Action Potential(s): 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) Action potentials have four properties important for (neurocommunication). First, they have a threshold for initiation. Second, the action potential is an all-or-none event. Third, the action potential is conducted without decrement. It has a self-regenerative feature that keeps the “amplitude” constant, even when it is conducted over great distances. Fourth, the action potential is followed by a ‘refractory period.’ For a brief time after an action potential is generated, the neurons ability to fire a second action potential is suppressed. The refractory period limits the frequency at which a nerve can fire action potentials. (Kandel3, 149) Potassium channels open and stay open a while, which allows the cell to actually get more negative than its resting membrane potential. That’s the refractor period because the action potential is not going to happen during the period. So, there’s a limit to how often any given cell can generate an action potential. (Campbell, BSP210) Also referred to as ‘fire,’ ‘firing,’ ‘spike,’ ‘spiking’ ‘impulse,’ ‘nerve impulse’ and ‘propagated signal.’

Author’s note: the phrase ‘action potential’ would seem to be an oxymoron and thus confusing for the lay reader. The ‘potential’ part has to do with the “voltage” level along the “cell membrane” of the neuron. If the voltage doesn’t reach a certain “threshold” level, no ‘action’ occurs. The neuron had the potential to fire, but it did not. As Dr. Kandel stated, the signal ‘propagates’ or moves along the whole length of the axon. It is really a sequence of events that occur, not a potential. If the sequence culminates in the release of “neurotransmitters” into the “synaptic clef,” neuroscientists say that the neuron has ‘fired’ or has ‘spiked.’

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

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)

Agonist: a chemical which combines with a “receptor” and initiates a physiological response. (Oxford) A drug or substance that binds to a receptor inside a cell or on its surface and causes the same action as the substance that normally binds to the receptor. (NCI1)

Antagonist: a substance (or organism) which interferes with or inhibits the action of another. (Oxford) In medicine, a substance that stops the action or effect of another substance. For example, a drug that blocks the stimulating effect of “estrogen” on a “tumor” cell is called an estrogen receptor antagonist. (NCIt)

Dual Signal Transmitter: both excitation and inhibition. (In Dr. Kandel’s research), the same cell excited some follower cells, inhibited others, and made a dual connection (which was both excitatory and inhibitory) to a third kind of cell. It always excited precisely the same cells, always inhibited another specific group of cells, and always made a dual connection with a third group. All of these three actions were accounted for by only one neurotransmitter -  “acetylcholine.” The reaction of acetylcholine with different types of receptors on the various follower cells, determined whether the synaptic action would be excitatory or inhibitory. (The Brain-Eric Kandel, 30)

Endorphins: natural "opiates" made by the body. These powerful "pain” relievers regulate pain naturally, in effect 'closing the valve' on the flow of painful signals traveling up the "spinal cord" to the brain. (Fields, 191) Triggered by pain and "stress," "bind" to their special receptors, altering pain sensations and “mood.” Regulate pain perceptions. ‘Runner’s high’ associated with “aerobic” exercise has been associated. In marathon runners, endorphin levels have been found to increase up to four times over their normal levels.  Primary roles: pain perception, positive emotions. Associated disorder includes opiate addiction. (Hockenbury, 48) Editor's note - opiates bind to the same receptors as endorphins.

Enkephalins: “peptides” occurring naturally in the brain and having effects resembling those of "morphine" or other opiates. (OxfordMed) One of the three major families of “endogenous” opioid peptides. The enkephalins are widespread in the “central” and “peripheral nervous systems” and in the “adrenal medulla.” (MeSH) Editor’s note - opiates also bind to the same receptors as enkephalins. Also referred to as ‘encephalin.'

Excitation: the process by which nerve cells use their “presynaptic terminals” to stimulate the next receiving nerve cell in line to transmit information onward. (Kandel, 71) The electrical activity in the brain is what tells us that a particular "neuron" has been "activated" at a particular time. It has "fired" as we say, and it has done so in order to “code” either a “sensory” event (example seeing some object or action), a “motor” act (example grasping an apple), or a “cognitive” process (example the memory of grasping an apple). (Iacoboni, 22) The task of every (pathway) in the nervous system is to add up the total excitatory and inhibitory information it receives and determine whether to pass that information along. (Kandel4, 160) The “depolarization” of a “postsynaptic neuron” increasing the likelihood that an "action potential" will be generated. (Kandel, 437) Some neurons function primarily to excite other neurons. (Kolb, 67) "Receptor" activation occurs when a chemical "binds" to a specific membrane protein initiating change. (Norman, 6/10/09) Verb - ‘excite.’ Adverb - ‘excitatory.’ Also referred to as ‘activation,’ and  ‘stimulation.’ Editor’s note - excitation may apply to a neuron, a “synapse,” a “signal,” a receptor, or a “neurotransmitter.”

