Biology · Coordination and control · Lesson 2"
---

# Who decides?

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

> **Key question:** Step on a pin and your foot is off the floor before you've said 'ouch'. Your brain wasn't the one who decided.


:::teacher-note

**In one sentence:** Students build stimulus–response pathways, name the CNS as the command centre with sensory and motor neurons, and explain why reflexes are fast (AC9S9U01, node 2, a hinge node).

**Students should be able to say:** I can build a stimulus–response pathway, choosing the receptor, command centre and effector. I can name sensory and motor neurons and the CNS. I can explain why a reflex is faster than a decided response.

**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

**You step on a drawing pin and lift your foot instantly. Which part decided to lift it?**

☐ The foot itself

☐ The leg muscles

☐ The spinal cord

☐ The brain

My prediction, and why:

{{lines-3}}

:::


:::teacher-note

**What happens:** The spinal cord. It's a reflex: the impulse goes from the receptor to the spinal cord and straight back out to the leg muscles. The brain finds out a moment later, which is when you feel the pain.

:::


## Explore

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

{{lines-2}}

## Explain: the nervous system


:::keep-together

### Neurons

**Match each part of the nervous system to its place in the pathway.**

A. The command centre

B. Turns a stimulus into an impulse

C. Carries impulses from the CNS to an effector

D. Carries impulses from a receptor to the CNS

{{columns 60% 40%}}

{{rows 1}}

| Part | Letter |
|---|---|
| `Sensory neuron` | |
| `Brain and spinal cord (CNS)` | |
| `Motor neuron` | |
| `Receptor` | |

:::


:::teacher-note

**Answer:** Sensory neuron → D. Carries impulses from a receptor to the CNS; Brain and spinal cord (CNS) → A. The command centre; Motor neuron → C. Carries impulses from the CNS to an effector; Receptor → B. Turns a stimulus into an impulse

:::


:::keep-together

### Reflexes

**Why is a reflex faster than catching a ball?**

☐ Pain impulses travel faster than light impulses

☐ Leg muscles are faster than arm muscles

☐ The impulse goes through the spinal cord only, with no time spent deciding in the brain

☐ Reflexes use hormones

:::


:::teacher-note

**Answer:** The impulse goes through the spinal cord only, with no time spent deciding in the brain

:::


:::keep-together

### Effectors

**In the chips pathway, the effector is a gland, not a muscle. What does an effector have to be?**

☐ Always a muscle

☐ A nerve

☐ A receptor

☐ A muscle or a gland: something that carries out the response

:::


:::teacher-note

**Answer:** A muscle or a gland: something that carries out the response

:::


:::typical

## Elaborate: reflex or not?


:::keep-together

**Reflex (automatic, no decision) or a decided response?**

Groups: Reflex, Decided response

- Blinking when something flies at your eye: {{field-15}}
- Answering a question in class: {{field-15}}
- Pupils narrowing in bright light: {{field-15}}
- Choosing a snack: {{field-15}}
- Knee jerking when tapped below the kneecap: {{field-15}}
- Catching a ball: {{field-15}}

:::


:::teacher-note

**Answer:** Blinking when something flies at your eye → Reflex; Answering a question in class → Decided response; Pupils narrowing in bright light → Reflex; Choosing a snack → Decided response; Knee jerking when tapped below the kneecap → Reflex; Catching a ball → Decided response

:::

:::


:::wider

## Elaborate: damage to the pathway


:::keep-together

**A person's spinal cord is damaged in their upper back. A doctor taps below their knee. What would you expect?**

☐ Both work normally

☐ The knee can't jerk, but they feel the tap

☐ Nothing changes

☐ The knee-jerk may still work if the reflex arc below the damage is intact, but the person may not feel the tap

:::


:::teacher-note

**Answer:** The knee-jerk may still work if the reflex arc below the damage is intact, but the person may not feel the tap

Doctors use reflex tests to find where along the spinal cord damage is.

:::

:::


## Evaluate: look back

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

**Revision summary**

- The **central nervous system** (brain and spinal cord) is the command centre.
- **Sensory neurons** carry impulses from receptors to the CNS; **motor neurons** carry them to effectors (**muscles or glands**).
- A **reflex** is automatic and fast: the impulse goes through the spinal cord without waiting for the brain.

## Exit ticket

:::keep-together

**Name the command centre in a reflex and say why reflexes are fast.**

{{lines-3}}

:::


:::teacher-note

**Answer:** The spinal cord; the impulse doesn't have to travel to the brain and wait for a decision.

:::