Systems/Circuits Dendrites Impact the Encoding Capabilities of the Axon

This studyhighlights anewandpowerful direct impact of thedendritic tree (the input regionofneurons) on the encoding capability of the axon (the output region). We show that the size of the dendritic arbors (its impedance load) strongly modulates the shape of the action potential (AP) onset at the axon initial segment; it is accelerated in neurons with larger dendritic surface area. AP onset rapidness is key in determining the capability of the axonal spikes to encode (phase lock to) rapid changes in synaptic inputs. Hence, our findings imply that neuronswith larger dendritic arbors have improved encoding capabilities. This “dendritic size effect”was explored both analytically as well as numerically, in simplified and detailed models of 3D reconstructed layer 2/3 cortical pyramidal cells of rats and humans. The cutoff frequency of spikes phase locking tomodulated inputs increased from100 to 200Hz inpyramidal cells of young rats to 400–600Hz in human cells. In the latter case, phase locking reached close to 1 KHz in in vivo-like conditions. This work highlights new and function-ally profound cross talk between the dendritic tree and the axon initial segment, providing new understanding of neurons as sophisti-cated nonlinear input/output devices. Key words: cable theory; dendritic modeling; linking dendrites to axon; spike sharpness; spike tracking of input modulatio