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"External" action potentials can be recorded from within a complex tissue such as the brain or spinal cord by means of a sharp electrode, for example, a wire insulated except at the tip. Such an electrode, considered to be "seeing" the potential at a point, is a "focal" electrode. The voltage at the point is recorded with respect to an "indifferent" electrode placed farther away in a relatively inactive area.

Under these conditions, the currents associated with a nerve impulse are distributed throughout the volume in which the active neuron is contained. Therefore this sort of recording is said to be under conditions of volume conduction. In a volume-conduction situation, current flowing away from the focal electrode provides positive signal, and current flowing toward the focal electrode yields the opposite (negative) sign.

The voltage drop seen under volume-conduction conditions by means of an electrode tip placed near an active nerve in the recording chamber is rather small because the resistance of the pathway external to the nerve is low. The external resistance along the isolated nerve can be increased, while still retaining the form of the volume-conduction record, if the nerve is held in the arrangement shown in the figure.

The "focal" electrode "looks" directly at a point on the nerve; the small volume of solution surrounding the nerve on each side of that point provides a high-resistance path for the action currents. The "indifferent" electrode is placed anywhere outside the tube so both ends of the T are electrically accessible to it. The directions of the leading and trailing currents determine the sign of the deflections in the record.

As the impulse advances from the right, the leading currents flow out of the tube away from the focal electrode, toward the right, providing a positive deflection (1). As the impulse approaches the focal electrode, the leading currents move to the left limb of the tube. When the leading and the trailing currents are both directed toward the focal electrode, the signal is a large negative phase (2) in the record. Finally, as the impulse moves out of the tube, toward the left, the trailing currents are also directed toward the left, away from the focal electrode, producing the final positive deflection (3) of the record.

Draw a simple diagram to help you predict the form of the record if (a) the impulse dies out at the focal electrode and (b) the impulse arises at the focal electrode. Can you determine which direction the impulse is moving?

 
Figure 30. Volume conduction recording.

 
 
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