Excitability Change: the change in the "threshold" (polarity) of a nerve call that follows activity - for example, from positive to negative, or from negative to positive. (Kandel, 437)

Excitatory Interneuron: can be thought of as an ‘amplifier.‘ The output of the (postsynaptic neuron to which it is connected) is amplified. (LeDoux, 53)

Excitatory Neurotransmitter: a signal that excites a neuron. If a neuron receives enough excitatory signals from other neurons, it will fire off its own signal. (Doidge, 54) When excitatory (signals) try to turn on a (postsynaptic neuron), preexisting inhibition of the (postsynaptic neuron) (may) have to be overcome. The balance between excitatory and inhibitory (signals) to a neuron determines whether it will fire. (LeDoux, 51) In the motor neuron, excitatory neurotransmitters lower the “resting membrane potential,” a value, of the post synaptic neuron from -70 millivolts to -55 millivolts. (Therefore) -55 millivolts is the "threshold" for firing an action potential. (Kandel, 98) About 80 percent of the “signaling” in the brain is carried out by two neurotransmitters that balance each other’s effect. The major excitatory neurotransmitter “glutamate” stirs up activity to begin the signaling cascade, and the major “inhibitory" neurotransmitter  “GABA” clamps down on activity. (Ratey, 37) Also referred to as 'excitatory signal.'

Excitatory Synapse: indicates a synapse that depolarizes its "target," increasing the chance that the neuron will fire. (Kandel, 437)

Firing Rate: the rate at which a cell fires. A function of certain electrical and chemical characteristics of the cell. (LeDoux, 64) Some cells fire action potentials regularly and others fire in recurrent brief bursts or 'trains.' (The Brain-Eric Kandel, 30) Rate varies. Sometimes the rate correlates with clearly definable external factors, like the presence of color in a face. In the “peripheral nervous system” more spikes mean more heat, a louder sound, or stronger (faster) muscle contraction. (Discover, Aug 2007, 56)

Single Neuron Insufficiency Principle: a single neuron individual firing rate is insufficient to sustain a particular function or behavior mediated by the cortex. (Nicolelis, 168) The arrival of transmitter from a single presynaptic terminal is typically not sufficient to produce an action potential in the postsynaptic cell. Only if the postsynaptic cell is bombarded with transmitter molecules from many presynaptic terminals at about the same time - within milliseconds- will an action potential results. (LeDoux, 47)

Tonically Active: (neurons that) are firing all the time. (LeDoux, 50)

Inhibition: the process by which some cells use their terminals to stop the receiving cells from relaying information. (Kandel, 71) The cessation of an activity. Some neurons function primarily to inhibit other neurons. (Kolb, 66-67) Nerve cells use their “presynaptic terminals” to stop the (post synaptic neurons) from relaying information. A stable, predictable, coordinated response to a particular “stimulus.” (Doidge, 71) The balance between "excitatory" and inhibitory inputs to a neuron determines whether it will "fire." (LeDoux, 51) Some drugs inhibit (and some enhance) the release of a particular “neurotransmitter” molecule and thus influence the “synaptic transmission” process. (The Brain-Leslie Iversen, 78-79) A very useful device in neural (pathways). It adds tremendously to the specificity of information processing, filtering out random excitatory inputs, preventing them from triggering activity. (LeDoux, 52) Verb - ‘inhibit.’ Adverb - ‘inhibitory.’ Also referred to as ‘suppression.’ Editor’s note - may apply to a neuron, a “synapse,” a “signal,” a neurotransmitter, a “protein receptor,” or a “gene."

Inhibitor: in chemistry and biochemistry - a substance which slows down or prevents a particular reaction or process, or diminishes the activity of some “reactant” or “catalyst”. In genetics, a gene whose presence prevents the “expression” of some other gene at a different “locus.” (Oxford)

Inhibitory Interneuron: releases a "transmitter" from its terminal that decreases the likelihood that the “postsynaptic cell” will fire. These neurons play an important role in counterbalancing the excitatory activity of other neurons. They are often firing all the time. Can be thought of as ‘filters.’ (LeDoux, 50-51) 

Inhibitory Neurotransmitter: one released from the presynaptic neuron whose work increases the “resting membrane potential” of the postsynaptic neuron from -70 to -75 millivolts making it much more difficult for the neuron to fire. (Kandel, 98) A signal that inhibits a neuron. When a neuron receives enough inhibitory signals from other neurons, it becomes less likely to fire. (Doidge, 54) Also referred to as 'inhibitory signal.'

