Spatial summation in L5 pyramidal cells

Hysell Oviedo and Alex Reyes


Dendrites have various voltage and time dependent conductances whose distribution and properties vary with distance from the soma. To examine how these conductances affect summation of inputs in the subthreshold range and while the neuron is firing, we performed simultaneous, whole-cell recordings from the soma and two locations on the apical dendrite of pyramidal neurons in slices of rat (>P21) sensorimotor cortex. Transient current was injected through each electrode to evoke EPSP-like potentials. The currents injected at the dendrite were adjusted so that the resultant EPSPs measured with the somatic electrode were identical. A computer simulation calculated a composite current for a hypothetical population of presynaptic cells, which was then injected into the cell. A specified number (n) of inputs was injected at the two dendritic locations individually and then simultaneously (2n); the resulting responses were compared to the predicted linear sum. In the subthreshold range summation changed from linear to supralinear as n was increased. In the suprathreshold range summation of firing rates changed from supralinear to sublinear. Sublinear summation at high rates was due to changes in dendritic conductances rather than to saturation effects since current injection at the soma evoked higher firing rates. Similar observations were obtained when the inputs were injected under dynamic clamp (to mimic the conductance changes caused by synaptic input) although summation in the subthreshold and suprathreshold ranges tended to be less supralinear. Efficacy of inputs was always least when the inputs were injected at the site farthest to the soma, and the greatest when the inputs were either injected at the site closest to the soma or split between the two dendritic locations.