How a fly knows which way it’s facing

Students see that a ring of neurons holds the fly’s heading as one patch of activity, and that inhibition from the ring neurons keeps that patch in place.

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.

Excitation and inhibition

The clearest example in the three lessons. The turn neurons excite the compass and push the bump along; the ring neurons inhibit it and hold the bump still.

Synapses and neurotransmitters

The ring neurons are predicted to release GABA, a transmitter that inhibits. The compass and turn neurons release acetylcholine, which excites.

How a network’s structure determines behavior

The compass neurons are wired in a ring, and that shape is what lets one bump of activity stand for one direction.

1. 5 min

   Opening question

   Ask: “Close your eyes and turn around once. How do you still know which way you are facing?” Then say that a fly does this with a small ring of cells, and the class is about to push them around.

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 heads for a gap of light, a draft turns him, and he steers back on course. The story sets up the question: how does a brain know which way its body is facing?

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

   The fly has no map and no GPS. What could it use to hold a straight course?

2. Brain

   Which senses would a compass in here need to listen to?

3. Circuit

   These cells form a ring. Why might a ring be a good shape for a compass?

4. Neuron

   This is one heading cell. If it fires, what do you think it says about where the fly is facing?

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

   Tap Stimulate to fire the left turn neurons (the turning signal). Then watch the compass dial.

   A patch of green compass neurons lights on one side of the ring, and the needle of the dial points at it. That patch is the bump.

2. Step 2

   Now tap Stimulate on the right turn neurons (the other signal). Keep your eyes on the bump.

   The bump moves around the dial. The two turn signals push it in opposite directions.

3. Step 3

   Silence the ring neurons (the landmark carriers). They bring in what the fly sees. We will fire the right turning signal for you.

   With the violet ring neurons off, the same signal leaves the bump wandering instead of settling on one spot.

4. Step 4

   Tap the ring neurons (the landmark carriers) button again to switch them back on. We will send the same turning signal.

   The bump holds its place again. Compare the dial with the step before.

You silenced the landmark carriers (the violet cells). Why did the bump drift?

The right choice: Nothing told the compass where the landmarks were, so the bump slid.

Nothing told the compass where the landmarks were, so nothing held the bump in place. The turn signal still arrived, because the green cells lit up, so what was missing was an anchor and not an input. For older students: in this model the anchor is the inhibition from the ring neurons.

1. Why is a ring a good shape for cells that keep track of direction?
2. The ring neurons quiet other cells instead of exciting them. How can switching cells off help a network work better?
3. People also lose their sense of direction in the dark or in fog. What does that have in common with the silence step?

• Students may think: The fly has a magnetic compass, like the one in a phone.

  How to address it: This compass is a pattern of activity that the fly updates from its own turns and from what it sees. Nothing in this circuit senses magnetism.

• Students may think: More active cells always make a better signal.

  How to address it: Compare the last two steps: taking inhibition away made the compass worse. Inhibition is part of how the circuit works.

• Students may think: The bump is an object that travels through the brain.

  How to address it: No cell moves. Neighboring cells take turns being active, like a wave going around a stadium crowd.

The full list, with numbers

Turn neurons.

Nerve cells that fire when the fly turns and push the bump around the compass. Scientists call them P-EN2 neurons.

Compass neurons.

A ring of nerve cells that keeps track of which way the fly is facing. Scientists call them E-PG neurons.

Activity bump.

The patch of active compass neurons. Where it sits on the ring shows which way the fly is facing.

Ring neurons.

Nerve cells that bring what the fly sees, such as landmarks, to the compass. They quiet the compass neurons, and that holds the bump in place.

Landmark.

Something the fly sees that stays in place, like the sun or a tree. The fly uses it to keep its sense of direction steady.

Protocerebral bridge.

A bar-shaped part of the fly’s brain, the protocerebral bridge. The compass neurons and the turn neurons both run through it.

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