Relations between EEG phenomena and potentials of single cortical cells. II. Spontaneous and convulsoid activity.

Abstract 1. 1. The relations between spontaneous EEG waves, convulsoid potentials (induced by i.v. Metrazol injection or epicortical stimulation) and intracellularly recorded activity were analyzed in the motor cortex of cats under Nembutal. 2. 2. During regular surface negative spindle waves a close positive correlation exists between cellular depolarization and surface negativity. The cellular depolarizations are composed of several EPSPs which can be easily distinguished in some cells. Sometimes they begin to summate during a slight surface positivity preceding the negative wave. The summated post-synaptic depolarization is preceded by a period of synaptic silence, indicating an extracellular, probably extracortical, trigger mechanism. IPSPs are seen only if much spike activity is present. 3. 3. Another type of spindle wave (sometimes seen in isolation) is characterized by a relatively short and weak surface negative wave followed by a longer positive potential. The cellular activity shows more or less pronounced depolarization and spike activity during or just preceding the negative wavelet and an IPSP during the surface positive potential. The IPSP may begin during the peak or falling phase of the surface negative potential. The presence of active inhibition in these cases has been proved by intracellular stimulation. 4. 4. If biphasic positive-negative spindle waves are present their relation to the cellular activity depends upon the potential gradient of the wave. If the positive-negative gradient is steep, cellular activity with a relatively high discharge rate is located on the positive phase; if the gradient is relatively small the slow summation of EPSPs reaches its peak during the negative wave (see Fig. 9). 5. 5. At the beginning of Metrazol-induced seizures slow surface negative waves occur simultaneously with cellular depolarization. During the peak of the seizure, when mainly large biphasic positive-negative potentials are seen in the EEG, massive cellular depolarization, leading quickly to high frequency grouped spike discharges, occurs with the first surface positive phase. Cellular repolarization coincides with the positive-negative transition of the surface potential. The biphasic EEG potential is frequently followed by a longer positive potential which corresponds to a cellular IPSP. At the end of the seizure isolated IPSPs are frequently seen. 6. 6. Strong high frequency epicortical stimulation leads to a long lasting depolarization of cortical cells (down to 20–30 mV) with inactivation of the spike generator. After the end of the stimulation the membrane potential shows regular oscillations, the spike generator recovers and the cell polarizes slowly. Finally, oscillating depolarizations are interrupted by long polarizations. Some cells do not show a long lasting depolarization after the stimulus but are subsequently synaptically depolarized during the depolarizing oscillations. The depolarizations occur simultaneously with large surface positive potentials of the cortical after-discharge. 7. 7. Possible mechanisms relating EEG waves to cellular potentials are discussed within the framework of transcortical and soma-apical dendrite potential distributions during different phases of physiological and pathological cortical activity.