Influence of the sodium channel band on retinal ganglion cell excitation during electric stimulation – A modeling study

• The axonal sodium channel band lowers electrode threshold currents.
• Electrodes distant from the band can initiate spikes within the band via intra-axonal current.
• The density of sodium channels strongly influences the site of spike initiation.

Electric stimulation using retinal implants allows blind people to re-experience a rudimentary kind of vision. The elicited percepts or so called ‘phosphenes’ are highly inconstant and therefore do not restore vision properly. The better knowledge of how retinal neurons, especially retinal ganglion cells, respond to electric stimulation will help to develop more sophisticated stimulation strategies. Special anatomic and physiologic properties like a band of highly dense sodium channels in retinal ganglion cells may help to achieve a focal activation of target cells and as a result better restoration of vision. A portion of retinal ganglion cell axons, about 40 μm from the soma and between 25 and 40 μm in length, shows a specific biophysical property. Electrode locations close to a band of highly dense sodium channels which were identified immunochemically show lowest thresholds during electric stimulation. The (modeled) thresholds for this kind of structure result in lowest thresholds as well. The influence on the location where action potentials are generated within the axon is far reaching. When a stimulating electrode is positioned far outside the actual band region the site of spike initiation still remains within the sodium channel band. These findings suggest to further examine the key mechanisms of activation for retinal ganglion cells because focal activation without influencing passing axons of neurons located far away can improve the outcome of electric stimulation and therefore the development of retinal implants.