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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.
B. I. KHODOROV