During synaptic transmission, neurotransmitters bind to receptors on the postsynaptic neuron, leading to the opening of specific ion channels. For Excitatory Postsynaptic Potentials (EPSPs), the primary ions that cross the neuron membrane are sodium ions (Na+). When these channels open, Na+ ions flow into the neuron, making the inside of the cell more positively charged and moving it closer to the action potential threshold.
In contrast, for Inhibitory Postsynaptic Potentials (IPSPs), the main ions involved are chloride ions (Cl−). The influx of Cl− ions or the efflux of potassium ions (K+) results in hyperpolarization of the neuron, making it less likely to fire an action potential.