Unit Activity in the Medulla Oblongata of Fishes

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UNIT ACTIVITY IN THE MEDULLA OBLONGATA OF FISHES

BY S. WOLDRING AND M. N. J. DIRKEN Physiological Institute, Groningen, Netherlands

(Received 17 July 1950)

(With Plate 2 and one Text-figure)

Adrian & Buytendijk (1931) have recorded potential waves in respiratory rhythm from the isolated brainstem of the gold-fish. 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 lines 05 mm. apart, the crossing of the commissura infima Unit activity in the medulla oblongata of fishes 219 Paid the mid-line, caudal to the tip of the 4th ventricle, serving as a point of reference in the various experiments. The respiratory potentials have been recorded photo- graphically, together with the movements of the gills which were connected to a lever carrying a small mirror. Most of the unit activity has been found in the central part of the medulla oblongata, whereas large parts of the vagal and facial lobes have proved to be almost free of electrical activity. Volleys of spike potentials in respiratory rhythm can be detected in two small strips to the right and the left of the mid-line at the frontal border of the facial lobe just behind the acoustic tubercles. PI. 2 a shows the discharges of a single respiratory unit picked up in the medulla oblongata 0-5 mm. anterior to the frontal border of the facial lobe, 0-5 mm. to the right of the mid-line and 1-25 mm. below the dorsal surface.

Text-fig. 1. Brain-stem of the carp after removal of the cerebellum (c). v., lobus vagus; l.m., lobus impar; t.a., acoustic tubercle; IV, fourth ventricle; f.l.p., fasciculus long post.; black dots, respiratory activity; + electrical activity on acoustic stimulus.

In the record upward deflexion of the curve corresponds to inward movement of the gills; the record shows that bulbar activity is associated with inward movement of the gills. The frequency of the discharge is highest at maximal adduction and decreases during abduction of the gills. With maximal abduction discharges are absent. PL 2b represents the discharge pattern 0-25 cm. ventral to the spot where the former record was taken. Here several units contribute to the volley, which shows the same general behaviour as the single unit activity of PL 2 a. The structures giving rise to these discharges were partly covered by the lobus facialis and were found 2 mm. below the dorsal surface. Those picked up anterior to this lobe were met at a depth of 1-1-5 mm- The region of respiratory activity lies c. 0-5 mm. from the mid-line and has an extension of 1-1*5 min- m tne antero- posterior direction. In one case a further respiratory volley was picked up from 22O S. WOLDRING AND M. N. J. DlRKEN a spot anterior to the left acoustic tubercle under the attachment of the cerebellurnB (dots in Text-fig, i). In the acoustic tubercles, about 1-2 mm. from the mid-line (Text-fig. 1), a volley of spike potentials was elicited by acoustic stimulation. Even whispering at a distance of 3 m. proved to be effective. On examining the microscopic sections several groups of rather large cells were found in the two strips near the mid-line where nearly all respiratory volleys had been detected. These cells lie ventro-lateral to the fasciculus longitudinalis posterior and partly give rise to outgoing fibres (see Text-fig. 1, right side, representing longitudinal and transverse sections of this particular region). As the needle tracks were not discernible on account of the softness of the tissue, it was not possible to verify exactly the position of the electrodes. The depth to which the electrodes were inserted, however, agrees with that of these large cells. Presumably we have recorded the discharges of the neurons of the cranial motor nerves VII, IX and X, whose nuclei lie in a row parallel to the fasciculus longitudinalis posterior according to Kappers, Huber & Crosby (1936). The segmental arrangement of these nuclei innervating the gill muscles has been emphasized by Hyde (1904). The lobi vagi and the lobus facialis did not exhibit much electrical activity; spike potentials were rather scarce here. This is readily understood in view of the sensory function of these out-growths which belong to the gustatory system (Kappers, et al. 1936). No relation with respiratory movements was found.

SUMMARY In the medulla oblongata of carp respiratory potentials were found in two small strips 0-5 mm. to left and right of the mid-line at the level of the caudal border of the acoustic tubercles and at a depth of about 2 mm. below the dorsal surface. Acoustic potentials were discovered in the acoustic tubercles. No particular unit activity was found in the vagal and facial lobes.

REFERENCES ADRIAN, E. D. & BUYTENDIJK, F. J. J. (1931). Potential changes in the isolated brainstem of the goldfish. J. Physiol. 71, 121. DIRKEN, M. N. J. & WOLDRING, S. (1950). Unit-activity in the bulbar respiratory center. J. Neuro- physiol. (in the Press). HYDE, J. H. (1904). Localization of the respiratory center in the skate. Amer. J. Physiol. 10, 236. KAPPERS, C. U. A., HUBER, G. C. & CROSBY, E. C. (1936). The Comparative Anatomy of the Nervous System of Vertebrates Including Man. New York. Macmillan & Co. WOLDRING, S. (1950). Thesis Groningen.

EXPLANATION OF PLATE 2 Records of the electrical activity of respiratory cells in the medulla oblongata of the carp. Registration of gill-movements: adductions of the gills is represented by an upward movement of the signal- line. Time signal: \ sec. a, single unit, discharging in respiratory rhythm, b. respiratory volley of several units. For further details see text. JOURNAL OF EXPERIMENTAL BIOLOGY, 28, 2 PLATE 2

WOLDRING AND DIRKEN—UNIT ACTIVITY IN THE MEDULLA OBLONGATA OF FISHES