When neurotransmitter-gated ion channels change conductance in a postsynaptic membrane, they produce a postsynaptic potential: a local change in membrane voltage.

For a channel that passes one dominant ion, opening the channel tends to move the membrane toward that ion's equilibrium potential. A channel that passes several ions has a reversal potential $E_{\rm rev}$: the membrane voltage at which the net current through that channel population is zero.

A simple current model is

$$\boxed{I_{\rm syn}=g_{\rm syn}(V_m-E_{\rm rev})},$$

where $g_{\rm syn}$ is the synaptic conductance. The sign convention for current may vary, but the physical rule does not: opening the channel drives $V_m$ toward $E_{\rm rev}$.

Excitatory postsynaptic potentials

An excitatory postsynaptic potential (EPSP) increases the probability that the postsynaptic cell will reach action-potential threshold.

For example, opening a mixed cation channel with a reversal potential near 0 mV in a neuron resting near $-70$ mV drives the membrane toward 0 mV and therefore depolarizes it.

Inhibitory postsynaptic potentials

An inhibitory postsynaptic potential (IPSP) or inhibitory synaptic conductance reduces the probability of firing.

Inhibition can occur by hyperpolarizing the membrane, for example by opening K$^+$ channels whose reversal potential is more negative than the current voltage.

But inhibition does not have to produce a large hyperpolarization. If a Cl$^-$-conducting synapse has a reversal potential close to the resting voltage, opening many such channels can hold the membrane near that voltage and reduce the effect of simultaneous excitatory currents. This is shunting inhibition.

Excitation and inhibition depend on the electrical context

A receptor is not excitatory simply because it passes positive ions, nor inhibitory merely because it passes Cl$^-$. The effect depends on

• the channel's ion selectivity;
• its reversal potential;
• the current membrane potential;
• where the resulting voltage lies relative to action-potential threshold.

The most reusable rule is therefore: synaptic conductance pulls membrane voltage toward its reversal potential, and excitation or inhibition describes how that shift changes spike probability.