There was, however, a striking correlation of the shape of the action potential at the soma with its amplitude in the dendrite larger, faster-rising, and narrower somatic action potentials exhibited more attenuation in the distal dendrites (300-410 microm from the soma). In addition, action-potential backpropagation was not dependent on a number of electrophysiological parameters (input resistance, resting potential, voltage sensitivity of dendritic spike amplitude). Quantitative morphometric analyses revealed that the dichotomy in action-potential backpropagation occurred in the presence of only subtle differences in either the diameter of the primary apical dendrite or branching pattern. In experiments combining dual somatic and dendritic whole cell recordings with calcium imaging, the amount of calcium influx triggered by backpropagating action potentials was correlated with the extent of action-potential invasion of the distal dendrites. However, in dendritic recordings distal to 300 microm from the soma, action potentials in most cells backpropagated either strongly (26-42% attenuation n = 9/20) or weakly (71-87% attenuation n = 10/20) with only one cell exhibiting an intermediate value (45% attenuation). Using paired somatic and dendritic whole cell recordings, we find that in the dendrites proximal to 280 microm from the soma, single backpropagating action potentials exhibit <50% attenuation from their amplitude in the soma. Despite previous reports describing action-potential propagation in the proximal apical dendrites, the extent to which action potentials invade the distal dendrites of CA1 pyramidal neurons remains controversial. In hippocampal CA1 pyramidal neurons, action potentials are typically initiated in the axon and backpropagate into the dendrites, shaping the integration of synaptic activity and influencing the induction of synaptic plasticity.
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