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fic. 122. A strength-duration curve (explained in the text)

of man and warm-blooded animals, the stomach of a frog, the foot of a snail, have the same shape. The differences between them are quantitative; in the nerve and muscle fibres of vertebrates chro-naxie is measured in thousandths and ten-thousandths of a second, but in the so-called slow tissues like in the muscle fibres of the foot of a snail or the stomach of a frog, in hundredths of a second.

These findings led investigators to conclude that excitable tissues differ from one another in their time constant.

The measurement of chronaxie — chronaximdry - is widely used not only in experimental research but also in clinical practice (Bourguig-non, Ufland, and others). In particular, by measuring the chronaxie of a muscle the neuropathologist can discover an injury to the fibres of a motor nerve. The point is that with an electrical stimulus applied to a muscle the current also passes through the nerve fibres and nerve endings contained in it. The stimulation threshold and chronaxie of nerve fibres being lower than those of muscle fibres, excitation first arises in the nerve fibres during stimulation of a muscle, and is transmitted from them to the muscle fibres. Hence it follows that when measuring the chronaxie of normal human muscle, it is actually the chronaxie of the nerve fibres that supply it which is determined. If, however, the nerve is damaged, or the cells in the spinal cord innervating the muscle are dead, the nerve fibres degenerate (p. 70) and a stimulus applied to the muscle reveals the longer-duration chronaxie of the muscle fibres.

THE STEEPNESS OF INCREASE IN STIMULUS STRENGTH.

ACCOMMODATION

The level of the threshold of nerve or muscle excitability depends not only on stimulus length but also on the steepness of its increase. The stimulus threshold is at its lowest value with rectangular current impulses characterized by maximum rapidity of increase in strength. If, however, stimuli increasing linearly or exponentially are used instead, thresholds prove to increase in inverse proportion to the rate of increase in current strength (Fig. 123).

When the steepness of the rise is reduced below a certain minimum, no action potential appears, no matter how great the final strength of the current, because of the fact that during the period of increase in stimulus strength active changes occur in' the tissue raising the threshold and interfering with stimulation.

This phenomenon of adaptation of excitable tissue to a slowly increasing stimulus is known as accommodation. The higher the rate of accommodation the more steeply the stimulus must increase so as not to lose its effect.

Accommodation develops not only during stimulation of excitable tissues with an electric current but also during the application of mechanical and thermal stimuli, etc. Quick tapping of a nerve

fig. 123. Accommodation curves of a nerve fibre, measured by means of linearly increasing currents Abscissa, duration of increase in current strength; ordinate, values of thresholds in milliamperes. The inclination of the thin lines to the abscissa corresponds to the rate of increase of the stimuli