Fig. 16.

Unmyelinated axons have higher stimulation thresholds and are activated less reliably than myelinated ones. Both bundles consisted of 15 axons with diameter 3 μ m and were stimulated with a bipolar electrode of radius 235 μ m and pole distance 1 mm using a biphasic pulse of frequency 1 kHz, duration 1 ms and duty cycle 0.5. The extracellular medium was a nerve of diameter 240 μ m bathed in oil. a Unmyelinated axons started to be activated at 1 mA and showed a peak in activation ratio at about 3 mA. b Myelinated fibres had a sharp activation threshold at a much lower current of about 0.15 mA and stayed activated for higher currents. Only when incrementing the stimulation current in very small steps of about 10 nA c a slight tortuosity-induced increase in stimulation threshold became visible for them as well