G. Dennis Rains,
Kutztown University of Pennsylvania
| absolute refractory period | The period after an action potential during which another action potential cannot be activated. This is due to the temporary inactivation of Na+ channels.
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| acetylcholine | A major neurotransmitter in the brain. It is found at the junction between motor neurons and muscle. It is also the major neurotransmitter in the parasympathetic nervous system.
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| action potential | The wave of influx of Na+ ions into the axon, which is the mechanism by which a signal is communicated down the length of the axon.
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| active zones | Regions of the presynaptic membrane where neurotransmitter is released.
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| afterpotential | The brief hyperpolarization that precedes the reestablishment of the resting potential after an action potential.
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| aspartate | An amino acid neurotransmitter.
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| astrocytes | Large, star-shaped glia cells that surround the brain's vasculature, forming the blood-brain barrier.
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| autoreceptors | Receptors on the presynaptic membrane that bind neurotransmitter and thereby provide the axon with information about the concentration of neurotransmitter in the synapse.
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| axon hillock | The part of the neuron that forms the junction between the cell body and the axon. This part of the neuron has the lowest threshold for triggering an action potential.
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| axon terminal (bouton) | The end of the axon. Also called the terminal bouton.
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| blood-brain barrier | A barrier that protects the brain by allowing only certain molecules to pass into it from the general circulation. It is formed by astrocytes (a type of glia) that surround the brain's vasculature.
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| cell body (soma) | The part of the neuron containing the cell nucleus and many of the organelles that are critical for the functioning of the cell. Also called soma.
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| conditioned response | The response evoked by the conditioned stimulus.
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| conditioned stimulus | When a neutral stimulus (one that does not produce a particular response) precedes an unconditioned stimulus on a number of occasions, it may come to evoke a response identical to (or similar to) the response evoked by the unconditioned stimulus. When this learning has occurred, the neutral stimulus has become a conditioned stimulus.
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| conformational state | The spatial configuration (arrangement) of the atoms in a molecule. Molecules that have the same atoms can have different conformational states, and these different states can have different chemical properties.
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| Dale's law | The principle that a single neuron generally does not release more than one small molecule neurotransmitter.
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| degrading enzyme | An enzyme that destroys (breaks down) a neurotransmitter. An example is acetylcholinesterase, which breaks down the neurotransmitter acetylcholine.
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| dendrite | The usually highly branched part of the neuron extending from the cell body that receives signals (binds neurotransmitter) from other neurons.
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| depolarization | Reduction in membrane poten-tial (decrease in the negativity of the inside of the neuron).
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| diffusion force | The force that tends to equalize the concentration of a particular molecule or ion.
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| distributed representation | See distributed processing.
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| dopamine | A small molecule monoamine neurotransmitter.
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| dopamine theory of schizophrenia | The idea that schizophrenia is caused by dopamine overactivity.
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| electrochemical equilibrium | The point at which diffusion force and electrostatic force for a particular ion are equal, so that there is no net movement of the ion from one compartment to the other.
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| electrostatic force | The force that manifests itself in like charges repelling and opposite charges attracting.
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| electrotonic potentials | The graded changes in membrane potential that are the result of passive ion fluxes following the opening of specific membrane channels in response to the binding of neurotransmitter to the postsynaptic membrane. EPSPs and IPSPs are electrotonic potentials. Also called graded potentials.
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| epinephrine | A small molecule monoamine neurotransmitter.
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| equilibrium potential | The difference in charge between two compartments when a particular ion is in electrochemical equilibrium.
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| excitatory postsynaptic potential (EPSP) | An instance of a small (a few millivolts), local depolarization of a neuron that occurs as the result of the binding of neurotransmitter to a postsynaptic receptor.
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| extracellular fluid | Fluid in the spaces between cells.
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| facilitating interneurons | Interneurons that bring about sensitization in Aplysia by exerting presynaptic facilitation on sensory neurons via axoaxonal connections
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| gamma-aminobutyric acid (GABA) | a major inhibitory neurotransmitter in the central nervous system.
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| gap junction channels | The type of transmission across synapses in which structural connections between two neurons allow a direct current flow between them. This type of transmission is much less common than transmission via the release of neurotransmitter. Also called electrical synapses.
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| gating | The change in the state of a membrane channel that results in an alteration of the degree to which the channel will allow its particular ion to pass. Changes in channel gating alter the membrane's permeability (conductance) to a particu-lar ion.
