1. Changes in the resting potential and the effective membrane resistance were measured in 77 cells in cat precruciate cortex during the transition from cortical slow wave phase to EEG arousal. 2. These 77 neurones were classified into the recipient cells of the following five different actions on the EEG arousal: (1) postsynaptic excitation (E cells), (2) postsynaptic inhibition (1 cells), (3) disinhibition (DI cells), (4) disfacilitation (DF cells) and (5) disfacilitation followed by excitation (DF-E cells). 3. The location of E cells ranged from laminae I to V, but the majority was found in lamina II. Most I cells were located in the upper half of lamina III, and a few in lamina V. DF, DI and DF-E cells existed deeply from the lower half of lamina III to laminae V-VI. 4. Slow pyramidal tract (PT) cells (n=6) all belonged to the E cell group, whereas fast PT cells were divided into the DF (n=10) and DF-E cell groups (n=4). 5. It is postulated that the EEG arousal is initiated with a direct excitation of laminae I-II cells, followed by excitation and inhibition to the upper lamina III cells and further processed to laminae III-VI cells with indirect excitation, inhibition, disinhibition and disfacilitation. The model of four vertical transmission relays is proposed to depict the cascade pattern of information being processed through the cortex during the EEG arousal.In a preceding paper (INUBUSHI et al., 1978) it was shown that the EEG arousal involved reactions of almost all cortical neurones toward depolarization (D-type), hyperpolarization (H-type) or both in a sequence (mixed type). The depolarizing response could be induced by bombardment of excitatory inputs (postsynaptic excitation), withdrawal of inhibitory inputs (disinhibition) or both. Likewise, the hyperpolarizing response could be produced by the postsynaptic inhibition, withdrawal of excitation (disfacilitation) or both. The present study attempts to determine which of these four actions is dominant in a cell examined during

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1. Intracellular recordings were made from 92 neurones in the precruciate cortex of encephale isole and midpontine pretrigeminal preparations of the cat. 2. All but only one of these cells showed appreciable changes in the membrane potential during the transition from the cortical slow wave phase to the EEG arousal occurring spontaneously or induced by stimulating the midbrain reticular formation. Thus, 38 cells were depolarized (D-type cells), 48 cells hyperpolarized (H-type cells) and 5 cells showed an early hyperpolarization and a later depolarization (mixed type). 3. The latency of intracellular responses to reticular stimulation was shorter in the D-type cells than the H-type or mixed-type cells, and shorter for each of the D-and H-types in the cells of the superficial layers than those of the deep layers. 4. The D-type cells were distributed widely through laminae I to V, but the majority was sampled in lamina II. The H-type cells were located in laminae III-VI with the mode at the upper half of lamina III. The mixed-type cells were mostly located in laminae V and VI. 5. Antidromically identified slow pyramidal tract (PT) cells (n =9) all belonged to the D-type, and fast PT cells either to the H-(n=11) or the mixed type (n 4) . 6. These results suggest that the EEG arousal is a state composed of both excitatory and inhibitory responses of cortical cells which are processed from the superficial to the deep layers.EEG arousal (MORUZZI and MAGOUN, 1949) has been ascribed to one of the main manifestations of arousal of the brain, and may accompany the orienting or investigatory response which provides a turning point in animal behaviour (PAVLOV, 1927 ;MORUZZI, 1962 ;MAGOUN, 1963 ;SOKOLOV, 1963). Recent electrophysiological studies of EEG, extracellular units or field potentials recorded from various neocortical regions have revealed that neuronal responses on the EEG arousal

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