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