A nerve cell receives many synapses from other neurons (and sometimes from itself). Each time one of these axons conducts an action potential, the presynaptic terminal releases neurotransmitters that can open “chemically gated” ion channels on the postsynaptic neuron (the neuron that receives the terminal). The opening of the ion channels produce local, graded changes in the resting potential of the neuron. If it depolarizes the cell (reduces the potential difference between the inside and outside of the neuron), the small change is called an excitatory postsynaptic potential (or EPSP). If it hyperpolarizes the cell (makes the cell’s internal potential more negative with respect its exterior), then it is called an inhibitory postsynaptic potential (or IPSP). All of the EPSPs and IPSPs add to change the membrane potential. A nerve impulse is generated when the membrane potential reaches a critical threshold. This threshold is typically about –55 millivolts. If the neuron does not reach this critical threshold it does not fire an action potential.