FORM OF THE COMPOUND ACTION POTENTIAL

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.