Thalamocortical activity exhibits two distinct states: (a) synchronized rhythmic activity in the form of delta, spindle, and other slow waves during EEG-synchronized sleep and (b) tonic activity during waking and rapid-eye-movement sleep. Spindle waves are generated largely through a cyclical interaction between thalamocortical and thalamic reticular neurons involving both the intrinsic membrane properties of these cells and their anatomical interconnections. Specific alterations in the interactions between these cells can result in the generation of paroxysmal events resembling absence seizures in children. The release of several different neurotransmitters from the brain stem, hypothalamus, basal forebrain, and cerebral cortex results in a depolarization of thalamocortical and thalamic reticular neurons and an enhanced excitability in many cortical pyramidal cells, thereby suppressing the generation of sleep rhythms and promoting a state that is conducive to sensory processing and cognition.

Spindle waves are a prototypical example of synchronized oscillations, a common feature of neuronal activity in thalamic and cortical systems in sleeping and waking animals. Spontaneous spindle waves recorded from slices of the ferret lateral geniculate nucleus were generated by rebound burst firing in relay cells. This rebound burst firing resulted from inhibitory postsynaptic potentials arriving from the perigeniculate nucleus, the cells of which were activated by burst firing in relay neurons. Reduction of gamma-aminobutyric acidA (GABAA) receptor-mediated inhibition markedly enhanced GABAB inhibitory postsynaptic potentials in relay cells and subsequently generated a slowed and rhythmic population activity resembling that which occurs during an absence seizure. Pharmacological block of GABAB receptors abolished this seizure-like activity but not normal spindle waves, suggesting that GABAB antagonists may be useful in the treatment of absence seizures.

1. A network model of thalamocortical (TC) and thalamic reticular (RE) neurons was developed based on electrophysiological measurements in ferret thalamic slices. Single-compartment TC and RE cells included voltage- and calcium-sensitive currents described by Hodgkin-Huxley type of kinetics. Synaptic currents were modeled by kinetic models of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), gamma-aminobutyric acid-A (GABAA) and GABAB receptors. 2. The model reproduced successfully the characteristics of spindle and slow bicuculline-induced oscillations observed in vitro. The characteristics of these two types of oscillations depended on both the intrinsic properties of TC and RE cells and their pattern of interconnectivity. 3. The oscillations were organized by the reciprocal recruitment between TC and RE cells, due to their manual connectivity and bursting properties. TC cells elicited AMPA-mediated excitatory postsynaptic potentials (EPSPs) in RE cells, whereas RE cells elicited a mixture of GABAA and GABAB inhibitory postsynaptic potentials (IPSPs) in TC cells. Because of the presence of a T current, sufficiently strong EPSPs could elicit a burst in RE cells, and TC cells could generate a rebound burst following GABAergic IPSPs. Under these conditions, interaction between the TC and RE cells produced sustained oscillations. 4. In the absence of spontaneous oscillation in any cell, the TC-RE network remained quiescent. Spindle oscillations with a frequency of 9-11 Hz could be initiated by stimulation of either TC or RE neurons. A few spontaneously oscillating TC neurons recruited the entire network model into a "waxing-and waning" oscillation. These "initiator" cells could be an extremely small proportion of TC cells. 5. In intracellular recordings, TC cells display a reduced ability for burst firing after a sequence of bursts. The "waning" phase of spindles was reproduced in the network model by assuming an activity-dependent upregulation of Ih operating via a calcium-binding protein in TC cells, as shown previously in a two-cell model. 6. Following the global suppression of GABAA inhibition, the disinhibited RE cells produced prolonged burst discharges that elicited strong GABAB-mediated currents in TC cells. The enhancement of slow IPSPs in TC cells was also due to cooperativity in the activation of GABAB-mediated current. These slow IPSPs recruited TC and RE cells into slower waxing-and-waning oscillations (3-4 HZ) that were even more highly synchronized. 7. Local axonal arborization of the TC to RE and RE to TC projections allowed oscillations to propagate through the network. An oscillation starting at a single focus induced a propagating wavefront as more cells were recruited progressively. The waning of the oscillation also propagated due to upregulation of Ih in TC cells, leading to waves of spindle activity as observed in experiments. 8. The spatiotemporal properties of propagating waves in the model were highly dependent on the intrinsic properties of TC cells. The spatial pattern of ...

We review here the development of Hodgkin-Huxley (HH) type models of cerebral cortex and thalamic neurons for network simulations. The intrinsic electrophysiological properties of cortical neurons were analyzed from several preparations, and we selected the four most prominent electrophysiological classes of neurons. These four classes are "fast spiking", "regular spiking", "intrinsically bursting" and "low-threshold spike" cells. For each class, we fit "minimal" HH type models to experimental data. The models contain the minimal set of voltage-dependent currents to account for the data. To obtain models as generic as possible, we used data from different preparations in vivo and in vitro, such as rat somatosensory cortex and thalamus, guinea-pig visual and frontal cortex, ferret visual cortex, cat visual cortex and cat association cortex. For two cell classes, we used automatic fitting procedures applied to several cells, which revealed substantial cell-to-cell variability within each class. The selection of such cellular models constitutes a necessary step towards building network simulations of the thalamocortical system with realistic cellular dynamical properties.