Synapse: from the Greek synaptein, meaning to bind together. Location where communication between neurons takes place. Where the “axon terminals” of the sending neuron reach out to, but do not quite touch, the end of the “dendrite” branch of the receiving neuron. A neuron uses its dendrites to receive "signals" from other nerve cells and its “axon” to send signals to other cells. (Kandel, 65)

Individual neurons themselves are complex entities with unique morphologies and thousands of “inputs” and “outputs.” Their interconnections, the synapses, come equipped with learning “algorithms” that modify their strength. (Koch, 10) The synapse is sandwiched by many surrounding structures including “glial cells,” other axons and dendrites, and other synapses. (Kolb, 153) Each neuron makes anywhere from a thousand to ten thousand synapses with other neurons. A synapse can be either on ("excitatory") or off ("inhibitory"). A piece of the brain the size of a grain of sand would contain one billion synapses. (Ramachandran, 8) While synapses themselves don’t account for everything the brain does, they do participate crucially in every act or thought that we have, and in every “emotion” we express and experience. (LeDoux, 64) Editor's note - adjective: ‘synaptic,’ verb: ‘synapse,’ meaning the action of synaptic transmission. Also referred to as ‘neuronal synapse.’

Chemical Synapse: an (extracellular) junction where “messenger molecules” are released when stimulated by an action potential. (Kolb, 153) A synapse where one cell releases “neurotransmitters” into the “synaptic cleft," and the neurotransmitter binds to receptors of the other cell’s "membrane." (Patestas, 29)

Electrical Synapse: a synapse where ions or small molecules may go from one cell into another cell by traversing small, contiguous channels present in the cell membranes of the two cells. (Patestas, 28) Fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next. (Kolb, 153) In electrical synaptic transmission, electrical signals are communicated as a "current" flow across electrical synapses. (MeSH) Also referred to as a 'gap junction.’

Postsynaptic Neuron: the neuron that receives signals from another neuron at a synapse. The signals affect the excitability of the post-synaptic cell. (Kandel, 445) (It’s) membrane is on the transmitter-input side of a synapse. (Kolb, 153) Also referred to as ‘target,’  ‘target cell,’  ‘postsynaptic cell,’  ‘receiving cell,’ and ‘receiving neuron.’

Synaptic Receptor(s): specialized (cell surface) proteins on the outer surface of the target cell membrane, that recognizes and captures a neurotransmitter that has been released into the synaptic clef. (Kandel, 91) Large protein molecules that act as ‘biological machines.’ In each receptor there is a passageway (“channel”) that can open into the dendrite (or into the “cell body”) of the receiving neuron when a neurotransmitter is detected. When the tunnel through the receptor opens briefly, charged “ions” floating in solution leak out, reducing the “voltage” inside the postsynaptic neuron. If the voltage change is big enough, the voltage drop triggers the postsynaptic neuron to "fire" an (action potential) out of its own axon to signal the next neuron in the “circuit.” (Fields, 20) “Glutamate receptors” tend to be located out on the dendrites, especially on the (dendritic) spines, whereas “GABA receptors” tend to be found on the cell body, or on the part of dendrites close to the cell body. (LeDoux, 56) Also referred to as ‘receptor,’ ‘receptor protein,’ and ‘postsynaptic receptor.’ Editor's note - not to be confused with "sensory receptors.”

GABA Receptor: cell-surface protein that binds “gamma-aminobutyric acid” and triggers changes that influence the behavior of cells. GABA receptors control “chloride channels.” (MeSH) A variety of tranquilizing drugs, including “barbiturates,” “alcohol,” and general “anesthetics,” bind to GABA receptors and produce a calming effect on “behavior” by enhancing the receptors’  "inhibitory" functions. (Kandel, 99) When GABA receptors are occupied, the inside of the cell becomes more negative due to the influx of negative ions, especially "chloride," through a passage in the receptor. This makes it harder for glutamate released from other terminals to sufficiently trigger an action potential. (LeDoux, 55) Editor’s note - a GABA receptor may be a “ionotropic receptor” or a “metabotropic receptor.”

Glutamate Receptor: when glutamate binds to the outside part (of this receptor), a passage opens up through the receptor, allowing positively charged ions in the “extracellular fluid” to move inside the cell, which changes the chemical balance between outside and inside. If enough glutamate receptors are occupied at the same time, the voltage inside becomes sufficiently positive, and then an action potential occurs. (LeDoux, 55) Editor’s note - a glutamate receptor may be an “ionotropic receptor” or an “metabotropic receptor.”

Ionotropic Receptor: one of two major categories of receptors. A protein that spans the cell surface-membrane, and contains a neurotransmitter “binding site” and an “ion channel” through which ions can pass. The binding of the appropriate neurotransmitter directly opens or closes the ion channel to the movement of ions. (Kandel, 449) Editor’s note - ionotropic receptors are usually closed until a “ligand” binds to the receptor. They are examples of “ligand-gated ion channels.”

Metabotropic Receptor: a protein on the cell surface that binds a neurotransmitter or “hormone” (the "first messenger") and then “activates” a chemical inside the cell (the "second messenger") that initiates a cell-wide response. (Kandel, 442) Editor’s note - metabotropic receptors do not have ion channels, but if a neurotransmitter binds to them, it may cause the opening of an ion channel somewhere else on the postsynaptic neuron membrane.

Opiate Receptor: cell membrane proteins that bind “opioids” and trigger intracellular changes which influence the behavior of cells. The “endogenous” “ligands” for opioid receptors in mammals include three families of peptides, the “enkephalins,” “endorphins,” and ‘dynorphins.’ (MeSH) Editor’s note - these are receptors involved in the perception and integration of “pain” and emotional experience. They are found in certain regions of the brain that bind opiate drugs. They are also concentrated in spinal cord areas. Also referred to as ‘opioid receptor.’

Presynaptic Neuron: the neuron that sends signals to another neuron at a synapse. (Kandel, 445) A nerve cell by which a wave of excitation is conveyed to a synapse. (GHR) (It’s) membrane is on the transmitter-output side of a synapse. (Kolb, 153) “Synaptic vesicles” are concentrated at presynaptic terminals. They actively (capture) transmitter molecules from the “cytoplasm.” In at least some synapses, transmitter release occurs by fusion of these vesicles with the presynaptic membrane, followed by (release) of their contents. (MeSH) Editor’s note - axons can connect to another neuron’s dendrites (‘axodendritic’) or directly to another neuron’s cell body (‘axosomatic’). They can also synapse directly onto muscles in neuromuscular junctions (‘axoaxonal’). Also referred to as ‘presynaptic cell,’  ‘sending cell,’ and ‘sending neuron.’

Synaptic Cleft: small gap at the synapse between one neuron’s axon and another neuron’s dendrite. (Kandel, 65) A tiny gulf of saltwater that bathes every cell in the body. (Fields, 19) Infinitesimally narrow (25 billionths of a meter). The (invention of the) "electron microscope" proved that every synapse in the body has a gulf of separation between the presynaptic and the postsynaptic neurons. A message passes across the synapse in about one-tenth of an eye blink, but compared with the two hundred mile per hour speed of the neural impulse, the synapse slows information flow much like a tool booth on a turnpike. (Fields, 20) Also referred to as ‘synaptic gulf.’