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(irritability), the ability of living cells, from the simplest unicellular organisms to the nerve cells of man, to sense environmental changes and to respond to these changes (stimuli) with excitation. Excitability is linked to the existence in the cell membrane of special molecular structures possessing specific sensitivity to various stimuli, such as electric current and chemical, mechanical, and thermal agents. Since the main component of excitation in nerve and muscle cells (fibers) is action potential, the excitability of these cells is usually judged by the threshold current strength or threshold shift in potential, which is sufficient for action potential to arise. The threshold current strength is depen-dent upon the duration of the stimulus. The threshold shift in potential, however, does not change with variation in the duration of the stimulus. The generation of action potential is based on increased permeability of the cell membrane to sodium ions. Accordingly, any agent that prevents the sodium permeability of a membrane from increasing causes excitability to decrease. Local anesthetics used in medicine, such as Novocain, cocaine, and dicaine, as well as various narcotics, such as ether barbiturates, are based on this mechanism.

The term “excitability” is also frequently used in medical and biological literature to characterize the state of the nerve centers in the brain and spinal cord, for example, the respiratory and vasomotor centers. In such cases excitability is judged by the lowest intensity of a stimulus that is required to elicit a reflex response.


References in periodicals archive ?
In this collaborative study, researchers will test whether Retigabine can reduce excitability of upper and lower motor neurons.
In general, the excitability of neuron can be characterized by the magnitude of the depolarizing current necessary to bring the membrane potential to the threshold leading to initiation of the action potential.
b]s measured in our highly tractable animals were the lowest reported for healthy moose, and consistent with the view that level of excitability influences [T.
Estragole is a phenylpropanoid, and some substances of this class are local anesthetics: they block excitability predominantly through direct inhibitory effects on [Na.
It was found that the operant conditioning resulted in an increase in the excitability of the B51 neuron, which did not occur in the respondent procedure (Lorenzetti et al.
Material is arranged in sections on biophysical chemistry, metabolism, second messengers, and ultrastructure; membrane potential, transport physiology, pumps, and exchangers; membrane excitability and ion channels; synaptic transmission and sensory transduction; and muscle and other contractile systems.
The Blue Correspondence Ironically it was only a fortnight earlier that the Birmingham Daily Post had reported on the excitability of Everton's Victorian followers.
However, the signal transduction mechanisms involved in increase in the neuronal excitability leading to the epileptic seizures are not known.
SWV has been shown to reduce central excitability and increase central inhibition (8).
Although the mechanism of action is not firmly established, the drug may act as an anticonvulsant by reducing excitability through the stabilization of neuronal potassium channels in an 'open' position, " the statement said.