There are two basic models that have been used to explain the observed phenomena of volume conduction. The first involves the use of a waveform moving down a nerve sitting in a medium. The second relies upon the concepts of what is called "solid-angle geometry." Although both models are relatively straightforward, because our goal here is merely to provide a basic understanding of volume conduction, we will omit the discussion on the solidangle geometry model, which can be obtained in the suggested reading.
Figure 1 demonstrates the depolarization of an axon moving from left to right and what will be observed depending on the exact location of the recording electrodes, both in terms of distance from the nerve (Levels 1, 2, and 3) and location along the nerve relative to the depolarizing wave front (Ovals A, B, and C). For purposes of discussion, it is easier to envision the waveform remaining stationery and the electrode moving from left to right across the diagram. In A, the waves of depolarization are initially moving away from the current source as they fan out in the medium, slightly against the direction of electrode movement (most obvious toward the extreme left side of A). By convention, this will produce a downward deflection on the oscilloscope. As we move through A, however, the current lines begin to move in the same direction as the electrode movement and the tracing on the oscilloscope begins to move upward. At the "O" line (the equipotential point), there is no net current flow across the electrode, and the oscilloscope tracing returns to baseline. In B, current flow is now toward the sink, in the same direction as the moving recording electrode, producing further upward movement of the tracing producing the rising peak of the main negative spike. In C, the current abruptly switches direction as we cross the point of maximal depolarization. Current again now moves in the opposite direction of the electrode and the oscilloscope shows a downward movement (but it now has to start at the apex of the negative peak, not at baseline, so it remains in negative territory). After crossing the equipotential line again, we
Fig. 1. A model of the effects of volume conduction on a recorded neuron or muscle potential. For discussion purposes, it is simpler to imagine the depolarization frozen in time and the electrode slowly being moved through positions A to D. Levels 1, 2, and 3 simply represent what would be seen depending on the proximity of the recording electrode to the nerve. Focusing on a single level (e.g., level 2) is simplest. By convention, if current lines are traveling opposite the direction of electrode movement, this will produce a downward deflection on the oscilloscope; if traveling in the same direction, it will produce a upward deflection. In A, the lines of current fanning out beyond the electrode source are initially traveling opposite the electrode movement, producing a downward (positive deflection), but then gradually start to travel in the same direction as the electrode, returning the deflection back to baseline, as the electrode "arrives at" the "zero" or equipotential line. In B, the current continues to travel in the same direction as the electrode movement and the oscilloscope traces an upward (negative) potential. As the electrode crosses the current sink (where the actually depolarization of the neuron is occurring), the lines of current abruptly change direction and the oscilloscope potential begins to move downward in C. This downward movement continues past the equipotential line and into D. However, within D, the fanning out of current lines gradually begins to move in the same direction as the electrode, producing an upward deflection on the oscilloscope and returning the tracing to baseline. Note that the potential is asymmetric, reflecting the fact that neuronal repolariza-tion is a slower process with less dense current lines.
Fig. 2. A monophasic potential produced by a neuronal recording in the complete absence of volume conduction. Because there is no fanning out of current lines observed in Fig. 1, the initial and final positivities do not occur, leaving only the negative, slightly asymmetric monophasic negative potential.
enter D, where the current continues to flow in the direction opposite electrode movement initially, and the oscilloscope traces out the trailing positive peak. However, at some point in D, the fanning out current lines begin to move in relatively the same direction as the electrode, and the tracing moves up and returns to zero as the current reaches undetectable levels. It is also worth noting that the repolarization of the axon is a slower process than the depolarization (note how the lines of current flow are more densely packed on the left side than the right side of the diagram), leading to a sharper initial positive peak as compared with the final positive peak. When closer to the axon (the different levels), the peaks appear sharper than when further away from it.
It is worth considering what a potential would look like in the absence of a volume conductor. Using the same model, the major difference would be that no initial positivity nor final positivity would be identified, because the current would not fan out from the source generator. This would leave a monophasic negative spike (Fig. 2). This spike would have some asymmetry, however, given the fact that repolarization is slower than depolarization. In summary, the middle negative phase of a recorded potential will be present with or without volume conduction, but the end positivities are purely a consequence of volume conduction.
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