Nerve Physiology

The BIPHASIC action potential results from the recording system which uses two surface electrodes of opposite polarity. Electrical stimulation of the nerve gives rise to a compound nerve action potential (CNAP). The resulting wave of depolarisation is conducted towards the two recording electrodes.

As this passes the first electrode, the oscilloscope beam is deflected in the upward direction. It returns to an isoelectric baseline as the wave of depolarisation passes between the recording electrodes.

The duration of the isoelectric interval is dependent on the distance between the two recording electrodes. As it passes the second recording electrode it is deflected in the opposite direction.

If the nerve is crushed between the two recording electrodes, with a pair of forceps, the wave of depolarisation is prevented from reaching the second electrode and the results is a MONOPHASIC action potential.

Though similar in appearance, this compound action potential is NOT to be confused with illustrations of action potentials recorded across the membrane of a single nerve cell.

Reverseing the polarity would mean that the wave of depolarisation would reach the - electrode first causing a downward deflection and then the + electrode causing an upward deflection.

Crushing the nerve would prevent the depolarisation wave from reaching the second (-) electrode so the downward deflection would disappear. The upward deflection would not be affected and the CNAP would now be monophasic.

The duration of the isoelectric interval is proportional to the distance between the two recording electrodes.

The latency is the time from the start of the stimulus artefact to the start of the CNAP. This measures 1.4 divisions. Each division = 0.4 ms. Therefore the latency is 1.4 x 0.4 = 0.56 ms. The amplifier does not affect the time axis.

The CNAP measures 4.4 divisions. Each division = 250 mV. Therefore the CNAP measures 250 x 4.4 = 1100 mV. However the amplifier has amplified the trace 1000x so the actual CNAP amplitude is 1100 / 1000 = 1.1 mV.

What would be the effect on the CNAP of changing the polarity of the recording electrodes?
The trace would invert
No effect
The CNAP amplitude would decrease
The CNAP amplitude would increase

What would be the effect of damaging the nerve (e.g. crushing with forceps) between the two recording electrodes?
The trace would invert
No effect
The CNAP amplitude would decrease
The CNAP would become monophasic

What would be the effect of increasing the distance between the two recording electrodes?
The interval between the upward and downward deflections would decrease
The downward deflection would disappear
The interval between the upward and downward deflections would increase
No effect

From the trace of the monophasic action potential (at the top of the page) calculate the latency of the CNAP in ms (remember the amplifier gain is x1000). Is it...
0.56 ms
0.014 ms
56 ms
1.4 ms

From the trace of the monophasic action potential (at the top of the page) calculate the CNAP amplitude in mV (remember the amplifier gain is x1000). Is it...
1100 mV
4.4 mV
1.1 mV
137.5 mV

Nerve Physiology DEMO