Brain functions, like perceptions, thoughts, and feelings, are the result of communication between neurons. To efficiently communicate and process information, neurons form connections with each other. Through these connections, they exchange information via messenger molecules. There are many messenger systems in the brain. They consist of two major players: a set of molecules that act as keys (called ligands) and another set of molecules that act as locks (called receptors). Each receptor has a specific ligand like each lock has a specific key. All messenger systems also have molecules called enzymes to break down excess ligands to keep the messengers always in the right amount. Most of the messenger systems communicate in one direction, where the ligand is released from neuron 1 and binds to receptors on neuron 2. There are also some that communicate in the opposite direction, from neuron 2 back to neuron 1.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2020.00041