FIG. 7.

DBS-excited GPe fibers of passage coursing through GPi. A: a population of 83 GPe fibers of passage were distributed within the pallidum such that their axonal collaterals were positioned randomly within the posterior GPi. B: since these fibers collateralized within GPi with multiple branches, action potentials could develop in these collaterals and propagate both antidromically and orthodromically. Arrows show the site of action potential initiation in the representative orange and red GPe fibers. C: increasing the voltage amplitude from −2 to −5 V more than doubled the number of GPe fibers activated by the stimulation regardless of whether the collaterals were myelinated or unmyelinated. At higher voltage settings, however, the percentage of activated fibers in which the action potential was initiated in the collaterals was less for unmyelinated than that for myelinated branches. Increasing the unmyelinated compartment diameter increased (D) the percentage of the GPe fiber population that was activated and (E) the percentage of fibers with action potentials first generated in their branches. F: since the site of action potential initiation varied within the population, the temporal profile of action potential innervation at the initial segment, the terminal ends of each branch, and the primary axon terminations targeting STN also varied across the GPe neuron population. These effects are shown for 2 representative GPe models, illustrated in B. Unmyelinated branch compartments are colored, whereas nodal compartments along the main axon are shown in black.