Deep brain stimulation has been used for over a decade to relieve the symptoms of Parkinson's disease, although its mechanism of action remains poorly understood. To better understand the direct effects of DBS on central neurons, a computational model of a myelinated axon has been constructed which includes the effects of K⁺ accumulation within the peri-axonal space. Using best estimates of anatomic and electrogenic model parameters for in vivo STN axons, the model predicts a functional block along the axon due to K⁺ accumulation in the submyelin space. The functional block occurs for a range of model parameters: high stimulation frequencies (>130 Hz); high extracellular K⁺ concentrations (>3 × 10⁻³ M); low maximum Na⁺/K⁺ ATPase current densities (<0.026 A m⁻²); low diffusion coefficients for K⁺ diffusion out of the submyelin space (<2.4 × 10⁻⁹ m² s⁻¹); small periaxonal space widths of the myelin attachment sections (<2.7 × 10⁻⁹ m) and perinodal/internodal sections (<8.4 × 10⁻⁹ m). These results suggest that therapeutic DBS of the STN likely results in a functional block for many STN axons, although a subset of STN axons may also be activated at the stimulating frequency.