To fire the initial signal, click on the black square in the bottom-left corner. The dark blue/dark red zones represent the synaptic gap. The "neuron" with the dark red circle will take a red/yellow signal and output a blue signal. The "neuron" with the dark blue circle will take a blue/yellow signal and output a red signal. The "neuron" with the grey circle will "extend" a signal.
For my psychology project. The black line represents the axon. The circle represents the dendrites (where the neurotransmitter is received) The square represents the synaptic terminal(s) of the neuron.