Natural waking and sleep states: A view from inside neocortical neurons

In this first intracellular study of neocortical activities during waking a nd sleep states, we hypothesized that synaptic activities during natural st ates of vigilance have a decisive impact on the observed electrophysiologic al properties of neurons that were previously studied under anesthesia or i n brain slices. We investigated the incidence of different firing patterns in neocortical neurons of awake cats, the relation between membrane potenti al fluctuations and firing rates, and the input resistance during all state s of vigilance. In awake animals, the neurons displaying fast-spiking firin g patterns were more numerous, whereas the incidence of neurons with intrin sically bursting patterns was much lower than in our previous experiments c onducted on the intact-cortex or isolated cortical slabs of anesthetized ca ts. Although cortical neurons displayed prolonged hyperpolarizing phases du ring slow-wave sleep, the firing rates during the depolarizing phases of th e slow sleep oscillation was as high during these epochs as during waking a nd rapid-eye-movement sleep. Maximum firing rates, exceeding those of regul ar-spiking neurons, were reached by conventional fast-spiking neurons durin g both waking and sleep states, and by fast-rhythmic-bursting neurons durin g waking. The input resistance was more stable and it increased during quie t wakefulness, compared with sleep states. As waking is associated with hig h synaptic activity, we explain this result by a higher release of activati ng neuromodulators, which produce an increase in the input resistance of co rtical neurons. In view of the high firing rates in the functionally discon nected state of slow-wave sleep, we suggest that neocortical neurons are en gaged in processing internally generated signals.