In mammalian neurons, the axon initial segment (AIS) electrically connects the somatodendritic compartment with the axon and converts the incoming synaptic voltage changes into a temporally precise action potential (AP) output code. Although axons often emanate directly from the soma, they may also originate more distally from a dendrite, the implications of which are not wellunderstood. Here, we show that one-third of the thick-tufted layer 5 pyramidal neurons have an axon originating from a dendrite and are characterized by a reduced dendritic complexity and thinner main apical dendrite. Unexpectedly, the rising phase of somatic APs is electrically indistinguishable between neurons with a somatic or a dendritic axon origin. Cable analysis of the neurons indicated that the axonal axial current is inversely proportional to the AIS distance, denoting the path length between the soma and the start of the AIS, and to produce invariant somatic APs, it must scale with the local somatodendritic capacitance. In agreement, AIS distance inversely correlates with the apical dendrite diameter, and model simulations confirmed that the covariation suffices to normalize the somatic AP waveform. Therefore, in pyramidal neurons, the AIS location is finely tuned with the somatodendritic capacitive load, serving as a homeostatic regulation of the somatic AP in the face of diverse neuronal morphologies.T he axon initial segment (AIS) specifies in vertebrate neurons a single domain for the final integration of synaptic input and the initiation of action potentials (APs) (1, 2). To rapidly produce large inward and outward currents mediating the AP, the AIS contains a complex arrangement of cytoskeletal and transmembrane proteins clustering high densities of voltage-gated sodium (Nav) and potassium (Kv) channels in the axolemma (2-4). Although the composition of ion channels is critical for initiation and regulation of firing patterns, there are emerging insights that the AIS is not operating in isolation but is also subject to activity-dependent changes in size and location constrained by the local dendritic branch geometry and the passive cable properties (5-7). Experimental studies linking changes in AIS length and neuronal output showed that an increased length facilitates AP generation (6,8). In these cases, the net increased excitability is a logical consequence of the larger Nav conductance. However, predicting the impact of AIS location on neuronal output is more complex. Experimental studies showed that an activity-dependent distal shift of the AIS is associated with decreased AP output (5). In contrast, model simulations showed that shifting the AIS distally promotes excitability (9). One of the critical factors influencing AIS excitability is the large somatodendritic membrane area acting as current sink for sodium current generated in the AIS (10-12). In this view, a distal anatomical location of the AIS increases electrical compartmentalization and facilitates axonal AP generation. Indeed, the local depolarization in the...

In most vertebrate neurons, action potentials are triggered at the distal end of the axon initial segment (AIS). Both position and length of the AIS vary across and within neuron types, with activity, development and pathology. What is the impact of AIS geometry on excitability? Direct empirical assessment has proven difficult because of the many potential confounding factors. Here, we carried a principled theoretical analysis to answer this question. We provide a simple formula relating AIS geometry and sodium conductance density to the somatic voltage threshold. A distal shift of the AIS normally produces a (modest) increase in excitability, but we explain how this pattern can reverse if a hyperpolarizing current is present at the AIS, due to resistive coupling with the soma. This work provides a theoretical tool to assess the significance of structural AIS plasticity for electrical function.