Functional Implications of Active Currents in the Dendrites of Pyramidal Neurons

The original analyses of the transformation of synaptic inputs into voltage deflections at the soma were based largely on studies of motoneurons (Coombs et al., 1957a, b) and suggested that motoneuron dendrites were electrically passive. The view was that synaptic inputs summed algebraically, with excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) offsetting each other, and an output signal emitted if and only if the net summed voltage reached at a spike initiation zone at the axon hillock exceeded a threshold. The notion that the input-output transform accomplished by neurons was based upon the simple summation of inputs at a single integration point was so strongly embedded that when Eccles et al. (1958) found evidence for active dendritic spikes in axotomized motoneurons, they regarded the nonlinear boosting and multiple regions of integration implied by these events to be undesirable (further discussion in Sections 2 and 4 below).