During the course of work on the effects of polarizing currents on the electrical activity of the rat cerebral cortex, we found that prolonged changes in the level of cortical activity could be produced by relatively brief periods of polarizing. This paper describes the after-action of transcortical polarizing current upon the activity evoked by stimulating the forepaw and upon spontaneous firing in cortical neurones. METHODS Experimental AnimalsAbout 200 albino rats of 200-250 g body weight were used. Each rat was anaesthetized with intraperitoneal urethane (360O solution, 0 5 c.c./100 g) and fixed in a rigid head-holder. The cortical surface was exposed by making a trephine hole 4 mm dia. and attaching a polyethylene cup to the skull as described by Bindman, Lippold & Redfearn (1962a) or by drilling 500 , dia. holes with an electric drill. Body temperature was maintained within 0.20 C of a given temperature, usually 350 C, with a 12 V heater driven by an OC 35 transistor, controlled by a thermistor in the rectum.Stimulation and polarization For evoked potential studies, stimuli were delivered at 2 sec intervals to the skin of the forepaw by means of two fine stainless steel needles inserted beneath the skin. The pulse generator gave 200 ,Lsec square waves which were isolated from earth and fed to the preparation by a 1:1 transformer.,The polarizing circuit consisted of a battery across which was connected a potentiometer. A series swamping resistance of 10 MQ was used in order to minimize any effects due to variation in resistance of the electrodes or the preparation. A galvanometer having a deflexion of 4 cm/,uA enabled the current flowing in the circuit to be measured. ElectrodesGlass micro-pipettes, usuallyfilled with 10%/' NaCl solution (and some filled with 1.8 % NaCl and some with 11 O0 KCI), were connected to the pre-amplifier by Ag-AgCl wire. Tip diameters were from 0 5 to 20 , according to requirements. In some experiments non-polarizable wick electrodes or agar gel ones were used for polarizing. * M. R. C. Scholar. 24Physiol. 172

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The passage of small direct currents through nerve cells gives rise to changes in their excitability. For example an outward current flow through the membrane of the soma gives rise to depolarization and a consequent fall in the threshold for excitation of the neurone. When a cell lies in an applied external electrical field a potential gradient exists between the cell body and its distant processes. If this gradient is such that the cell body is negative relative to its processes, the cell becomes more excitable; it becomes less excitable when the cell body is relatively positive.

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