The interconnectedness produces the synchrony of neurons during the oscillation Third, VPM and nRT neurons are highly interconnected-both within and between the thalamic nuclei. The action potentials are transmitted from VPM to S1 bfd to form EEG spikes as well as to nRT, which provides feedback inhibition to VPM. In the bursting mode, a VPM neuron pacemaker-like current (I h) activates upon hyperpolarization and the resultant depolarization activates a T-type calcium channel (T-type current) that transiently further depolarizes the neurons allowing a brief period of repetitive action potentials. Second, VPM neurons fire in two modes-tonic and bursting. First, excitatory neurons in VPM project to the inhibitory GABAergic neurons in nRT, which then provide feedback inhibition to VPM. Three key anatomical/physiological features of this cortico-thalamo-cortical (CTC) circuitry promote oscillations. In rodent models, typical absence seizures also activate discrete, interconnected cortical and thalamic regions including the somatosensory barrel cortex (S1 bfd) and the thalamic reticular (nRT) and ventroposterior medial (VPM) nuclei (for in-depth reviews, see Fogerson and Huguenard, 2016 Luttjohann and van Luijtelaar, 2015). However, functional MRI (fMRI) ( Carney et al., 2012 Gotman et al., 2005), magnetoencephalography (MEG) ( Miao et al., 2014 Stefan et al., 2009 Tenney et al., 2014 Westmijse et al., 2009), and high-density EEG ( Holmes et al., 2004) studies revealed that generalized seizures engage distinct brain networks that often involve the thalamus and frontal and parietal cortices. Generalized seizures such as typical absence seizures engage widespread regions in both cerebral hemispheres extremely rapidly and thus give the appearance on clinical EEG of activating the entire cortex instantaneously. Joseph Jankovic MD, in Bradley and Daroff's Neurology in Clinical Practice, 2022 Typical Absence Seizures
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