Unit activity in the medulla oblongata of fishes.

(With Plate 2 and one Text-figure) Adrian & Buytendijk (1931) have recorded potential waves in respiratory rhythm from the isolated brainstem of the goldfish. The waves could continue for 1 hr. or more. During an investigation of the electrical activity of single cells in the central nervous system these results prompted us to explore with microelectrodes the medulla oblongata of fishes for unit activity in the respiratory centre. Carp, varying in size from 15 to 30 cm. (nose to tail) have been used in all experiments. The actual size of the animal is, however, of little importance as the brain volume does not differ appreciably in bigger and smaller carps, the surplus space in the skull of the bigger specimen being occupied by a jelly-like substance. The animals were anaesthetized by keeping them in a 1% urethane solution till the reflexes disappeared. They were then fixed to a splint by means of a bandage; a cannula was tied into the mouth to lead a current of tap-water through the gills. When required, 10% urethane solution was added. In later experiments the anaesthetized fish was placed in a basin with only the top of the skull above water-level, as the mouth-cannula was found to hinder the respiratory movements. Unhampered movement of the mouth is an essential part of the respiration especially in the bigger carps. The skull was trepaned and firmly fixed by a special clamp. The brain was then freed from the embedding jelly and the cerebellum, which covers the bulb, was cut off at the insertion into the brain-stem. The medulla oblongata is thereby exposed as shown in Text-fig. 1 (left side), with the vagal lobes to right and left and the 4th ventricle, partly covered by the lobus facialis. The electrical activity of the nervous tissue was studied by inserting bipolar microelectrodes, consisting of two enamelled platinum wires of 50 fx, gauge each, glued together and cut perpendicularly to their longitudinal axis. The electrodes are connected to the input of a condenser-coupled push-pull amplifier, whose output is led to a Philips cathode-ray oscillograph, type GM 3156. A power amplifier and loudspeaker are connected f6r acoustic control of the action sounds. The apparatus is fully described by Woldring (1950) and Dirken & Woldring (1950). The electrodes were inserted by means of a Zeiss-micromanipulator, specially adapted to our purpose. The bulb was searched for unit potentials along antero-posterior and transverse …