The 30-millisecond escape

Students follow a reflex from the eyes to the leg muscles in about 30 milliseconds, and show that the fast escape depends on one link, the giant fiber.

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.

Reflex arcs

A whole reflex pathway in three parts: the sensory side (the looming neurons, which report what the eyes see), a relay (the giant fiber), and motor neurons (the jump neurons). Unlike a knee-jerk reflex, this relay runs through the brain.

How a network’s structure determines behavior

The signal narrows from hundreds of looming neurons onto the giant fiber. That bottleneck is why silencing one kind of cell removes the behavior.

Synapses and neurotransmitters

The giant fiber is thick, which makes its signal fast. In a real fly it also uses electrical synapses, the fastest kind; the model has chemical synapses only.

1. 5 min

   Opening question

   Ask: “Why is it so hard to swat a fly?” Collect a few ideas, then say that the class will time the fly’s escape and find the wire behind it.

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 is resting when a swatter swoops in, and he jumps clear in about 30 milliseconds. The story sets up the question: what in a brain can be that fast?

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

   Which parts of this body does the fly need in order to escape?

2. Brain

   The signal starts in the eyes and ends in the legs. Where does it have to pass on the way?

3. Circuit

   Hundreds of blue cells, and just two red lines, one on each side. What do you notice about the route?

4. Neuron

   This one cell is unusually thick. Why would a thick wire help a fast escape?

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 swatter swoops at the fly. Tap Stimulate to fire the looming neurons (the swatter spotters).

   The blue looming neurons light first, and the red giant fiber follows almost at once.

2. Step 2

   Tap Stimulate again. Follow the giant fiber (the escape wire) down into the nerve cord (like a spine).

   Activity runs down the giant fiber into the nerve cord, and the yellow jump neurons fire within 30 milliseconds of fly time.

3. Step 3

   Now silence the giant fiber (the escape wire). We will send the swatter. Watch the jump neurons (the leg kickers).

   Blue still lights, red stays dark, and yellow never fires. The fly saw the swatter, but the message did not reach its legs.

4. Step 4

   Tap the giant fiber (the escape wire) button again to switch it back on. We will send the same swatter.

   Yellow fires again as soon as the giant fiber is back on.

You silenced the giant fiber (the escape wire). Why did the leg kickers stay dark?

The right choice: The escape wire carries the jump signal to the leg kickers.

The giant fiber carries the jump signal from the looming neurons to the jump neurons. With it silenced, the fly still sees the swatter, because the blue cells light up, but the signal stops before the legs. Real flies also have slower ways out, as the lesson’s last step says.

1. This escape is faster than a blink. Which features of the pathway make it so fast?
2. Why might it help an animal to have one fast escape route and several slower ones?
3. Compare this pathway with a human reflex you know, such as pulling your hand away from something hot. What is the same, and what is different?

• Students may think: The fly decides to jump.

  How to address it: The fastest jump works like a reflex: the wiring carries the signal straight through. Slower escapes leave a little time to aim the jump, but no time to think it over.

• Students may think: If the fly did not jump, it did not see the swatter.

  How to address it: The silence step separates the two. The looming neurons fire, so the fly saw it. Seeing and acting are different stages of one pathway.

• Students may think: One neuron cannot matter in a whole brain.

  How to address it: Here one kind of cell carries the entire fast signal. Wherever a pathway narrows to a few cells, each of them matters a great deal.

The full list, with numbers

Looming neurons.

Nerve cells behind the fly’s eyes that fire when something grows quickly in view, like a swatter coming closer. Scientists call them LC4 and LPLC2.

Giant fiber.

A very thick nerve cell that runs from the brain down into the nerve cord. It carries the fly’s fastest escape signal. A fly has two, one on each side.

Nerve cord.

The fly’s version of a spinal cord: a bundle of nerve cells in its chest that controls the legs and wings.

Jump neurons.

Motor neurons that make the jump muscles of the middle legs contract, so the fly springs into the air.

Reflex arc.

The route a reflex takes: from sensory neurons, through one or more relay cells, to the motor neurons that move a muscle.

Sensory neuron.

A neuron that picks up something from the world, such as light or a smell, and reports it to the brain.

Motor neuron.

A neuron that carries a command out to a muscle and makes it contract.

Electrical synapse.

A direct channel between two neurons that passes a signal with almost no delay. The map behind the lessons records chemical synapses only.

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