1. Cycle 7: Thalamocortical Oscillations
Various thalamocortical oscillations,
involving both intrinsic and network mechanisms.
We’ll focus on
1) Connectivity of thalamus-cortex
2) Intrinsic and network mechanisms
3) Delta oscillation, spindles

2. Cycle 7: Thalamocortical Oscillations
See RotB p.178, or: Figure 5: Schematic diagram showing first order and higher order relays. A first order thalamic relay represents the first relay of peripheral or subcortical information of a particular type to a first order or primary cortical area. A higher order relay relays information from layer 5 of one cortical area to another cortical area; this can be between first order and higher order cortical area or between two higher order cortical areas. The difference is the driver input, which is subcortical for a first order relay and from layer 5 of cortex for a higher order relay. Abbreviations: FO, first order; HO, higher order; LGN, lateral geniculate nucleus; MGNd and MGNv, dorsal and ventral portions of the medial geniculate nucleus; PO, posterior nucleus; Pul, pulvinar; TRN, thalamic reticular nucleus; VP, ventral posterior nucleus.

3. Cycle 7: Thalamocortical Oscillations
Thalamocortical relay neuron, 2 modes of firing
Figure 3: Various properties of the low threshold Ca2+ spike; redrawn from (Sherman and Guillery, 2006). A,B: Intracellular recording of a relay cell from the lateral geniculate nucleus of a cat in vitro. At an initial membrane potential of -59 mV, IT is inactivated and thus the cell responds in tonic mode (A). Here, the response to a depolarizing 0.3 nA current injection is a steady stream of unitary action potentials. At an initial membrane potential of -70 mV, IT is de-inactivated and thus the cell responds in burst mode (B). Now, the very same current injection activates the low threshold Ca2+ spike, which in turn activates, in this case, a burst of 8 conventional action potentials. C: Initial response of cell in A,B to various levels of current injection from different initial membrane potentials. At levels that inactivate IT and produce tonic firing (-47 mV and -59 mV), a fairly linear relationship ensues. At levels that de-inactivate IT and produce burst firing (-77 mV and -83 mV), a nonlinear relationship in the form of a step function is seen at low stimulus intensities.

4. Cycle 7: Thalamocortical Oscillations
Corticothalamic (relay) neuron, key intrinsic mechanism of oscillation
IT of T-type calcium channel generates Ca++ wave + burst, ‘oscillatory’ mode
Ih”inward rectifying” current (Buzsaki calls “pacemaker”) provides slow depolarization to trigger voltage-dependent T-type calcium channel.

5. Cycle 7: Thalamocortical Oscillations
Schematic view of five sections through the thalamus of a monkey; redrawn from Sherman and Guillery (2006). The nuclei filled in yellow are first order nuclei, and those filled in blue are higher order. TRN, thalamic reticular nucleus (this is not a relay nucleus);
Figure 1: Schematic view of five sections through the thalamus of a monkey; redrawn from Sherman and Guillery (2006). The sections are numbered 1 through 5 and were cut in the coronal planes indicated by the arrows in the upper right mid-sagittal view of a monkey brain. The major thalamic nuclei in one hemisphere are shown for a generalized primate. The nuclei filled in yellow are first order nuclei, and those filled in blue are higher order (see text for definition of first and higher order thalamic relays). Nearby nuclei or structures left blank are not parts of thalamus. Abbreviations: AD, anterodorsal nucleus; AM, anteromedial nucleus; AV, anteroventral nucleus; CM, centermedian nucleus; CN, caudate nucleus; H, habenular nucleus; IL, intralaminar (and midline) nuclei; LD, lateral dorsal nucleus; LGN, lateral geniculate nucleus; LP, lateral posterior nucleus; MD, mediodorsal nucleus; MGN, medial geniculate nucleus; PO, posterior nucleus; PU, pulvinar; TRN, thalamic reticular nucleus (this is not a relay nucleus; see text for details); VA, ventral anterior nucleus; VL, ventral lateral nucleus; VPI, VPL, VPM, are the inferior, the lateral and the medial parts of the ventral posterior nucleus.

6. Cycle 7: Thalamocortical Oscillations
Figure 2: Schematic representation of actions of voltage dependent T (Ca2+) and K+ conductances underlying low threshold Ca2+ spike; redrawn from (Sherman and Guillery, 2006). The sequence of events is shown clockwise in A-D, with the membrane voltage changes shown in E. A: At a relatively hyperpolarized resting membrane potential (roughly -70 mV), the activation gate of the T channel is closed, but the inactivation gate is open, and so the T channel is deactivated and de-inactivated. The K+ channel is also deactivated. B: With sufficient depolarization to reach its threshold, the activation gate of the T channel opens, allowing Ca2+ to flow into the cell. This depolarizes the cell, providing the upswing of the low threshold spike. C: The inactivation gate of the T channel closes after roughly 100 msec ("roughly", because closing of the channel is a complex function of time and voltage), inactivating the T channel, and the K+ channel also opens. These combined actions repolarize the cell. D: Even though the initial resting potential is reached, the T channel remains inactivated, because it takes roughly 100 msec of hyperpolarization to de-inactivate it; it also takes a bit of time for the various K+ channels to close. E: Membrane voltage changes showing low threshold Ca2+ spike.
Thalamic reticular neuron, key intrinsic mechanism of oscillation

7. Cycle 7: Thalamocortical Oscillations
Figure 4: Schematic diagram of circuitry for the lateral geniculate nucleus. The inputs to relay cells are shown along with the relevant neurotransmitters and postsynaptic receptors (ionotropic and metabotropic) Abbreviations: LGN, lateral geniculate nucleus; BRF, brainstem reticular formation; TRN, thalamic reticular nucleus.

8. Cycle 7: Thalamocortical Oscillations
Thalamic delta (1-4 Hz) results from intrinsic oscillations of thalamic relay neurons
low-threshold Ca2+ current (IT)
hyperpolarization activated cation current (Ih).
Seen during deep SWS, when cells are hyperpolarized and in a state of low conductance/low synaptic activity
relay neurons deinactivate IT (McCormick and Pape, 1990).
Periods of delta-like oscillation in thalamocortical neuron in decorticated cats can start from subtle fluctuations of the membrane potential. The amplitude of such activity increases and decreases without changes in frequency.
Synchrony between different thalamic relay neurons during delta activity has not been found in decorticated cats (Timofeev and Steriade 1996). it is unlikely that thalamic delta activity could play a leading role in the initiation and maintenance of cortical delta rhythm. However, the presence of a corticothalamic feedback in intact-cortex animals could synchronize thalamic burst-firing at delta frequency and generate field potentials.