One can measure a single trans-membrane potential by inserting a glass pipette into one cell and recording the potential changes with respect to an extracellular reference electrode. This intracellular technique is used, for example, to record the resting membrane potential of a muscle in the Resting Membrane Potential Lab.
The intracellular recording technique does allow for very accurate assessment of the electrical activity of a single cell, but it is very difficult to do in vertebrate nerve fibres and can involve considerable damage to the membrane around the electrode tip.
A far less demanding technique, extracellular recording, involves placing one electrode in close proximity to the excitable cell and the reference electrode at some location in the extracellular fluid. This arrangement records potential changes at the membrane surface rather than across the membrane.
Extracellular positioning of the recording electrode
Action potentials recorded extracellularly differ from those recorded intracellularly in several important respects. The size of any one action potential will be obviously reduced. The shape of the waveform for any one action potential will depend on the exact geometry of its contact with the electrode. Extracellular techniques are therefore better suited where one only wants to know that an action potential has occurred or to record the activity of an entire population of cells.
Biphasic recording
In this lab you will study the response of the sciatic nerve of the frog to electrical stimulation using two pairs of stainless steel wire electrodes that are both in contact with the nerve, therefore recording the potential difference between two points on the nerve. There are 12 such recording electrodes in the nerve bath, but only two can be connected to the differential amplifier at any one time.
Modified from ADinstruments. All Rights Reserved.
Delivering a sufficiently large stimulus to the nerve will result in an action potential that is quite a bit larger than a single intracellular action potential but looks remarkably similar.
This compound action potential (CAP) is the algebraic summation of all the action potentials produced by all the fibres that were fired by that stimulus. The nerve is made of thousands of axons whose size, myelination and position with respect to the stimulating and recording electrodes all affect the size of their contribution to the compound action potential.