What Is the Most Realistic Single-Compartment Model of Spike Initiation?
(A) The cooperativity hypothesis (adapted from ). Collective activation curve of Na channels as a function of coupling strength λ (0: no coupling). When cooperativity is strong enough, collective activation becomes step-like: Na channels open all at once when a threshold voltage is exceeded. (B and C) The active backpropagation hypothesis (adapted from ). (B) When a spike propagates forward along an active 2 mm-long cylinder of 1 μm diameter (model), its onset becomes progressively sharper. (C) Onset rapidness as a function of distance from soma where the spike is measured, for three models all including a 50 μm long axon (1 μm diameter) and a soma: soma with the same diameter as the axon (“Uniform axon”), large soma (“Soma+Axon”), and large soma and dendrites (“Dend+Soma+Axon”).