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.
— Eric Kandel, In Search of Memory

Synaptic Transmission: sending a "signal" from one "neuron" to another across a "synapse" via a "neurotransmitter" molecule. (Lewis, 150) Any process in which a “presynaptic cell” transfers a signal to a “postsynaptic cell,” by either release of a neurotransmitter or by passage of an electrical current via specialized channels. This process is essential to all neuronal functions. (NCIt)

The “presynaptic neuron” releases a neurotransmitter that “diffuses” across the “synaptic cleft” and “binds” to specific “synaptic receptors,” “activating” them. The activated receptors “modulate” specific “ion channels” and/or “second-messenger” systems in the postsynaptic cell. In electrical synaptic transmission, electrical signals are communicated as a "current" flow across “electrical synapses.” (MeSH) The process is described as follows: When an “action potential” in a neuron reaches the “axon terminal,” it causes a neurotransmitter molecule to be released into the synaptic cleft. That neurotransmitter moves across the synaptic cleft to the target cell, where it is recognized and captured by specialized receptors on the outer surface of the target “cell membrane." This process was first discovered by Henry Dale and Otto Loewi. (Kandel, 91) "Synaptic vesicles" accumulate inside the axon right next to the cell membrane at the tip. Like cellular water balloons, one or more synaptic vesicles are smashed against the cell membrane of the axon by the force of the electrical impulse when it arrives, releasing the vesicle’s contents into the (synaptic cleft). The "messages" are not (actually) floated across the synaptic gulf in spherical bottles; instead, their contents are dumped into the gulf and “diffuse” across to the opposite shore. (Fields, 19-20) Also referred to as ‘neurotransmission’ and ‘neuronal transmission.’


Action Potential: an all-or-none, pulse-like change from the "resting membrane potential." The primary means to rapidly communicate information between neurons, and from neurons to muscles. (Koch, 329) These propagate along the whole length of the axon to the synaptic terminals. (Kandel, 449) An electric signal generated within the neuron that travels along the axon until it reaches the axon terminal. (The Brain-Eric Kandel, 30) This electrical signal is different from the flow of electricity in a copper wire, which initially moves close to the speed of light (186,000 miles per second), but deteriorates badly over long distances. Axons of nerve cells conduct electricity much more slowly than wires do, and they do so by means of a novel, wavelike action. Unlike signals in wires, they do not decrease in strength as they move along. All of the action potentials generated by a single nerve cell are about the same shape and amplitude, regardless of the strength, duration, or location of the stimulus that elicits them. (Kandel, 76-77) About 100 mV in amplitude and 0.5 - 1.0 mSec in width. (Koch, 329) Also referred to as ‘spike,’ ‘impulse,’ ‘nerve impulse’ and ‘propagated signal.’

Conduction: movement of an electrical impulse along a nerve fiber. (Lawrence)

Conductivity: a measure of the property or power of a substance of conducting heat or “electricity.” Conductance per unit volume; the property of tissue of conveying nerve impulses. (Oxford)

Current: the movement or flow of electricity. (Soares, 11) A flow of electricity: the rate of this, measured as quantity of charge per second. (Oxford) The flow of “electrons” in a “conductor.” Electrons enter a conductor, which provides a path of the current to flow. (Shultz, 14)

Ionic Hypothesis: the action potential is caused by the movement of sodium “ions” into the cell. (Kandel, 83) Once an action potential has been generated in one region of the axon, the current it generates excites the neighboring region to generate an action potential. The resulting chain reaction ensures that the action potential is "propagated" along the entire length of the axon. Hypothesis formulated by Alan Hodgkin and Andrew Huxley. For their work, Hodgkin and Huxley shared the Nobel Prize in Physiology or Medicine in 1963. The generality and predictive power of the ionic hypothesis unified the cellular study of the nervous system: it did for the “cell biology” of neurons what the structure of “DNA” did for the rest of biology. (Kandel, 88-89) 

Polarity: the electrical condition of a body as positive or negative. (Oxford)

Polarization: the action of inducing electrical polarity. (Oxford) Verb - 'polarize.'

Depolarization: decrease in electrical charge across a membrane, usually due to the inward flow of sodium ions. (Kolb, 124) The sudden surge of charged particles across the membrane of a nerve cell that accompanies a change in the membrane and cancels out, or reverses, its resting (membrane) potential to produce an action potential. (OxfordMed) Verb - 'depolarize.'

Hyperpolarization: increase in electrical charge across a membrane, usually due to the inward flow of "chloride" ions or the outward flow of "potassium" ions. (Kolb, 124) Changes the membrane potential of a nerve cell toward a more negative value. Hyperpolarization decreases the likelihood that a neuron will generate an action potential and is therefore "inhibitory." (Kandel, 439) Verb - 'hyperpolarize.'

Potential: a difference in "voltage." (Kandel, 80). The quantity of energy required to move a charge from a given point to a reference point of zero potential. (Oxford)

Resting Membrane Potential: the difference in electrical charge between the inside and the outside surfaces of a (axon) membrane. In (a neuron's) resting state, in the absence of any neural activity, there exists a steady potential across the (axon) membrane. All signaling is based on changes in this, resting potential. It results from an uneven distribution of sodium, potassium, and chloride ions. The resting potential (in most mammalian nerve cells) is -70 millivolts, with the inside of the cell having a greater negative charge than the outside. (Kandel, 81) Also referred to as 'resting potential.'

Threshold Potential: voltage on a (neuron) membrane at which an action potential is triggered. (Kolb, 124) Also referred to as 'threshold."

Propagation: to give birth. (Kolb, 128) Cause to grow in numbers or amount; extend the bounds of; spread from place to place. Extend the action or operation of; transmit in some direction or through some medium. (Oxford) Verb - 'propagate.' Editor’s note - in describing information transfer between neurons, propagation is a process by which nerve impulses begin and then travel down the neuron. For example, signals propagate along the whole length of the axon to the synaptic terminals.

Transient: passing away with time, not durable or permanent, temporary, transitory. A transient variation in current or voltage, especially at the beginning of a signal. A very brief surge. (Oxford) Also referred to as ‘temporary.’

Voltage: a measure of the force or difference in potential that tends to give rise to an electric current. Expressed in volts. Also referred to as ‘electromotive force.’ (Oxford)

Neurotransmitter 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)

Neurotransmitter 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, circuits can be strengthened or weakened, in effect allowing the circuits 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)