There is an optimal Pulse Width at which a specific nerve is most excitable. This is called the chronaxie threshold. It is preferable to keep the pulse width as close to this value for the related nerve or nerve plexus, as the Peripheral Nerve Stimulator allows, then increase only the current.
It should be noted that, although there are many published values for chronaxie for various excitable tissues, the range of variability for a given tissue type is quite large. It is generally assumed, however, that nerves can be classified according to their chronaxie thresholds as follows:
||Predominantly motor neurons. They also have
the following sensory functions: Proprioception,
hair receptors, vibratory sensors and high
||Deep pressure and touch, pricking pain and cold
||Crude touch and pressure, tickle,
aching pain, cold and warmth
From the above table, it would seem reasonable to deduce that the ideal pulse width to facilitate a motor nerve response (A alpha), would be around 100Îźs. If one sets the nerve stimulator at 100Îźs and increases the amplitude to 5mA giving a total charge of 500nC one would not get the same muscle response as if the setting is at 500Îźs and 1mA, also giving a total charge of 500nC. In the second case even though the total charge transferred to the nerve is the same, because of the chronaxie threshold of 100Îźs for the nerve, much of the energy transferred to the nerve after the 100Îźs is wasted on the nerve.
This is clearly shown by the graph below. The strength-duration curve (green) indicates the current necessary at the different pulse widths to facilitate a contraction. The energy cost or total charge is shown by the blue curve. It can be seen that the stimulation is the most energy efficient at the chronaxie pulse of +- 80Îźs width as would be expected. It should be noted how the energy cost increases when pulse width increases.
As a preference, keep the nerve stimulator at a 100Îźs pulse width and adjust the current. If the nerve stimulator is already set at 20mA and the Nerve Mapping Probe does not elicit any neuromuscular response, increasing the pulse width to 300Îźs will offer 3 x more charge, however bear in mind that the net effect on the nerve will not constitute a contraction which is 3 times more powerful.