A model for responses to activation by axodendritic synapses.

A simple mathematical model of synaptic activation shows that the response to synaptic activation depends inversely on the size of the subsynaptic process. This provides a theoretical foundation for: the relationship between excitability and cell size; a possible source of plasticity in nerve cell behavior; and the hypothesis that postsynaptic responses to activation at axodendritic synapses are of large amplitude. The last-mentioned idea provides for flexible nonlinear interaction in dendritic regions because the diminution of postsynaptic potentials (PSPs) by prior potential becomes significant at high levels of depolarization. Digital-computer simulations of nerve cell input-output behavior for axodendritic activation based on these ideas reveal: frequency-transfer curves for axodendritic activation saturate; activations combined on different dendritic branches sum approximately linearly while those on the same branch occlude; simultaneous activation of several synapses on a previously inactive dendritic branch results in a large "peak" response at the onset of stimulation; and such an initial peak may be markedly mitigated by a prior depolarization of the branch. The third-mentioned finding may represent a widespread mode of hypersensitivity to stimulus onset in neural systems and in particular may contribute to the "on" responses of sensory channels, and the fourth suggests that depolarizing synapses at extreme peripheries of dendritic fibers might in some cases serve an inhibitory function.