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A <synapse> is a signaling junction between a <neuron> and another <biological cell>. The sending side is presynaptic and the receiving side postsynaptic. At a <chemical synapse>, released <neurotransmitter> acts on a <receptor> in the receiving <cell membrane>. At an <electrical synapse>, <gap junctions> provide a direct path for <electric current> between cells. See <Neuronal Dynamics: From Single Neurons to Networks and Models of Cognition by Wulfram Gerstner and others/1.1 Elements of Neuronal Systems>.

In a common sequence at a <chemical synapse>, an arriving <action potential> opens <voltage-gated ion channels> selective for <calcium>. <Calcium> entry triggers fusion of <synaptic vesicles> with the presynaptic <cell membrane>, releasing <neurotransmitter> into the <synaptic cleft>. Binding to a <receptor> changes postsynaptic <electrical resistance and conductance>, directly through an <ion channel> or through an intracellular signaling pathway. The resulting change in <membrane potential> is a <synaptic potential>. See <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Ion Channels and the Electrical Properties of Membranes>.

A useful conductance approximation writes the inward-positive synaptic <electric current> as
$$
I_{\mathrm{syn}}=g_{\mathrm{syn}}(t)(E_{\mathrm{syn}}-V),
$$
where $g_{\mathrm{syn}}\geq0$ describes the activated <electrical resistance and conductance>, $V$ is the postsynaptic <membrane potential>, and $E_{\mathrm{syn}}$ is the <reversal potential>, the voltage at which this current vanishes. Opening these channels tends to pull $V$ toward $E_{\mathrm{syn}}$; it does not prescribe a current of fixed sign independently of $V$.

For a worked example, let $V=-65\,\mathrm{mV}$ and $g_{\mathrm{syn}}=1\,\mathrm{nS}$. A <reversal potential> of $0\,\mathrm{mV}$ gives $I_{\mathrm{syn}}=65\,\mathrm{pA}$, an inward <electric current> that raises <membrane potential>. A <reversal potential> of $-75\,\mathrm{mV}$ instead gives $-10\,\mathrm{pA}$, which lowers <membrane potential>. These are illustrative calculations, not measurements. A <reversal potential> of $-60\,\mathrm{mV}$ can raise $V$ from $-65\,\mathrm{mV}$ yet oppose reaching a threshold of $-50\,\mathrm{mV}$: a depolarizing input need not be excitatory.

An excitatory <synapse> favors a subsequent <action potential>, whereas an inhibitory <synapse> opposes it. The effect depends on the <receptor>, the permeating <ions>, their gradients and the <reversal potential>; a <neurotransmitter> does not have one fixed excitatory or inhibitory sign in every context. Even with $E_{\mathrm{syn}}=V$, an added conductance can reduce responses to other inputs by lowering effective <electrical resistance>, an effect called <shunting inhibition>. See https://www.ncbi.nlm.nih.gov/books/NBK11117/[Purves and colleagues: excitatory and inhibitory postsynaptic potentials]. This dependence matters when translating a biological <synapse> into a signed weight or an imposed <electric current> in <computational neuroscience>.


 Synapse (source code)