For a nerve-teste circuit, what causes the shock at low voltage?
WARNING - Note that used improperly the equipment described below could cause death (or just possibly worse). read ALL the safety warnings available before playing.
One man says:

01-18-2004, 09:35 PM
Be real careful here. I design medical grade TENS and MicroTENS
systems as well as defibrillators and if you don't know what you are
doing you can cause injury or death. Improper safeguards can cause
burns, muscle damage, even cardiac arrest. I would be very leary of
following any schematics found in magazines or the internet. If you do
venture into the endeavor make damn sure you use a medical grade or
UL544 type isolation transformer if you plan on running the device on
mains power.


You are unlikely to achieve sensation at 5V without the use of conductive pads, such as are made for ECG use - or in the skin probes if you are VERY keen (and not too smart?) and maybe not even then.
What you describe is often done using a so called &quot;TENS Unit&quot;.
&quot;T.E.N.S.&quot;  = Transcutanaceuos neural stimulation.
These units usually produce voltages in the 10s of volts to 100+ Volt range.
Here is a circuit of a 555 based TENS unit. Note that they use a 10:1 stepup transformer AND the way it is used it can produce voltages of MORE than 10 x 9V due to &quot;flyback action.

The article is from Elektor mag. They say:

How hard can it be to make such a device ourselves? The simple circuit uses a CMOS 555 timer to produce a brief pulse which feeds a 1:10 miniature transformer. Together with a 4.7 nF capacitor the transformer makes a parallel resonant circuit: the resonance leads to a considerable increase in the output voltage. The pulse width can be adjusted using a potentiometer, here shown combined with the on-off switch. Wider pulses produce higher output voltages. Since a peak voltage of up to 200 V can be produced, the transformer must have adequate insulation: Conrad Electronics type 516260-62 is suitable. A low-cost phono socket at the output gives reliable connection to the electrode cable.
The adhesive electrodes shown in the photograph (disposable and permanent types are available) can be obtained from pharmacies and medical suppliers. They generally have connectors compatible with 2 mm banana plugs, and so it is possible to make up the necessary cable yourself. To treat responsive parts of the body, such as the arm, the potentiometer need not be turned up far to obtain the necessary sensation. Less sensitive parts, such as the knee or foot, need a rather higher voltage and hence a correspondingly higher potentiometer setting.

Author: Klaus Rohwer - Copyright: Elektor Electronics Magazine
PLUS
Several circuits not marvellous.
This circuit  is from the above page and is notable in using a single winding flyback inductor rather than using a transformer.
TENS article scanned pages
Good discussion page - same as the warning one in first paragraph.
There is ongoing debate about the genuine effectiveness of TENSS units despite a vast user experience base and many peer reviewed scientific papers demonstrating that they do and don't produce statistically significant results. If they don't work for you you could try using the unit as a pipe descaler or pulsed battery charger ;-).