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| glutamate | An amino acid neurotransmitter.
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| glycine | An amino acid neurotransmitter.
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| habituation | A simple form of learning in which an animal learns to suppress a response to a recurring neutral (neither rewarding nor harmful) stimulus that had evoked a response when it was first presented.
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| hyperpolarization | Increase in membrane poten-tial (increase in the negativity of the inside of the neuron).
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| inhibitory postsynaptic potential (IPSP) | An instance of a small (a few millivolts), local hyperpolarization of a neuron that occurs as the result of the binding of neurotransmitter to a postsynaptic receptor.
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| interneurons | Neurons neither directly receiv-ing information from the environment (sensory receptors and primary sensory neurons) nor directly causing muscle contractions (motor neurons). The vast majority of neurons in the brains of mammals are interneurons.
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| intracellular fluid (cytoplasm) | Fluid within the cell.
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| ion | A charged atom or molecule.
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| ionotropic receptors | Receptors that, when a neurotransmitter binds to them, directly change the gating of an ion channel in the postsynaptic membrane.
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| length constant | The quantification, for a given postsynaptic membrane, of the extent of spread of an EPSP. This depends on the particular characteristics of the membrane.
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| lipid bilayer | The double layer of lipid molecules that forms the membrane of neurons.
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| long-term habituation | Habituation that lasts for a relatively long period of time (hours, days, or weeks).
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| membrane potential | The potential difference (difference in charge) between the two sides of a membrane (the inside and the outside of a cell).
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| metabolic pump | A biochemical mechanism that actively moves an ion across a membrane in the direction opposite to that dictated by electrochemical forces.
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| metabotropic receptors | Receptors that, when they bind their neurotransmitter, activate a second messenger.
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| microglia | A type of glia characterized by small, irregularly shaped cells that invades and removes damaged tissue.
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| modulation | The capacity to respond flexibly and to varying degrees, taking into account a complex array of factors. Modulation is an essential characteristic of complex nervous systems.
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| monoamine | A molecule that has a single amine (NH2). The neurotransmitters epinephrine, norepinephrine, dopamine, and serotonin are monoamines.
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| monoamine oxidase inhibitors (MAOs) | A class of drugs that enhances monoamine activity by inhibiting the breakdown of monoamines by monoamine oxidase.
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| motor neurons | Neurons with their cell bodies in the anterior horn of the spinal cord that innervate muscle. They are the last link in the efferent chain from brain to muscle.
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| myelin | The lipid sheath formed around certain nerve fibers, serving to enhance the speed and efficiency of neuronal transmission. It is formed by oligodendroglia in the central nervous system and Schwann cells in the peripheral nervous system.
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| neurites | A generic name for the extensions radiating out from the cell body (i.e., dendrites and axon).
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| neuroglia (glia) | Cells found in the central and peripheral nervous systems. In the central nervous system they provide structural and nutritive support for neurons. Certain types of glia have specialized functions, such as forming the blood-brain barrier. Some types of glia in both the central and peripheral nervous systems surround axons, forming the myelin sheath that increases the speed and efficiency of signal transmission down the axon. Also called neuroglia.
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| neuron hypothesis (neuron doctrine) | Another name for the neuron theory.
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| neuropeptides | Short chains of amino acids that, like neurotransmitters, are released at the axon terminal. The effects of neuropeptides are typically longer lasting than those of neurotransmitters, suggesting that they may be involved in long-term processes such as learning and memory.
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| neurotransmitters | The small molecules that are the immediate mechanism by which neurons communicate with each other. They are released into the synapse by the presynaptic membrane and then diffuse across the synapse, eventually binding on receptor sites on the postsynaptic membrane.
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| neutral stimulus | A stimulus that does not produce a particular response.
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| nodes of Ranvier | Periodic gaps in the myelin sheath that surrounds many axons in the central and peripheral nervous systems.
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| norepinephrine | A small molecule monoamine neurotransmitter.
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| nucleus | (1) A concentration of neuron cell bodies within the central nervous system. (2) A spheroid body within a cell containing a number of organelles and the cell's complement of DNA.
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| oligodendrocytes | Glia cells that form the mye-lin sheath around axons in the central nervous system.
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| permeability (conductance) | the degree to which a membrane allows a particular molecule (or ion) to pass through it.
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| phenothiazines | A class of drugs that blocks dopamine receptors and reduces the frequency and magnitude of psychotic symptoms in some individuals with schizophrenia.
