# Study of Electrical Neural Stimulation Effects Using Extraneural and Intraneural Electrodes
## 4 Discussion  
Computer simulation using a hybrid FEM-Neuron model is a useful approach for studying the electrical response of neural tissues subjected to electric current stimulation. This work adopts the same modeling approach as the literature [4, 8], to investigate different stimulation approaches and to evaluate the effects of the stimulation configuration of two different types of electrodes, taking into account the topography of the nerve. For this purpose, FEM simulation was used to evaluate electric potential distribution into the nerve model subjected to a current stimulus from neural electrodes and the MRG axon model was adopted to evaluate the nerve fiber activation in terms of the generation and propagation of action potential. The main differences are related to the different types of electrode models used (i.e., ds-FILE intraneural and CUFF with four rings and four active sites on each ring) and to the purposes of our study. The aims were to compare the activation properties of nerve fibers in the fascicles using the two different electrodes, accounting also for the nerve fascicle topography and to observe the relation between the fascicles that have activated fibers and the sensation intensity reported by a human subject in an experimental trial. Considering a given configuration of electrode active sites, it is possible to find how the nerve fascicles that have activated fibers are located, how many nerve fascicles they are, and their order of activation according to their position compared with the location of active sites. Moreover, by comparing simulation results with experimental data, for an increase in intensity sensation perceived by a human subject, it is possible to know where the activated fascicles are located and in which order they are activated when the stimulus increases. Therefore, the percentage of activation in nerve fascicles has been studied by considering a different number of fibers randomly located in each fascicle. From the simulation results obtained using cuff electrodes in five different models (Figure 3), different maps of activation can be observed. Observing the fiber percentage of activation in each fascicle, it is possible to note that more than one fascicle has a PA above 10% for the minimum value used in the experimental trials of 480 nC (_blue_\(-\)labeled fascicles), for all the five models considered. The other fascicles related to a PA above 10% when the electric current of active sites reaches amplitudes of 720 nC and 960 nC are, respectively, labeled in orange and yellow. The PA of fascicles in each one of the five nerve models with cuff also depends on the fascicle's dimension and location. For this reason, it is possible to observe in Figure 3a-e that 7, 17, 13, 11, and 14 fascicles are activated, respectively, with a PA above 10%, when the minimum value is 480 nC, at which the patient perceived the sensation in the experimental trial. These observations can be made for the electrode stimulation configuration considered in our experimental trials, i.e., electric current is sent through two active sites, one close and the other located longitudinally along the nerve, Figure 2a. This configuration type, of bipolar stimulation with active sites located longitudinally along the nerve length, could also be the reason for the particular fascicle activation behavior shown in Figure 3. For example, observing in detail Figure 3b, (similar behavior can be observed in other subplots of Figure 3), and it is possible to note that at the minimum value considered of electric charge (i.e., 480 nC), fascicle 19, that is far from the active sites, has a PA value around 80%, fascicle 15 around 40%, and fascicle 12 has a PA value below 10%. This behavior can be related to the longitudinal configuration of the active sites to perform bipolar stimulation. The two different active sites are positive and negative current sources. Close to the active sites, there is a higher strength both for positive and negative current compared with locations far from the active sites.