When an action potential reaches the synaptic terminal of the axon, it encounters a region called the synapse, where two neurons come close together but do not touch one another. A minuscule gap, called a synaptic cleft, separates the synaptic terminal of the first or pre-synaptic neuron from the dendrite or cell body of the second or post-synaptic neuron. The arrival of an action potential at the synaptic terminal causes the synaptic terminal to become positively charged and release neurotransmitters into the synaptic cleft. The neurotransmitters diffuse rapidly across the synaptic cleft and bind to receptors in the membrane of the dendrites or cell body of the post-synaptic cell. Each type of receptor in the post-synaptic membrane binds to a specific type of neurotransmitter, and after binding to a neurotransmitter, the receptor causes specific types of ion channels in the post-synaptic membrane to open. Desion channels are opened. Ions flow across the cell membrane of the post-synaptic neuron along their concentration gradients. This flow of ions into the post-synaptic neuron causes a post-synaptic potential in the dendrites or cell body, depending on the channels open and the ions that flow in. The synaptic potentials can be either excitatory, making a neuron less negative inside and more likely to fire an action potential or inhibitory, making it more negative and less likely to fire. An excitatory post-synaptic potential is called an EPSP. An inhibitory post-synaptic potential is called an IPSP. A synapse that produces EPSPs in the post-synaptic cell is called an excitatory synapse, while a synapse producing IPSPs is an inhibitory synapse. Post-synaptic potentials cannot travel far in a neuron. After a few millimeters at most, the ions leak back across the membrane and the signal is lost. However, post-synaptic potentials travel far enough to reach the cell body, where they determine whether or not an action potential will be produced.