Single cell spike activity in the olfactory bulb.

Studies of single-cell spike discharges in the olfactory bulb of the rabbit indicate the presence of three classes of neurons as characterized by their discharge patterns. Cells of class I discharge continuously and spontaneously; class II cells discharge intermittently in bursts, in synchrony with the passage of air through the nose. Cells of classes I and II are unmodified during olfactory stimulation. It appears there are many cells in the olfactory bulb whose discharge patterns are unrelated to excitation of the olfactory receptors by odors. Cells of class III respond to appropriate odors; the response of such cells to some odors and not others indicates that odor specificity is a fundamental characteristic of the olfactory system. c E VIDENCE indicating the existence of three classes of neurons in the olfactory bulb of the rabbit has been presented in a preliminary report (I). The electrical activity of one class (III) is altered by certain odorous stimuli. In contrast, the activity of the other classes (I and II) is not modified by odors. Classes I and II differ in that cells of one class discharge continuously whereas those in the other discharge in synchrony with the respiratory cycle. The present report describes in detail the electrical activities of the three different classes of cells in the olfactory bulb. METHODS AND APPARATUS All experiments were performed on young adult Champagne rabbits under deep urethane anesthesia. Two kinds of preparations were used. In one, the trachea was left intact, permitting the animal to breathe normally. In the other, it was transected and a separate cannula was inserted into each end. The postnasal cannula was attached to a reduced pressure apparatus which permitted air to be drawn through the nose at intervals and frequencies independent of the animal’s own inhalation cycle. The dorsal aspect of the olfactory bulbs was exposed through a small hole in the skull, and the dura was removed surgically or by tryptic digestion. Monopolar Received for publication August 8, 1955. l Work carried out at Brookhaven National Laboratory under the auspices of the U.S. Atomic Energy Commission. 2 Present address: Dept. of Physiology, School of Medicine, University of Colorado, Denver, Colo. micropipette electrodes were used, the tips varying from 3-6 p in diameter. Animals with intact tracheas breathed either room or purified air. When an animal breathed room air, odors were presented by placing a cotton pledget soaked with a particular odorant directly in front of the animal’s nostrils. Purified air was made available by means of a loose-fitting glass mask placed over the nose. The mask was joined by glass tubing with ball and socket joints to an all-glass purification and odor injection apparatus. For purification, compressed air was dried by passing through anhydrous calcium chloride and Drierite. The air was then introduced into the purification unit where it passed through a dry ice-butyl acetate trap and glass cannisters containing activated charcoal and silica gel. The air then passed into the nosemask. An odor was introduced by passing the air stream though a liquid odorant contained in a gas diffusion bottle prior to entering the nose-mask. Either ordinary or purified air could be drawn through the nostrils of animals with a postnasal cannula. Odors were made available in the same way as for animals with an intact trachea.