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UTILIZATION TIME

The minimum time during which an electric current must act upon tissue to cause a wave of excitation is inversely proportional to its voltage and strength.

If this minimum time (for example, for a d-c impulse) is plotted in milliseconds along the abscissa, and the current voltage or strength along the ordinate, a strength-duration curve is obtained, which has been studied in detail in experiments on various nerves and muscles by Hoorweg, Weiss, and Lapicque, and more recently by Nasonov and his co-workers (Fig. 122).

Examination of the curve shows primarily that a current below a definite minimum strength or voltage does not cause excitation, however long its action. Lapicque called the minimum strength (or voltage) of a current capable of producing excitation the rheobase (the OA ordinate). The minimum time (ОС segment) that a current equal to the rheobase must act to induce an action potential is designated by the term utilization time, which implies that further prolongation of the effect of current has no value or is useless in generating action potential.

Intensification of the current leads to a shortening of the minimum time of stimulation, but not without limits. As may be seen from the graph, with very short-lived stimuli the strength-duration curve becomes parallel to the axis of the ordinate, which means that they produce no excitation however strong they may be.

The strength-duration curve takes the form of an equilateral hyperbola and can be described by the empirical formula i = j + &, where i is the current strength, t its duration and a and b are constants determined by properties of the tissue.

In practice it is difficult to determine the utilization time (the ОС segment) as the value of the rheobase undergoes continuous slight variations reflecting variations in the functional state of the membrane at rest. For that reason, Lapicque (1909) proposed measuring another, conventional value, which he called chronaxie, the least time required for a current equal to double the rheobase (OD) to produce excitation in a tissue. Utilization time and chronaxie characterize the rate at which a stimulus causes excitation.

On the diagram in Fig. 122, OF on the abscissa corresponds to the chronaxie. As may be seen, the point E obtained w’ith a doubled rheobase strength of stimulus lies on the steep slope of the curve and can therefore be measured accurately. Chronaxie is determined by means of special instruments, chronaximeters.

It has been found experimentally that the strength-duration curves for quite different tissues, for example, the nerves and muscles