Up
Previous
Next
Spatial summation in L5 pyramidal cells
Hysell Oviedo and Alex Reyes
NYU
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.