Comparative Observations on Motor and Sensory Fibers with Special Reference to Repetitiousness’

While experimenting with spinal root preparations of the bullfrog it was found that during the passage of a constant current a repetitive response was obtained more commonly from the sensory than from the motor fibers. The present paper presents evidence indicating that this difference is characteristic, and also experiments having as their end attempts to disclose other differences between motor and sensory fibers, in particular such as might prove to be of interest relative to the difference in repetitiousness. PREPARATION AND METHODS IN GENERAL. The root pairs (usually the VIIIth or IXth, the VIIth occasionally), together with the trunk formed by their union, were studied either immediately upon removal or after refrigeration in Ringer’s solution. In the refrigerator irritability was retained for as long as two weeks, and responses to all appearances continued normal. The preparations were suspended vertically in the moist chamber, under the tension of their own weight, from threads tied to the root ends. The roots deviated from the vertical only by enough to keep them apart on their respective electrodes. The positions of all supports and electrodes were adjustable from without the moist chamber. The electrodes were of silver excepting when used to convey constant current; then in most of the experiments calomel half-cells were employed. Except where otherwise stated, stimuli were break shocks from coreless Porter coils. The methods of expressing shock and constant current strengths are those described in previous publications (Erlanger and Blair, 1936; Erlanger and Gasser, 1937). There will be found also descriptions of our amplifying and recording techniques. The root preparation is peculiarly suited to a comparative study of the properties of motor and sensory fibers, since in the trunk the two kinds of fibers are exposed to the same conditions, while from the roots their conducted responses can be recorded separately. A comparison can also be