A dye that glows brighter in the presence of calcium ions is loaded into the neurons. Neurotransmitters released by an upstream neuron in a network lead to rapid calcium influx in downstream neurons, seen here as a sudden burst of green. The influx triggers an electrochemical propagation of a signal down the length of the neuron, called the axon, and causes the subsequent release of more neurotransmitters, which signals the next neurons in the same network to fire.