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the property by which materials and products resist high temperatures without fusing. The degree of refractoriness is expressed by the pyrometric cone equivalent— the temperature in degrees centigrade at which a trihedral pyramid of the refractory material 30 mm in height with base sides measuring 8 and 2 mm softens so that the uppermost section of the pyramid touches the surface of the supporting pedestal.
REFERENCEPraktikum po tekhnologii keramiki i ogneuporov. Moscow, 1972.
a short-term decrease in the excitability of nervous or muscle tissue occurring immediately after the manifestation of action potential. It can be detected by the stimulation of nerves and muscles by paired electrical impulses. If the force of the first impulse is sufficient to produce action potential, the response to the second impulse will depend on the duration of the pause between the impulses. When the interval is very brief, there will be no response to the second impulse, regardless of the increase in the intensity of stimulation (absolute refractory period). Prolongation of the interval initiates a response to the second impulse, but this response is smaller in amplitude than the response to the first impulse. This phenomenon has been demonstrated in experiments on nerve trunks that consist of a large number of parallel nerve conductors. Another means of producing a response to the second impulse is to increase the intensity of the stimulating current. This has been done in experiments on single nerve fibers.
The period of decreased excitability of a nerve or muscle cell is called the relative refractory period. It is followed by the supernormal period, or phase of exaltation, which is a phase of increased excitability. This is followed by a period of somewhat decreased excitability—the subnormal period. These fluctuations in excitability are caused by the changes in the permeability of biological membranes that accompany the manifestation of action potential. The duration of each of the above periods is determined by the kinetics of these processes in the given tissue. In rapidly conducting nerve fibers, refractoriness lasts no more than 3 to 5 msec (milliseconds); in heart muscle, the period of changes in excitability can last up to 500 msec.
Refractoriness is one of the factors limiting the frequency of recurrence of biological signals, their summation, and the speed at which they are transmitted. The duration of refractory periods may be altered by changes in temperature or the action of some medicinal substances. These means are used to control the excitability of such tissue as heart muscle. Prolongation of the relative refractory period causes a decrease in the frequency of heart contractions and eliminates disturbances in cardiac rhythm.
L. G. MAGAZANIK