WASHINGTON >> Thanks to a mouse watching clips from “The Matrix,” scientists have created the largest functional map of a brain to date — a diagram of the wiring connecting 84,000 neurons as they fire off messages.

Using a piece of that mouse’s brain about the size of a poppy seed, the researchers identified those neurons and traced how they communicated via branch-like fibers through a surprising 500 million junctions called synapses.

The massive dataset, published Wednesday by the journal Nature, marks a step toward unraveling the mystery of how our brains work. The data, assembled in a 3D reconstruction colored to delineate different brain circuitry, is open to scientists worldwide for additional research — and for the simply curious to take a peek.

“It definitely inspires a sense of awe, just like looking at pictures of the galaxies,” said Forrest Collman of the Allen Institute for Brain Science in Seattle, one of the project’s leading researchers. “You get a sense of how complicated you are. We’re looking at one tiny part ... of a mouse’s brain and the beauty and complexity that you can see in these actual neurons and the hundreds of millions of connections between them.”

How we think, feel, see, talk and move are due to neurons, or nerve cells, in the brain — how they’re activated and send messages to each other. Scientists have long known those signals move from one neuron along fibers called axons and dendrites, using synapses to jump to the next neuron. But there’s less known about the networks of neurons that perform certain tasks and how disruptions of that wiring could play a role in Alzheimer’s, autism or other disorders.