Inhibitory Synapse: indicates a synapse that “hyperpolarizes” its target, decreasing the chance that the neuron will fire an action potential. (Kandel, 440)

Input: anything added into a system. (NCIt) Neurocommunication arriving at a single neuron or multiple neurons, a brain structure or a brain region. Inputs are just patterns that arrive in a sequence. (Hawkins, 127)

Convergence: (the property of) a single neuron receiving inputs from multiple sources. (LeDoux, 42) A given postsynaptic (neuron) is believed to receive relatively few synaptic contacts from any one presynaptic (neuron). As a result, much of the convergence that drives a postsynaptic cell toward "action potentials" comes from the convergence of different presynaptic (neurons) onto the postsynaptic neuron (that is, the near-simultaneous arrival of neurotransmitter from different presynaptic neurons). In order for the inputs to arrive in the postsynaptic “cell body” at about the same time, action potentials have to have been triggered in the various presynaptic cells at about the same time. (LeDoux, 47)

Receptive Field: the region of sensory space from which a “sensory neuron” gets its information. (Blakeslee, 214)

Messenger Molecules: (found) in a chemical “synapse,” housed in “synaptic vesicles” and released into the “synaptic cleft” (during “synaptic transmission”). (Patestas, 31) Also referred to as ‘ligands’ and 'signaling molecules.’

First Messengers: "neurotransmitters" like "glutamate" are considered first messengers. They are responsible for "signaling" between neurons. (LeDoux, 147)

Second Messengers: chemicals produced inside the cell when a neurotransmitter binds to a particular class of receptor on the cell surface. “Cyclic AMP” is a common second messenger in neurons. (Kandel, 448) (Part of) systems in which an “intracellular” signal is generated in response to an “intercellular” (first) messenger such as a hormone or neurotransmitter. (GHR) Pick up where first messengers leave off. Their job is to initiate chemical reactions within the cell on the basis of information provided from outside the cell during (synaptic) transmission by first messengers. "Calcium" is one of the major second messengers. When glutamate binds to its receptor, calcium takes over and directs the chemical reactions that strengthen "synaptic" connections, both in the short run and the long run. (LeDoux, 147) Calcium “protein channels” located in the “membrane” and “endoplasmic reticulum” allow calcium to act as a second messenger and initiate a cellular response. (Norman, 6/10/09)

Modulation: to adjust, or change. (NCIt) Variation in the activity or form of a cell in response to a changing environment. (Oxford) Modulatory neurons strengthen the connections between the sensory and motor neurons. (Kandel4, 113) Verb - 'modulate.' 

Neuromodulators: enhance or diminish the overall effectiveness of the synaptic connections and bring about enduring change. Examples include “oxytocin” and “vasopressin.” (Doidge, 118) Have slower and longer-lasting effects (than neurotransmitters). (LeDoux, 57) (Neuromodulatory) hormones are typically released from bodily organs into the bloodstream where they travel to the brain. There they can alter the efficacy of glutamate or "GABA" transmission by binding to specific receptors on cells. (LeDoux, 59)

Output: anything coming out of a system. (NCIt) Neurocommunication leaving a neuron, a brain structure, or a brain region. (Motor) command signals are sent (out) to the muscles. Having received a command to move, the muscles execute the movement. In turn, signals from the muscle spindles and joints are sent back up to the brain (as input) via the “spinal cord.” (Ramachandran, 44)

Divergence: (the property of) messages sent out from one cell affecting many others. Exists when a neuron (has) axons that branch and terminate on multiple targets. (LeDoux, 42)

Projection: the process by which groups of neurons in one region receive a strong driving input from a lower region in their hierarchy, then send it to another group located higher in the hierarchy. (Koch, 23)

Signal(s): conveyed message to a "target" cell. (Patestas, 17) A detectable physical quantity or “impulse.” (NCIt) Electrical discharges are the way brain cells send signals to one another. (Iacoboni, 22) Can be incoming or outgoing. (Norman, 6/10/09) Neurons send messages through an exchange of electrically charged chemicals. These messages are responsible for all functions of the 3-pound human brain. (Chudler, 17) Also referred to as “message.”

Transmission: the process by which a form of physical energy is converted into a coded “signal” that can be processed by the “nervous system.” (Hockenbury, 85) The conversion of a signal from a “receptor” into a specific “cellular response.” (Norman, 6/10/09) Verb - ‘transmit.’ Also referred to as ‘transduction,’  ‘sensory transduction,’ and ‘conveyance.’

Volume Transmission: Another kind of neurocommunication (not synaptic) where neuromodulators are released into the ‘extracellular’ space and their targets are outside of the synapse. First described in the 1980s. Examples include ‘neurohormones,’ ‘neurotropic factors,’ and ‘immune modulators.’ (Campbell, BSP210)