Fig. 1.

Schematic illustration of polarity effects based on modeling work by Ratty et al. (2001a,b). Top: For the healthy neuron, both anodic and cathodic polarities produce peripheral-to-central propagation at low levels (directional arrows pointing toward the right). At higher levels, the anodic phase also yields direct spiking in the central axon. Bidirectional arrows represent orthdromic and antidromic action potential propagation for high levels. Bottom: For the degenerated neuron with missing peripheral axons, a 5–6-fold increase in current strength is needed for spike initiation with the cathodic phase, as compared to the anodic phase (Rattay et al., 2001a).