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The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 514-527
Copyright ©2001 by the American Physiological Society
Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington 98195-7290
Oakley, J. C.,
P. C. Schwindt, and
W. E. Crill.
Dendritic Calcium Spikes in Layer 5 Pyramidal Neurons Amplify and
Limit Transmission of Ligand-Gated Dendritic Current to Soma. J. Neurophysiol. 86: 514-527, 2001. Long-lasting, dendritic, Ca2+-dependent action
potentials (plateaus) were investigated in layer 5 pyramidal neurons
from rat neocortical slices visualized by infrared-differential
interference contrast microscopy to understand the role of
dendritic Ca2+ spikes in the integration of
synaptic input. Focal glutamate iontophoresis on visualized dendrites
caused soma firing rate to increase linearly with iontophoretic current
until dendritic Ca2+ responses caused a jump in
firing rate. Increases in iontophoretic current caused no further
increase in somatic firing rate. This limitation of firing rate
resulted from the inability of increased glutamate to change evoked
plateau amplitude. Similar nonlinear patterns of soma firing were
evoked by focal iontophoresis on the distal apical, oblique, and basal
dendrites, whereas iontophoresis on the soma and proximal apical
dendrite only evoked a linear increase in firing rate as a function of
iontophoretic current without plateaus. Plateau amplitude recorded in
the soma decreased as the site of iontophoresis was moved farther from
the soma, consistent with decremental propagation of the plateau to the soma. Currents arriving at the soma summed if plateaus were evoked on
separate dendrites or if subthreshold responses were evoked from sites
on the same dendrite. If plateaus were evoked at two sites on the same
dendrite, only the proximal plateau was seen at the soma.
Just-subthreshold depolarizations at two sites on the same dendrite
could sum to evoke a plateau at the proximal site. We conclude that the
plateaus prevent current from ligand-gated channels distal to the
plateau-generating region from reaching the soma and directly
influencing firing rate. The implications of plateau properties for
synaptic integration are discussed.
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