Figure 4.

Silent inhibition. (A) The tuning of subthreshold responses in cortical neurons driven by synaptic input in response to sensory stimuli can sometimes look very different to the suprathreshold spiking output which tends to be more narrowly tuned. In layer 2 pyramidal neurons of the visual cortex in mice, differently oriented visual stimuli cause very subtle differences in subthreshold responses but wildly different AP firing (Jia et al., 2010). (B) A similar phenomenon has been shown in somatosensory cortex of rodents where paired ipsi- and contralateral hindlimb stimuli (P-HS) lead to identical subthreshold responses to contralateral hindlimb stimuli (C-HS) alone whereas as spike output is different (Palmer et al., 2012). (C) In the somatosensory case, at least, it could be shown definitively that the difference was due to a form of dendritic GABA_B-mediated silent inhibition. This comes about because the inhibitory action is predominately on voltage-sensitive dendritic conductances. Without dendritic inhibition, AP firing in the neuron activates dendritic conductances which contribute to the integrative process in the production of subsequently generated APs. (D) Dendritic inhibition in the absence of cell firing opens dendritic K⁺ channels which have a very weak influence on the soma but their blocking action on dendritic Ca²⁺ channels is hidden or silent because these channels are closed anyway in the absence of dendritic depolarization. (E) Only during cell firing can the absence of the contribution from dendritic conductances be observed. Adapted from (Palmer et al., 2012).