In this animation, presynaptic neurons fire onto a postsynaptic neuron. The color "yellow" represents the neural impulse (action potential) that travels down the length of the axon. When the postsynaptic neuron receives the messages, "excitatory postsynaptic potentials" (EPSPs) (represented by the color "green") result and are strong enough to cause the membrane potential at the axon hillock of the postsynaptic neuron to reach "threshold." When threshold is reached, that neuron fires and sends an excitatory message on to another postsynaptic neuron.  Click on the "Play" button to see a slowed version of the previous neurotransmission animation. Be sure to click on the "Rewind" button before you replay the animation.