Extracellular Recording From a Single Axon

To introduce the principles of extracellular recording, it is useful to begin with the simplest case—the extracellular recording of activity from a single nerve axon. Figure 5 illustrates both the general strategy and typical results. Two metal electrodes, A and B, are placed in close proximity to an isolated nerve axon. They do not impale the axon but are very near its outside surface. The electrodes are connected to a suitable electronic amplifier that is designed so that the voltage displayed is the difference between the voltage sensed at electrode B (VB) and the voltage sensed at electrode A (VA):

The recording device measures voltages B and A and subtracts the two to generate a visual display. In this example, the display is such that an upward deflection on the recording device reflects a positive difference (e.g., VB — VA = +), and a downward deflection reflects a negative difference (e.g., VB — VA = —).

Let us first examine the case where no action potential is propagating along the nerve axon (Fig. 5, trace 1). At rest, the membrane potential of the axon is approximately —60 mV inside with respect to the outside. Although this is not the usual convention, another way of stating it is that the axon is +60 mV outside with respect to the inside. Thus, the electrodes placed in the immediate environment of the cell membrane will both sense equal positive charges on the outside of the membrane. By taking the difference between the potentials at the recording electrodes, the electronic amplifier will record a potential difference of 0. This will correspond to no deflection on the recording device. To summarize, at rest,

Time

FIGURE 5 Experimental arrangement for the extracellular recording of electrical activity from a single nerve axon.

Time

FIGURE 5 Experimental arrangement for the extracellular recording of electrical activity from a single nerve axon.

Let us now initiate an action potential at some distant point to the left of the axon illustrated in Fig. 5 and allow that action potential to propagate along the nerve axon toward the recording electrodes. Consider the consequences of the changes in charge distribution when the wave of depolarization representing the propagating action potential first comes near the region of electrode A. Assume for the moment that we can freeze the action potential at this point in time. We have learned that during the peak of the action potential, the potential inside the cell becomes approximately +55 mV with respect to the outside. Stated in a different way, the outside of the axon will be negative with respect to the inside. So, electrode A will now sense a negative outside surface charge, but electrode B will sense the same positive outside charge (because the action potential has not yet propagated to electrode B). Thus, at point 2 (Fig. 5), the electronic amplifier will take the difference between the two potentials and determine a positive difference. The positive difference will correspond to an upward deflection of the recording device. To summarize, at point 2 (Fig. 5),

If we now unfreeze the action potential and allow the wave of depolarization to continue to propagate, it eventually will reach a point where the action potential is in the region of both electrodes A and B (trace 3, Fig. 5). We now freeze the action potential at the point at which each electrode senses equal parts of the action potential. The regions under both electrodes will be positive inside with respect to the outside or negative outside with respect to the inside. So, electrode B will sense a negative charge, and electrode A will sense the same negative charge. The electronic amplifier will subtract the two and determine that there is no difference. The recording device will return to its initial state and display no deflection. To summarize, at point 3 (Fig. 5),

The potential recorded at point 3 is an interesting case. The recording device is back to its initial state, but that does not mean that the recorded potential is equal to the resting potential. Rather, it means that electrodes A and B are recording the same potential.

What happens as the action potential continues to propagate along the axon? The area of excitation under the two electrodes will move away from electrode A and eventually will reach a point at which the excited region (the action potential) is predominantly in the vicinity of electrode B. As a result, the region under electrode A will repolarize so that the inside of the axon will become negative with respect to the outside, or the outside will be positive with respect to the inside (as it was initially). The region under electrode B will still be excited so that it will be negatively charged. Thus, at point 4 (Fig. 5), the difference between the potentials at the two electrodes is negative, and a negative difference corresponds to a downward deflection of the recording device: At point 4,

Eventually, the excited region will move away from the electrodes, and both electrodes will sense positive charge. The difference between the two will be zero, and the recording device will return to its initial state.

Thus, by using extracellular recording techniques, it is possible to obtain signals that correspond to the passage of an action potential along a nerve axon. Note that the form of the measured potential is very different from the action potential recorded with an intracellular microelectrode. In addition, it is only a rough reflection of the magnitude and duration of the underlying membrane permeability changes. This type of recording is used primarily as a simple index of whether or not an action potential has occurred.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment