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Apical tuft input efficacy in layer 5 pyramidal cells from rat visual cortex

Paul A. Rhodes and Rodolfo R. Llinás

1. The integration of synaptic inputs to the apical dendrite of layer 5 neocortical pyramidal cells was studied using compartment model simulations. The goal was to characterize the generation of regenerative responses to synaptic inputs under two conditions: (a) where there was an absence of background synaptic input, and (b) when the entire cell surface was subjected to a uniform blanket of synaptic background conductance such that somatic input resistance was reduced 5-fold.
2. Dendritic morphology corresponded to a layer 5 thick-trunked pyramidal cell from rat primary visual cortex at postnatal day 28 (P28), with distribution of dendritic active currents guided by the electrophysiological characteristics of the apical trunk reported in this cell type. Response characteristics for two dendritic channel distributions were compared, one of which supported Ca²⁺ spikes in the apical dendrite.
3. In the absence of background, synaptic input to the apical tuft was surprisingly effective in eliciting somatic firing when compared with input to apical oblique branches. This result obtained even when the tuft membrane was the least excitable in the dendritic tree.
4. The special efficacy of tuft input arose because its electrotonic characteristics favour development of a sustained depolarization which charged the apex of the apical trunk to its firing threshold; once initiated in the distal trunk, firing propagated inward to the soma. This mechanism did not depend upon the presence of depolarizing channels in tuft membrane, but did require an excitable apical trunk.
5. Rather than disconnect the tuft, background synaptic conductance enhanced the efficacy advantage enjoyed by input arriving there. This counterintuitive result arose because background reduced the subthreshold spread of voltage, and so diminished the ability of the excitation of various individual oblique branches to combine to charge the relatively thick adjacent trunk. In contrast, drive from the depolarized tuft is exerted at a single critical point, the apex of the distal trunk, and so was relatively undiminished by the background. Further, once initiation at the apex occurred, background had little effect on inward propagation along the trunk.
6. We conclude that synaptic input to the apical tuft of layer 5 cells may be unexpectedly effective in triggering cell firing in vivo. The advantage in efficacy was not dependent upon the characteristics of tuft membrane excitability, but rather stemmed from the geometry of the tuft and its junction with the distal apical trunk. The efficacy of tuft input was, however, critically dependent upon inward propagation, suggesting that modulation of membrane currents which affect propagation in the apical trunk might sensitively control the efficacy of tuft input.

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