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. 2017 Mar 28;7(4):34. doi: 10.3390/brainsci7040034

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© 2017 by the author.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Potential mechanistic targets in the cortico-reticulo-thalamocortical (CT-TRN-TC) system for transcranial electrical stimulation. This model includes three parts, which are assumed to work together: (i) The innervation of the intracortical circuitry by both the descending axonal branches (top-down process) of the axons running within the molecular layer and the ascending TC inputs (bottom-up process); (ii) functional interactions between glutamatergic and GABAergic neurons of the intracortical circuitry, which includes feedback and feedforward excitations (from CT and TC axon collaterals, respectively); and (iii) the layer VI CT pathway, one of the outputs of the intracortical circuitry, which innervates simultaneously the thalamic GABAergic reticular (TRN) and glutamatergic relay (TC) neurons. In this model, the TRN cells generate more lateral than feedback inhibition in the dorsal thalamus, which contains only TC neurons. The layer VI CT axonal projections are about ten–fold higher in number than the TC projections, thereby generating a great excitatory pressure on TRN and TC neurons. Furthermore, the apical dendrites of layer VI pyramidal neurons terminate in layers III–IV. Each neuron exhibits its own firing pattern that is state-, voltage-, synaptic- and time-dependent. The action potentials (APs) are drawn like a code bar. Under physiological condition, it is assumed that the APs are initiated at the axon hillock, the initial segment of the axon. The axon can also transmit, in addition to APs, analog signals (generated in the somatodendritic domain and represented by sinusoidal waves) along the axon (at least several hundreds of micrometers away from the soma) and can modulate AP-evoked transmitter release at the corresponding synapses. In this model, it is assumed that axodendritic (chemical synapses) and dendrodentritic electrical (via gap junctions) coupling exist between the two types (basket and chandelier) of GABAergic parvalbumin (PV) expressing cells. 5-HT, 5-HT3A receptor; CT, corticothalamic; SOM, somatostatin; ssc, spiny stellate cells; TC, thalamocortical; TRN, thalamic reticular nucleus.