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| postsynaptic membrane | The membrane (usually forming the outer surface of the dendrite) containing receptors to which neurotransmitter binds after it is released into the synapse.
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| presynaptic facilitation | Facilitation that results when axoaxonal inputs increase the magnitude of neurotransmitter release by the postsynaptic axon terminal. A typical mechanism for presynaptic facilitation is increasing Ca2+ influx at the postsynaptic axon terminal.
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| presynaptic inhibition | Inhibition that results from axoaxonal inputs that reduce the number of synaptic vesicles releasing neurotransmitter into the synapse (typically by reducing Ca2+ influx at the axon terminal).
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| presynaptic membrane | The membrane (usually forming the outer surface of the axon terminal) from which neurotransmitter is released into the synapse.
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| primary sensory neurons | Neurons that receive input from sensory receptors. They are the first link in the afferent chain from sensory receptor to brain.
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| receptor | A specialized protein molecule on the postsynaptic membrane that recognizes and binds neurotransmitter. See also sensory receptors.
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| receptor blockers | Drugs that diminish the effectiveness of a neurotransmitter by competing for binding sites at its receptors.
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| relative refractory period | The period after an action potential (and just after the absolute refractory period) when the neuron has an elevated threshold for firing (i.e., greater depolarization at the axon hillock is required). During this period, K+ channels remain open to reestablish the resting potential. Thus, any influx of Na+ is compensated for by an efflux of K+, making depolarization of the neuron more difficult.
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| resting channels | Channels that are open when the neuron is in the resting (not firing) state.
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| resting potential | The membrane potential when the neuron is at rest (not firing).
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| reticular hypothesis | The idea that the nervous system is a continuous structure, a network of tissue that constitutes an exception to the general rule that living tissue is made up of individual units or cells.
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| reuptake | The reabsorption of neurotransmitter across the presynaptic membrane.
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| Schwann cells | Glia cells that form the myelin sheath around axons in the peripheral nervous system.
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| second messenger | A response that occurs when neurotransmitter binding to a receptor activates a second molecule, which indirectly alters the gating of membrane channels through the initiation of a sequence of biochemical events. Second messengers can have far-reaching and long-lasting effects on the metabolic state of the neuron. Cyclic adenosine monophosphate (cAMP) is the best understood second messenger.
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| selective serotonin reuptake inhibitors | A class of antidepressant drugs that selectively blocks the reuptake of serotonin. Fluoxetine (Prozac) was the first drug of this class.
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| sensitization | A simple form of learning in which the vigorousness of a response to a neutral stimulus (one that is neither rewarding nor harmful) increases when it is preceded by a noxious (painful) stimulus.
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| sensory receptors | The first neurons to register the presence of a stimulus.
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| serotonin | A small molecule monoamine neurotransmitter.
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| short-term habituation | Habituation that lasts for a brief period (minutes).
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| sodium-potassium pump | A metabolic pump that transports Na+ out of the neuron and K+ into the neuron. This compensates for the slow leak of Na+ into the neuron (and resulting efflux of K+) during the resting state.
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| spatial summation | The cumulative effect of EPSPs occurring at different locations on the postsynaptic membrane.
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| synapse | The gap between neurons across which neurotransmitter diffuses.
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| synaptic vesicles | Membrane-bound spheres, found at the synaptic terminal, in which neurotransmitter molecules are stored before their release into the synapse.
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| temporal summation | The cumulative effect of EPSPs occurring close together in time.
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| threshold potential | The critical depolarization that, when achieved at the axon hillock, initiates an action potential.
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| time constant | The quantification, for a given postsynaptic membrane, of the duration of an EPSP. The time constant depends on the particular characteristics of the membrane.
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| transmitter-gated channels (ligand-gated channels) | Membrane channels that alter their gating in response to the binding of specific neurotransmitters to postsynaptic receptors. Also called ligand-gated channels.
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| tricyclic antidepressants | A class of antidepressant medications with a three-ringed structure that blocks the reuptake of monoamines (particularly norepinephrine and serotonin). An example is imipramine.
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| unconditioned response | A natural (unlearned) response to an unconditioned stimulus.
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| unconditioned stimulus | A stimulus that naturally causes a particular response.
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| voltage-gated channels | Membrane channels that alter their gating in response to changes in voltage.
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2002 McGraw-Hill Higher Education