Two States of Activity in Single Thalamocortical Neurons In Vitro.
Intracellular recordings from single thalamocortical neurons maintained in vitro reveal two distinct states of activity. Without any current injection, some of these neurons generate repetitive bursts of action potentials at 0.5 to 4 Hz, just as do the same neurons in the deeply sleeping animal. A close examination of these oscillations reveal that they are generated through the interaction of two distinct ionic currents: the low threshold Ca2+ current, or T-current, and a hyperpolarization-activated cation current known as the H-current.
The generation of low threshold Ca2+ spikes by the T-current was first demonstrated in thalamic neurons by Jahnsen and Llinas (J. Physiol. 349: 205-247, 1984) and also recorded in vivo by Deschenes, Steriade and colleagues (see Steriade and Deschênes, Brain Res. Rev. 8: 1-63, 1984).
Depolarizing thalmaocortical neurons with the intracellular injection of current (depol. current injection) results in the abolition of rhythmic burst firing and the occurrence of single spike activity, as in the transition from slow wave sleep to waking or REM sleep. This abolition of rhythmic burst firing results from the inactivation of the low threshold Ca2+ current and the lack of activation of the H-current.
Removal of the depolarizing current injection reinstates the rhythmic burst firing, as in the transition back to slow wave sleep.
Interestingly, these results indicate that even single thalamocortical neurons exhibit the two distinct states of the EEG: rhythmic burst firing during slow wave sleep and tonic activity during waking.