When a neuron sends a message to another neuron, a change is produced in the postsynaptic neuron.  That change may be excitatory or inhibitory. In the case of excitation, the change in the postsynaptic neuron is called an "excitatory postsynaptic potential" (EPSP). If the EPSP is strong enough to cause the membrane potential at the axon hillock to reach "threshold" then the postsynaptic neuron will also fire and send a message to other neurons. Some EPSPs are not strong enough, however, to cause the postsynaptic neuron to fire. In that case the membrane potential will quickly return to "resting potential" (normal). In the animation below, the presynaptic neuron will produce an EPSP in the postsynaptic neuron that quickly dissipates and does not cause the postsynaptic neuron to fire. Click on the "Play" button to see the animation. You must click on the "Rewind" button before you can replay the animation.

Let's slow this process down now so we can take a closer look:  CLICK HERE!