Fig. 8: Schematic illustration of TC coupling mechanism.

a Input to the apical dendrites of PTs (blue) that precedes the onset of direct sensory input from the thalamus determines how far the dendritic membrane potential is away from the Ca²⁺ AP threshold. b The more depolarized the Ca²⁺ domain is prior to the stimulus, the closer will the direct sensory input from the thalamus (green), which specifically targets this domain, shift it to the Ca²⁺ AP threshold. c This fast, but weak, TC-driven activation of the Ca²⁺ domain enables indirect sensory input streams (gray), which would otherwise be too weak, to drive sensory-evoked Ca²⁺ APs (e.g., TC→L4SPs→PT). Bursts of 3 APs rely on this transition of the TC-driven dendritic responses into Ca²⁺ APs. d Indirect sensory input streams (i.e., TC→L6CC→PT^32,35) are sufficient to evoke 1 AP responses (gray). Multiplexing of direct and indirect sensory inputs in basal dendrites is sufficient to modulate 1 AP responses with bursts of 2 APs (green). However, additional inputs to apical dendrites (sensory-evoked and/or nonsensory) are required to modulate sensory-evoked cortical output with bursts of 3 APs (blue). This multiplexing of basal with apical inputs into bursts of 3 APs arises from the specific innervation of the Ca²⁺ domain by the TC→PT pathway. We hence term this mechanism ‘TC coupling’.