Possible mechanism of generation of SEP far-field component in the brachial plexus in the cat.

By cutting the brachial plexus at a few levels on the side stimulated, it was verified that the first far-field component of SEPs with forepaw reference recording is generated in the brachial plexus in the cat. Incidentally, a reversal of the polarity of the earliest portion of the first component was observed with abduction of the upper arm to a high position on the side stimulated. X-ray photographs of the nerve course in such a high position disclosed that both the position and direction of the nerve course in the distal brachial plexus were changed substantially. On the other hand, bipolar derivations away from the bullfrog sciatic nerve volley revealed that a reversal of sign of the action potential could be observed by changing the direction and/or position of the nerve traveling impulse against two recording electrodes a long distance apart. These findings suggest that a reversal of polarity of the earliest portion of the first component in a high upper arm abducted position may be produced due to change in the direction and position of the nerve impulse against the two recording electrodes placed on the scalp and on the forepaw a along distance apart, that is, the forepaw electrode is neither the reference nor the indifferent, but is active to far-field components.