Abstract
Purpose :
A modeling study was conducted to examine how the low-threshold region of axon initial segments (AIS) can be utilized to focally stimulate the ganglion cell (GC) layer with epiretinal implants.
Methods :
The model neuron was derived from the morphology of a mouse OFF GC. Transmembrane currents were computed with a Hodgkin-Huxley type model using a backward Euler integrator. Low- and high-threshold sodium channels were incorporated with specific distributions along the axon. External potentials evoked by a monopolar disc electrode were calculated by the method of finite elements.
Results :
The response of multiple closely neighbored GCs elicited by 0.1ms cathodic pulses was computed. Activation thresholds were compared to the threshold of a passing axon from a GC located 400μm away. Thresholds for multiple neurons having their AIS close to the stimulating electrode were lower than threshold for the passing axon (3.18μA for a 20μm disc electrode diameter variation of passing axon (0.5,1,2μm caused 30,18,11 focally activated GCs). In contrast, the region which can be focally activated is rather independent of size of small electrodes (10,20,40μm disc diameter resulted in 18,19,20 focally activated GCs).
Conclusions :
According to the model assumption of about 2500 GCs per mm2 the low-threshold region in the proximity of the AIS can be utilized for direct stimulation of about 20 GCs close to the electrode without eliciting spikes in bypassing axons. Focal stimulation is not sensitive to electrode size but rather to the GC’s axon diameter. Further investigations of GC properties will be needed to explore how threshold variations depend on different GC types.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.