30. Januar, 2026, 11.00 Uhr: Anatomisches Kolloquium

Adaptive Dynamics of Single Neurons in Small Networks 

A hallmark of neuronal network activity is the selective recruitment of neurons into active ensembles that form transiently stable patterns of activity. In the mammalian hippocampus, the activation of such neuronal ensembles is orchestrated by network oscillations. A central question is how individual neurons are selected to participate in these patterns of coactivity. The prevailing concept holds that the activation of specific neurons results from the convergence and use-dependent plasticity of excitatory synapses. Within this framework, strong rhythmic perisomatic inhibition during network oscillations is thought to provide a global gain control mechanism for local neurons, as well as a temporal scaffold for embedded spatiotemporal activity patterns. 

According to the textbook view, pyramidal neurons integrate synaptic inputs arriving at their dendrites at the soma, from which the axon originates, and generate action potentials whenever excitation is sufficient to reach threshold at the axon initial segment. Recent work, however, has revealed that in approximately 50% of hippocampal pyramidal neurons the axon emerges not from the soma, but from a basal dendrite. This morphological asymmetry preferentially weights inputs to the axon-carrying dendrite, which may escape control by somatic inhibition and thus play a special role in the activation of these neurons. 

We investigated this potential mechanism during ripple oscillations in the rodent hippocampus, a network state known to recruit highly specific neuronal ensembles while engaging pronounced perisomatic inhibition. In awake, head-fixed mice, we found that CA1 pyramidal neurons with a dendritic axon origin exhibited an approximately four-fold higher firing rate during ripple oscillations compared to neurons with a somatic axon origin. This difference was not observed outside of ripples. Extra- and intracellular recordings in mouse brain slices, together with computer simulations, led us to hypothesize that the axon forms a functional unit with its parent dendrite. Consistent with this idea, experimental data and simulations demonstrate that excitatory input to the axon-carrying dendrite readily triggers action potentials even in the presence of strong perisomatic inhibition. In contrast, inputs to other dendrites are effectively uncoupled from the axon by perisomatic inhibition, preventing them from triggering action potentials. 

The inhibition-dependent recruitment of morphologically distinct pyramidal neurons thus represents a novel mechanism for state-dependent neuronal selection. Perisomatic inhibition may therefore serve not only as a global gain control, but also as a dynamic gating mechanism that enables the activation of ensembles drawn from different populations of principal cells. Given the prevalence of similar axon morphologies in other cortical and subcortical regions of the vertebrate brain, selection of active neurons based on axon origin may constitute a widespread organizational principle.