FAST CENTRAL FIBERS IN FISH: PROPERTIES OF MAUTHNER AND MLLER FIBERS OF MEDULLOSPINAL SYSTEM

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Abstract
Stimulation of the catfish (Ameiurus) medulla produces spike potentials in the cord which resemble those of frog A fibers in conduction rate, recovery cycle, and temp. coefficient of conduction rate. The conduction rate of the fastest of the cord potentials (50-60 m./sec. at 10-15[degree] C.) indicates that they occur in fibers larger than frog A fibers. Fibers of sufficient size to account for the cord potentials pass into the ventromedial bundle of the spinal cord from the median longitudinal fasciculus of the medulla, which contains the axons of the Mauthner and Miiller cells. The outside diams. of the 2 Mauthner fibers vary between 43 and 22 [mu] in the cord and medulla. The heavy myelin sheath accounts for about 1/2 of the total fiber diam. in the medulla and a progressively smaller fraction down the cord. The sheath of the 10 next largest fibers accounts for only about 0.3 of their 22.5-11.1 [mu] outside diam. That the potentials recorded from the cord arise in the fibers described is indicated not only by the correlation between fiber size and the properties of the potentials, but also by the decrease in the response if the stimulation is moved rostral to the main group of Mauthner-M[mu]ller cells in the medulla. Stimulation of the saccular otolith causes responses in the cord which apparently occur in the same fibers as the responses to medullary stimulation. Responses of single fibers, of various conduction rates from 60 to 15 m./sec, may be recorded with suitable medullary stimulation.