Inside a single neuron, a signal travels as an electrical event, a wave of changing voltage running the length of the cell. But neurons aren't fused together into one continuous electrical circuit — there's a physical gap between one neuron and the next, called the synapse, and no electrical current can simply leap across it. Getting a signal from one neuron to the next requires converting that electrical event into a chemical one, sending it physically across the gap, and converting it back into an electrical signal on the other side.
An electrical signal arrives, and triggers a chemical release
When an electrical signal, called an action potential, reaches the very end of a transmitting neuron's axon, it triggers voltage-sensitive channels to open, letting calcium ions flow into the cell. That calcium influx causes small membrane-bound sacs called synaptic vesicles, each packed with molecules called neurotransmitters, to fuse with the cell membrane and release their contents into the narrow gap between the two neurons — the synaptic cleft. This is the point at which the signal genuinely stops being an electrical phenomenon and becomes a chemical one: a burst of neurotransmitter molecules, physically diffusing across a gap only tens of nanometres wide, carrying the message the rest of the way by sheer molecular diffusion rather than by any electrical conduction.
Converting the chemical signal back into an electrical one
On the receiving side of the synapse, neurotransmitter molecules bind to specific receptor proteins embedded in the receiving neuron's membrane. Depending on which neurotransmitter and which receptor type is involved, this binding causes ion channels to open or close in ways that either make the receiving neuron more likely to fire its own action potential (an excitatory effect) or less likely to (an inhibitory effect). A single neuron typically receives synaptic input from thousands of other neurons simultaneously, and whether it ultimately fires its own signal depends on the combined, continuously shifting balance of excitatory and inhibitory chemical signals arriving at all its synapses at once — meaning the synapse isn't just a relay point passing a signal along unchanged, but the actual site where a huge amount of the nervous system's real computation and integration of information happens.
What we're still unsure about
The basic mechanics of synaptic transmission — vesicle release, neurotransmitter diffusion, and receptor binding — are extremely well characterised through decades of neuroscience research, down to the molecular level for many specific neurotransmitter systems. What remains a genuinely active area of research is understanding exactly how the vast, continuously shifting pattern of synaptic strength across billions of individual synapses gives rise to specific memories, thoughts, and behaviours at the level of the whole brain — neuroscientists can describe a single synapse's mechanism in fine detail without yet having a full account of how that mechanism, repeated at massive scale, actually produces coherent cognition.
This sits inside Synaptic Transmission & Neurotransmitters, one of seven topics in Neuroscience, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.