How a fly remembers a smell

Students trace a smell from the receptors to the cells that read its memory, and show that silencing one layer of a pathway cuts off everything after it.

A shared screen helps but is optional: a projector, a TV, or a computer screen the class can see. Students need any device with internet access, ideally one for each pair: a laptop, tablet, or phone. Nothing to install and no accounts. Each lesson opens with a short guide to reading the screen, and every scientific term can be tapped for an explanation.

Synapses and neurotransmitters

Each layer passes the signal to the next across synapses. Scientists think the memory itself is a change in the strength of the synapses from Kenyon cells to output neurons.

How a network’s structure determines behavior

The pathway is a chain of layers, so silencing the Kenyon cells darkens the output neurons even though the smell still gets in.

Excitation and inhibition

Stimulate excites cells and Silence holds them off: the lesson’s two tools. In this part of the circuit, each layer excites the next.

1. 5 min

   Opening question

   Ask: “How does your brain know that a smell is one you have met before?” Take a few guesses, then say that today the class will look for the answer inside a fly.

2. 10 min

   Demo with “Show me how it works”

   Show the lesson in class mode on a shared screen (a projector, a TV, or a computer screen) and press “Show me how it works”. Read each step aloud as it plays. With no shared screen, students open the same walkthrough on their own devices. It stops at the lesson’s question: leave that unanswered for now.

3. 20 min

   Students explore

   Students work in pairs on their own devices, with the worksheet. They write a prediction before each tap, run the lesson, answer its question, and try free play.

4. 10 min

   Discussion

   Collect the pairs’ answers to the lesson’s question, then use the discussion questions below. End with what the model leaves out.

Dror smells a ripe peach, recognizes the smell from the last time it led him to food, and follows it upwind. The story sets up the question: where in the brain is that memory kept?

The lesson opens with a short comic about Dror, the fly. Read the panels aloud or let students read them; it takes about a minute. Its last panel comes back on the win screen.

Project the lesson. Pause at each of the four levels and ask its question before you zoom in. The level buttons jump to any level, and the magnifier button reopens the zoom at any point of the lesson.

1. Fly

   Where on this body would you look for a nose?

2. Brain

   The brain fills most of the head. What do you think takes up so much of it?

3. Circuit

   The smell comes in at the orange cells. Which color do you think it reaches last?

4. Neuron

   The dot is one spike. How could one cell tell the rest of the brain about a smell?

The body and the brain come from two different flies, shown together at their true sizes. Say so to the class.

Before the silence step, students pick what they think will happen. Take a show of hands first. Nothing is scored, and a wrong guess costs nothing.

1. Step 1

   A fly smells with its antennae (its two feelers). Tap Stimulate to send a smell in. Watch the orange neurons light up.

   Only the orange smell receptors light up. The signal starts at one edge of the circuit, not everywhere at once.

2. Step 2

   Tap Stimulate again and follow the colors. Yellow projection neurons (the smell messengers) carry the smell inward. Blue Kenyon cells (the smell sorters) catch it next.

   The colors light in order: orange, then yellow, then blue. The signal moves through the layers one after another.

3. Step 3

   Tap Stimulate once more. Now watch the pink output neurons (the memory readers). They read the blue pattern.

   The pink output neurons light last, and only after the blue Kenyon cells.

4. Step 4

   Now silence the Kenyon cells (the smell sorters). We will send the smell for you. What disappears?

   Orange and yellow still light, blue stays dark, and pink almost vanishes. The smell got in but could go no further.

5. Step 5

   Tap the Kenyon cells (the smell sorters) button again to switch them back on. We will send the same smell.

   Pink returns as soon as the Kenyon cells are back on. Nothing was damaged: the signal simply had no path.

You silenced the Kenyon cells (the blue cells). Why did the pink cells go dark?

The right choice: The blue cells pass each smell’s pattern to the pink cells.

The Kenyon cells pass each smell’s pattern on to the output neurons. With them silenced, the smell still reaches the receptors and the projection neurons, but the output neurons get nothing to read. Send students who answer “the smell never got in” back to look at the orange and yellow cells.

1. The smell still entered the brain while the Kenyon cells were silenced. How do we know?
2. Suppose the fly learned that this smell means sugar. Where in this circuit would you expect something to change?
3. What would you expect to see if you silenced the yellow projection neurons instead? Test it in free play.

• Students may think: A memory sits inside one cell, like a file.

  How to address it: Go back to the step where pink reads blue. The memory is thought to be in the strength of many connections between the two groups, not in a single cell.

• Students may think: Silencing a cell kills or damages it.

  How to address it: Run the last step again: the pink light returns at once. Silenced cells are whole. They are just not firing.

• Students may think: The lesson shows the fly learning a smell.

  How to address it: It does not. The model shows where a memory would be stored, but its connections never change. Say so plainly, and ask what would have to be added.

The full list, with numbers

Antennae.

The two feelers on a fly’s head. A fly smells with them, the way you smell with your nose.

Smell receptors.

Nerve cells on the antennae that fire when a smell lands on them. Every smell starts here.

Antennal lobe.

The first stop for smells inside the fly’s brain, where the smell receptors hand their signal to the projection neurons.

Projection neurons.

Nerve cells that carry the smell signal from the antennal lobe deeper into the brain, to the mushroom body.

Mushroom body.

A mushroom-shaped part of an insect’s brain where smells are sorted and smell memories are stored.

Kenyon cells.

The small nerve cells of the mushroom body. Each smell switches on a different handful of them, so every smell gets its own pattern.

Output neurons.

Nerve cells that read the pattern of the Kenyon cells and pass it on to the rest of the brain. Scientists call them MBONs, short for mushroom body output neurons.

This section is not filled in yet. The match to the official biology curriculum is being confirmed with biology instructors.