Neuroscience glossary

Every scientific term the lessons use, explained in a sentence or two. Open a term to see where it shows up in a real fruit fly’s brain.

Basic terms

Neuron: A nerve cell. It receives signals from other cells and, when enough of them add up, sends a signal of its own.
Action potential (spike): A brief electrical pulse that an active neuron sends along its length, also called a spike or an action potential. In the lessons, each flash of light marks one.
Synapse: The junction where one neuron passes its signal to the next. The more synapses two cells share, the stronger the link between them.
Neurotransmitter: A chemical that a neuron releases at a synapse to pass its signal on. Some neurotransmitters excite the next cell, and others inhibit it.
Excitation: A signal that pushes the next neuron toward firing.
Inhibition: A signal that holds the next neuron back from firing. A network needs it to stay stable and precise.
Acetylcholine: The main excitatory neurotransmitter in a fly’s brain. In the lessons, cells that release it excite the cells they connect to.
GABA: An inhibitory neurotransmitter, in flies and in people. In the lessons, cells that release GABA quiet the cells they connect to.
Neural circuit: A group of neurons wired together to do one job, such as recognizing a smell or setting off a jump.
Connectome: A complete wiring diagram of a nervous system: every neuron and every synapse between them. The lessons run on the connectome of one real fruit fly.

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.

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.

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.