“Injecting one brief pulse of serotonin near the synapse strengthened the synaptic connection between the neurons for a few minutes by enhancing the release of glutamate from the sensory cell. This short-term-enhancement of synaptic strength is a functional change: it does not require the synthesis of new proteins. In contrast, five separate pulses of serotonin, strengthened the synaptic connection for days and led to the growth of new synaptic connections, an anatomical change that did involve the synthesis of new protein.”
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)
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) The synapse is sandwiched by many surrounding structures including “glial cells,” other axons and dendrites, and other synapses. (Kolb, 153) Editor's note - adjective: ‘synaptic,’ verb: ‘synapse.’ Also referred to as ‘neuronal synapse.’
Binding: neurotransmitters travel across the synapse like rafts across a river and attach themselves to receptors on the other side of the synapse. The "activation" of these receptors results in another electric event. (Goldberg2, 28)
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.’
Release: the arrival of a “nerve impulse” at an axon terminal causes the synaptic vesicles to discharge their neurotransmitter molecules into the synaptic space between the neuron that released them and the adjacent neuron. (The Brain-Leslie Iversen, 76)
Synaptic Potential: a local signal restricted to a certain space. Does not propagate actively. (Kandel, 449) A change in the “membrane potential” of a postsynaptic neuron. A synaptic potential can be either excitatory or inhibitory. If sufficiently strong, an excitatory synaptic potential will trigger an action potential in the postsynaptic cell. (Kandel, 450) Also referred to as ‘local signal.’
Synaptic Regulation: once synaptic transmission has been completed, the neurotransmitter molecules must be rapidly inactivated; otherwise they would act for too long. (The Brain-Leslie Iversen, 78) If neurotransmitters are not taken up efficiently, communication across a synapse will fail because the gulf will become saturated with stale messages. If neurotransmitters are taken up too quickly, the message will appear too briefly to have full effect on the postsynaptic cell. (Fields, 23) By regulating the flow of information across a synapse, (neural pathways) can be strengthened or weakened, in effect allowing the (pathways) to change their behavior from experience - that is to “learn.” (Fields, 22) All excitation has to be regulated, both to maintain normal functions and to prevent injury. (LeDoux, 53)
Enzymatic Degradation: "enzymes" in the synaptic cleft destroy (unneeded) neurotransmitters. (Kolb, 155) For example, the neurotransmitter "acetylcholine" is destroyed by the enzyme ‘acetylcholinesterase,’ which can cleave 25,000 molecules of the transmitter per second. (Fields, 22)
Reuptake: protein molecules in the "glial" membrane pump the neurotransmitter out of the synaptic cleft and into the "astrocyte" where it is reprocessed. After filtering out the neurotransmitter and recycling it, the astrocyte delivers the reprocessed substance back to the presynaptic terminal. The neuron then carries out a simple chemical reaction to convert the inert neurotransmitter back into active neurotransmitter and repackages it into “synaptic vesicles.” (Fields, 22) Reuptake has the advantage over enzymatic degradation in that the neurotransmitter molecules can be conserved through several cycles of release and recapture. (The Brain-Leslie Iversen, 78)
Synaptic Diffusion: some of the (unneeded) neurotransmitter simply diffuses away from the synaptic cleft and is no longer available to "bind" to "receptors." (Kolb, 155)
Synaptic Strength: the (effectiveness) of communication between a pair of connected neurons. If two neurons are strongly connected, the message between them comes in loud an clear, but if they are weakly connected, the messages are faint. (Cerebrum2009, 71)
Hebbian Learning: changes in the connection strength between two neurons caused by the fact that the postsynaptic cell was active when presynaptic inputs arrived. (LeDoux, 137)
Synaptic Structures: 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)
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.’
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) In a conventional neurotransmitter, ‘synaptic vesicles’ merge with the cell membrane and release the neurotransmitter into the synapse, which is called ‘exocytosis.’ (Campbell, BSP210) Synaptic vesicles are concentrated at presynaptic terminals. They (also) actively (capture) transmitter molecules from the “cytoplasm.” Neurotransmitter 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 (connect) 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.’