Orderly recruitment order of peripheral nerve stimulation with electrode array

Electrical stimulation of peripheral nerve activates large diameter fibers before small ones. Currently available techniques for achieving a natural recruitment order (from small to large fibers) require long-duration stimulation pulses (>500 mus) and large stimulation amplitude, which shorten battery life of the implanted stimulator and could lead to electrode corrosion. The hypothesis that small axons can be recruited before large ones with an electrode array and narrow pulsewidth (50mus) was tested in animal experiments using the cat lateral gastrocnemius (LG) model (N=3). The LG nerve innervates both LG and soleus muscle groups. The LG has fast-twitch fibers and is innervated by large axons (9-16mum), whereas the soleus has slow-twitch fibers and is innervated by smaller axons (8-12 mum). Peak twitch force of LG and soleus were plotted as a function of stimulation amplitude to indicate the recruitment curve. At 40% activation of LG, a conventional tripolar electrode activated only 8.7 plusmn 6.4 of soleus whereas the electrode arrays of 5 and 7 contacts activated 43% plusmn 11.6 and 48% plusmn 10.3 of soleus respectively, suggesting that the arrays could activate some small axons before fully recruiting larger ones. The electrode arrays also decreased significantly the recruitment curve slopes to only 10%-20% of the value obtained for the tripolar electrode. In conclusion, electrode array of 5 and 7 contacts can be used to modulate the recruitment order of peripheral nerve stimulation with a narrow pulse