Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials. They are caused by the presynaptic neuron releasing neurotransmitters from the terminal bouton at the end of an axon into the synaptic cleft. The neurotransmitters bind to receptors on the postsynaptic terminal, which may be a neuron or a muscle cell in the case of a neuromuscular junction. These are collectively referred to as postsynaptic receptors, since they are on the membrane of the postsynaptic cell. One way receptors can react to being bound by a neurotransmitter is to open or close an ion channel, allowing ions to enter or leave the cell. It is these ions that alter the membrane potential. Ions are subject to two main forces, diffusion and electrostatic repulsion. Ions will tend towards their equilibrium potential, which is the state where the diffusion force cancels out the force of electrostatic repulsion. When a membrane is at its equilibrium potential, there is no longer a net movement of ions. Two important equations that can determine membrane potential differences based on ion concentrations are the Nernst Equation and the Goldman Equation. Neurons have a resting potential of about −70 mV. If the opening of the ion channel results in a net gain of positive charge across the membrane, the membrane is said to be depolarized, as the potential comes closer to zero. This is an excitatory postsynaptic potential (EPSP), as it brings the neuron's potential closer to its firing threshold (about −55 mV). If, on the other hand, the opening of the ion channel results in a net gain of negative charge, this moves the potential further from zero and is referred to as hyperpolarization. This is an inhibitory postsynaptic potential (IPSP), as it changes the charge across the membrane to be further from the firing threshold.