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Glossary of Physiology Terms – N
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Nascent
Definition:
Newly synthesized or in the process of being synthesized



Nernst equation
Definition:
An equation used to calculate the equilibrium potential (Veq.) of an ion. The equilibrium potential for an ion is also referred to as the Nernst potential for that ion. It is the membrane potential at which no net movement of the ion in question occurs across the membrane.

General form of the Nernst equation

where Veq. is the equilibrium potential, R is the universal gas constant, T is the temperature in Kelvin, z is the valence of the ionic species, F is the Faraday's constant, and [X]o and [X]i are the extracellular and intracellular, respectively, concentrations of the ion in question.

See also:
Resting Membrane Potential - Nernst Equilibrium Potential
Derivation of the Nernst Equation



Nerve impulse
Definition:
Refers to the action potential of neurons. It is also referred to as the nervous impulse.

Related glossary terms/phrases:
Action potential

See also:
Neuronal Action Potential



Nervous impulse
Definition:
Refers to the action potential of neurons. It is also referred to as the nerve impulse.

Related glossary terms/phrases:
Action potential

See also:
Neuronal Action Potential



Net flux
Definition:
Net flux represents the amount of substance moved in or out of the cell. It is the mathematical difference between influx and efflux.

Net flux = Influx − Efflux

Similar to influx and efflux, net flux is reported as a rate. It is the net amount of substance that moves through a given area of the plasma membrane per unit time.

Related glossary terms/phrases:
Flux
Influx
Efflux
Unidirectional flux



Neurohormone
Definition:
Neurohormones are chemical messenger molecules that are released by neurons, but enter the bloodstream where they travel to distant target sites within the body. Therefore, neurohormones share characteristics with both neurotransmitters and hormones. Similar to neurotransmitters, neurohormones are released by neurons. Similar to hormones, neurohormones travel in the bloodstream.

Two well-known examples of neurohormones are oxytocin and the antidiuretic hormone (also referred to as vasopressin).

Related glossary terms/phrases:
Hormone
Neurotransmitter



Neurohypophysis
Definition:
Posterior pituitary gland

Related glossary terms/phrases:
Hypophysis
Adenohypophysis



Neurotoxin
Definition:
Neurotoxins are chemical molecules that have an adverse effect on neuron function and, thus, disrupt the normal function of the nervous system. Neurotoxins could be small molecules or peptides and can be derived from a variety of invertebrate and vertebrate animals, as well as plant species.

The following is a short list of some examples of neurotoxins:

α-Bungarotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

Chlorotoxin: A peptide neurotoxin that inhibits chloride channels.

α-Conotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

δ-Conotoxin: A peptide neurotoxin that inhibits voltage-gated sodium channels.

w-Conotoxin: A peptide neurotoxin that inhibits N-type voltage-gated calcium channels.

Picrotoxin: Inhibits GABAA receptor chloride channels.

Tetrodotoxin: Inhibitor of neuronal voltage-gated sodium channels.



Neurotransmitter
Definition:
Chemical messenger molecules released by neurons into the synaptic cleft.

Neurotransmitter molecules may be small molecules such as glutamate, γ-aminobutyric acid (GABA), acetylcholine, dopamine, norepinephrine, serotonin (5-hydroxytryptamine), and glycine. These are referred to as classic neurotransmitters. A neuron generally releases only one type of small classic neurotransmitter.

Neurotransmitters may also be peptide molecules, such as substance P, opioids, and somatostatin. These are referred to as neuropeptides.

Gas molecules, such as nitric oxide (NO), may also act as neurotransmitters.



Non-excitable cell
Definition:
Refers to cells that do not generate action potentials. With the exception of neurons, muscle cells, and some endocrine cells, all cells in the body are non-excitable.

See also:
Resting Membrane Potential - Introduction









Posted: Saturday, March 9, 2013
Last updated: Sunday, February 7, 2016