A Useful Electrode Arrangement For the Acquirement
Audio Brainstem Response from Rats.

A Short Communication
Jorgen Holm-Jensen

The method described below was introduced as a replacement for a time-consuming and troublesome procedure used originally by the National Institute of Occupational Health in Denmark. The old way included amputation of the scalp, cleaning and polishing of the scull prior to the mounting of small screws into the bone. The screws in turn served as electrodes for the sampling of ABR. Using the epidural electrodes, however, enabled measurements on fully conscious animals, a possibility not available with the new arrangement, which demands some kind of anaesthesia.

         Most electrophysiological amplifiers require a 3-point contact to the object and so does the Dantec 15C01 EMG-amplifier used here. The common earth and frame connection is established through a needle-electrode of stainless steel placed subcutaneously on the dorsal side of the tail close to the body. The two remaining links to the balanced input are taken to the inner lining of the mouth and subcutaneously into the scruff right behind the ears respectively.

         The mouth-electrode is the more elaborate of the two and is made of 8 turns 0.6 mm cross-diameter pure silverwire tightly wound bifilarly on an aircore 2.5 mm cross-diameter -- 4 turns either direction. In the middle of the coil, where the direction changes, an open loop extends 12 mm from the axis and leaves a gap 1.2 mm wide. The two ends of the wire proceed in the opposite direction to the loop and serve as contact-points. In order to add mechanical stability the ends are made to cross and twist creating an "X" some distance from the coil. During use the electrode is placed like a "teething stick" behind the front teeth so the loop touches the tongue and keeps it down.

         Preparing the crooked nape-electrode is a more straightforward matter (sorry). A 14 cm length of 0.6 mm cross-diameter silverwire is cut and its one end sharpened. The wire is then bent in two places: 1.5 cm and 3 cm from the tip. Bending is done in the same plane and the angles are slightly narrower than 90 degrees. Finally the short end is given an additional slight bending (5 degrees) out of the plane. This final adjustment comes naturally once the electrode is put to use for the first time. The long end provides a suitable point of contact for the EMG-amplifier.

         In order to obtain comparable ABR-recordings the electrodes have to be positioned in a reproducible way. Putting the mouth-electrode in place presents no problem. Mounting the nape-electrode is, however, a more delicate matter and for this purpose a few tools are needed. Those are a medium-sized, anatomical pair of pincers and a 2 ml syringe equipped with a very sharp 19 G needle. Before use the syringe is filled with a sterile 1% solution of lidocainehydrochloride. The following procedure will ensure a reasonably good positioning of the nape-electrode: Initially the branches of the pair of pincers are placed on the back of the head to make good contact with the skin in the immediate vicinity of the vaulted part of the earlobes. Next the branches are closed so they grip firmly around a fold of skin from the scruff, which is lifted a few mm's. A single drop of lidocaine-solution is applied to the surface of the skin closely to the end of one of the branches, before the needle in a single move is pushed through both layers of skin. The sharpened tip of the electrode is then stuck into the cavity of the needle and another drop of lidocainesolution released. Still in contact with the electrode the needle is retracted until the short end of the electrode has passed through the skin-fold and is again fully visible. The electrode is finally dismantled from the needle and its position and shape adjusted so the long end follows the bridge of the nose. To prevent the electrode from acting like a mechanical pick-up for vibrations, care should be taken to fix its long end somewhat.

Copenhagen 1997