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mA

with a rod, for example, causes excitation, whereas pressure slowly applied to the nerve with the same rod does not. An isolated nerve fibre can be stimulated by very rapid chilling, whereas gradual chilling excites no response.

In the laboratory linearly or exponentially increasing currents are used to measure accommodation, the index of the rate of accommodation being the minimum rate of increase of the current (known as the minimum gradient or critical slope) at which the stimulus retains its capacity to excite an action potential. The gradient is expressed either in absolute terms (in milliamperes per second) or in relative units (in rheobases per second). In the latter case the rheobase is measured with a rectangular current, and it is then calculated by how many rheobases the current must be increased so as not to lose its stimulating effect.

The rate of accommodation of various excitable formations varies widely. The highest is that of the motor nerve fibres of warm-blooded animals. Sensory fibres have a lower rate, while that of the fibres of heart muscle and of the smooth muscles of the intestine, ureters, and stomach, i. e. of formations tending to automatic activity, is very low.

LAW OF POLARITY OF STIMULATION

Direct current has a polar action on excitable tissue; when the direct current circuit is closed excitation always arises in a nerve or a muscle only at the cathode, and when the circuit is opened only at the anode. Pflueger, who discovered the phenomena, demonstrated them by means of the following experiment. Having destroyed a portion of .the nerve under one electrode, he placed a second electrode on the intact portion. When the cathode was in contact with the intact portion, excitation arose when the circuit was closed; when, however, the cathode was applied to the damaged portion and the anode to the intact part, excitation arose only when the circuit was opened. The stimulus threshold at the moment of opening the current circuit, when excitation arises at the anode, is considerably higher than at its closing, when excitation arises at the cathode.

Pflueger determined the appearance of excitation indirectly from the contraction of the muscle supplied by the stimulated nerve. Subsequently these phenomena, generalizedin the polarity of law of stimulation, were also confirmed directly, by recording action potentials immediately in the spots where direct current poles were applied to tissue.

The mechanism of the polar action of an electric current is now studied by stimulating nerve and muscle fibres and recording their electrical potentials by means of intracellular microelectrodes (Fig. 121). It has been found that an action potential arises only when the cathode comes into contact with the outer surface of the cell membrane and the anode is inside the cell. With a reversed arrangement, i. e. with an external anode and an internal cathode, excitation is not caused when the circuit is closed, however strong the current.