Chunk #9 — Activation of metabotropic glutamate receptors (mGluRs) or muscarinic Ach receptors (mAchRs) induces rhythmic bursting at α/θ frequencies in TC neurons
Given that the thalamus is involved in synchronized oscillations at both low and high cognitively-relevant frequencies, a key goal is to understand the intrinsic properties in thalamic neurons that are central to this involvement. Under normal conditions, TC neurons recorded intracellularly in vitro show two distinct modes of firing (Llinás and Jahnsen 1982). When these cells are relatively hyperpolarized (≤~−65 mV) a brief injection of positive current leads to a transient depolarization lasting around ~100–200 ms which is crowned by a high-frequency burst of action potentials (i.e. burst mode) (Fig. 1A, top left) (Llinás and Jahnsen 1982). This transient depolarization is typically referred to as a low-threshold Ca2+ potential (LTCP) or low-threshold spike (LTS) and is generated by a T-type Ca2+ current (Coulter et al. 1989; Crunelli et al. 1989; Hernandez-Cruz and Pape 1989; Suzuki and Rogawski 1989). In contrast, when TC neurons are relatively depolarized (≥~−60 mV), a brief injection of positive current leads to tonic firing or single spike activity (i.e. tonic or relay mode, see below) (Fig. 1A, top right) (Llinás and Jahnsen 1982). The discovery of
