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Among all these stimuli electric current has a special place, since (a) it can easily and accurately be dosed in strength, duration, and steepness of increase, and (b) it does not damage living tissue, while its action is quickly and completely reversible when it is strong enough to cause excitation. Study of the action of electrical stimulation on excitable tissue is of great interest to physiology because excitation is transmitted through the nerves and muscles by local currents arising between the excited and resting portions of tissue.

Under laboratory conditions and in certain clinical investigations nerves and muscles are excited with electrical stimuli of various kinds: rectangular, sinusoidal, linearly and exponentially increasing currents, induction shocks, condenser discharges, etc. (Fig. 119).

fig. 119. Electrical stimuli of various shape

d

a — rectangular pulse of current; b — linearly increasing current; c — exponentially increasing current; <f — making (1) and breaking (2) induction shocks: e — condenser discharge; f — sinusoidal current

The mechanism by which electricity stimulates is identical in principle for all types of stimuli, but is most distinctly seen when direct current of rectangular wave form is used.

For a stimulus to cause excitation it must have sufficient strength, duration and steepness of increase.

The significance of each of these parameters is described below.

THE STIMULUS THRESHOLD

The lowest strength of stimulation required to give rise to an action potential in excitable tissue is called the threshold of stimulation. Stimuli below threshold strength are known as subthreshold, while those above are called suprathreshold stimuli.

When an electric current is used as a stimulus the threshold is expressed in units of current strength or voltage.

The absolute value of the threshold varies with the properties and physiological condition of the tissue and with the method of stimulation.

There are two methods of applying an electric current to tissue, extracellular and intracellular. With the first both electrodes are placed on the surface of the tissue to be stimulated and the current enters the tissue in the region of the anode and passes out at the cathode (Fig. 120). The drawback of the method is the considerable

fic. 120. Scheme of the distribution of current iu tissue during stimulation via external (extracellular) electrodes. The batching represents muscle fibres with intercellular spaces between them

pic. 121. Stimulation and recording of potentials via intracellular microelectrodes (explained in the text)

branching off of current, only part of which passes through the cell membranes, the rest branching off into the intercellular spaces. Therefore a much stronger current has to be applied than is actually needed to produce excitation.

The other method, by means of an intracellular electrode, is more accurate. A microelectrode with a tip diameter around 0.5 micron is introduced into the cell, while another, conventional electrode is applied to the tissue surface (Fig. 121). All the current applied then passes through the cell membrane which permits accurate determination of the value of the threshold, which varies within the limits of 10^-’ to 10~* angstrom units for different cells. Intracellular stimulation is usually combined with registration of potentials through another intracellular electrode, as shown in Fig. 121.