Figure 1. Neuronal loops between the thalamus and cerebral cortex. Three types of neurons were intracellularly recorded and stained: corticothalamic, thalamic reticular and thalamocortical. Direction of axons is indicated by arrows. Insets represent their responses to thalamic and cortical stimulation (arrowheads point to stimulus artefacts). The corticothalamic neuron from cortical area 7 responded to thalamic stimulation of centrolateral intralaminar nucleus with antidromic (a) and orthodromic (o) action potentials (top superimposition). At more hyperpolarized levels (bottom superimposition), a response failed, but b response survived as subthreshold excitatory postsynaptic potentials (EPSPs). This is a typical neuron interposed in a corticothalamocortical loop. The thalamic reticular GABAergic neuron (recorded from the rostrolateral district of the reticular nucleus) responded to motor cortical stimulation with a high-frequency spike-burst, followed by a sequence of waves on a depolarizing (upward) envelope. These waves are within the frequency range of sleep spindles. The thalamocortical neuron (recorded from the ventrolateral nucleus) responded to motor cortex stimulation with a biphasic inhibitory postsynaptic potential (IPSP), generated by the thalamic reticular neuron, leading to a low-threshold spike (LTS), and followed by a sequence of hyperpolarizing spindle waves. Modified from Steriade (1999).

Figure 2. (a) Thalamic glomeruli and three types of inhibitory postsynaptic potentials (IPSPs) in thalamocortical neurons. Synaptic contacts between thalamic reticular (RE), thalamocortical (Relay), and local interneurons (I) are indicated by dots at the end of axons (ax). Note that RE axons contact not only relay cells but also local interneurons. Afferent (Aff) fibres to the thalamus contact dendrites (d) of relay cells and local interneurons. Territory delineated by dashed line, representing a thalamic glomerulus, is expanded below (arrow). The first presynaptic element consists of ascending afferent axons that have round and large (RL) vesicles and make asymmetrical synaptic contacts with dendrites of the relay cell (Rd) as well as dendrites of local interneurons. The second presynaptic element is the dendrite of the interneuron which is postsynaptic to the afferent axon, contains pleomorphic (P) vesicles, and makes symmetrical synaptic contacts on dendrites of the relay cell as well as on dendrites of other local interneurons (I₁d and I₂d are dendrites of two interneurons). In the latter case, contacts may be reciprocal. (b) Superimposition of intracellular recordings in a thalamic relay neuron showing three types of IPSPs: a, A and B. See also text. (b) Modified from Paré et al. (1991).

Figure 3. Similarities between the low-threshold spike (LTS) of thalamocortical neurons in vitro and in vivo. (a) Thalamic neuron recorded from dorsal thalamic slice of guinea-pig, maintained in vitro. 1, Subthreshold current pulse (lowest trace) produced a subthreshold depolarization of the cell; the same stimulus, delivered after an imposed direct current depolarization of the cell, produced repetitive, single-spike (tonic) firing. 2, After hyperpolarization of the cell, current pulse similar to that in (1) produced an LTS crowned by a high-frequency burst of action potentials. 3, Rebound LTS also occurred after hyperpolarizing pulses of different amplitudes. (b) Thalamocortical neuron from the cat ventrolateral (VL) nucleus, recorded in vivo. The top three traces depict the neuronal responses to stimulation of the internal capsule, at three different membrane potentials (note EPSP triggering LTS, eventually reaching the threshold for action potentials, at –75 mV). The bottom four traces (depolarizing and hyperpolarizing current pulses) show: tonic firing at –65 mV; passive response at –70 mV; spike-burst at –80 mV; and spike-burst at the break of a hyperpolarizing pulse, at –63 mV. (c) The same neuron was stained with neurobiotin; the soma had a diameter of 20 m and radiating tufted dendrites were characteristic for bushy thalamocortical neurons. Modified (a) from Llinás and Jahnsen (1982) and (b, c) Contreras and Steriade (1995).

Figure 4. Cellular mechanisms of clock-like thalamic oscillation, in vivo and in vitro. (a) Voltage dependency of oscillation. Shown in the intracellular recording in vivo of a cat thalamocortical neuron from the lateroposterior (LP) neuron after ablation of cortical areas projecting to the LP nucleus. The cell oscillated spontaneously at 1.7 Hz. A 0.5-nA depolarizing current pulse +d.c., between arrows), bringing the membrane potential to –63 mV, prevented the oscillation, and its removal set the cell back into the oscillatory mode. Three cycles marked by the horizontal bar in the upper trace are expanded below. (b) Spontaneous rhythmic burst firing in a cat lateral geniculate relay cell recorded in vitro, before and after block of the voltage-dependent  Na⁺ channel blocker tetrodotoxin. Modified from Steriade et al. (1993).