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Comparative Neurology and Psychology
PHYSIOLOGICAL EVIDENCE OF THE FLUIDITY OF
THE CONDUCTING SUBSTANCE IN THE
By O. P. JENKINS and A. J. CARLSON.
(From the Physiological Laboratory of Leland Stanford, Jr., University.)
In measuring the rate of the nervous impulse in the slug Ariolimax columbianus ' the fact of the remarkable extensibility of the pedal nerves, which were used for the pupose by us, was a matter of constant observation, as it gave us no little trouble in making the determinations. This slug reaches a large size, individuals being frequently met with which, when extended in the act of crawling, are 25 centimeters in length.
A workable distance of 8 cm. or more of the pedal nerve can be obtained in such slugs. Now this nerve in the uninjured living animal is extended during its act of crawling and contracted during its time of rest and during these changes it, obviously, remains functional. We found that when the pedal nerve is freed from its ganglia and allowed to contract without hindrance it would shorten to about one-half the length maintained in it when the animal was fully extended in the act of crawling. In a muscle-nerve preparation made as described in the paper referred to, this nerve could be repeatedly stretched to this extent and allowed to contract and at each of these positions and at intermediate ones, normal contractions were obtained. Thus the stretching of the nerve through these limits
· Jenkins, O. P. and Carlson, A. J. American Journal of Physiology, 1903, Vol. VIII, p. 251-268.
does not appear to affect its functional activity. If, however, the nerve was stretched one or two centimeters in excess of the length it reaches in the animal fully extended by its own act of crawling, this excess of stretching itself acted as a stimulus and both the elasticity and irritability of the nerve were speedily lost not to be subsequently regained. It became a matter of interest to determine what is the effect of stretching the nerve on the rate of conduction of the nervous impulse.
In the time from August, 1901, to May, 1902, the musclenerve preparations of 25 individuals, in connection with other work, were tested on this point, and all these showed without exception an increase in the latent period following the extension of the nerve from the contracted state, the height and rapidity of the muscular contraction remaining fairly constant. In order to determine more accurately the relation of the amount of extension of the nerve to its rate of impulse we took a series of records obtained from stimulating the central and peripheral points chosen on the pedal nerve in the contracted condition and in the extended condition. From these records rates for different amounts of extension in the same nerve were determined, allowing comparisons to be made. This series of experiments was carried on at the Hopkins' Seaside Laboratory at Pacific Grove in June, 1902.
1902. Except for the alternate stretching and relaxation of the nerve, the apparatus and method of experimentation were the same as used in the previous work on the slug. The muscle-nerve preparations from 16 large individuals in good condition were used and it will be seen from the summary in table IV that several pairs of records both in the extended and contracted condition of the nerve were usually obtained from each preparation.
Fig. 1. Ariolimax columbianus. Four pairs of successive records obtained on stimulating peripheral and central points in the stretched and relaxed condition of the pedal nerve in the same individual. curve from central, p. curve from peripheral point of stimulation.
No. 1. Stretched, length of nerve, 8 cm. rate, 36.8 cm.
4 cm. rate, 36.4 cm.
8 cm. rate, 33.6 cm. No. 4. Relaxed,
4 cm. rate, 30.8 cm.
To make sure that the point of peripheral stimulation was the same in the extended and the contracted condition of the nerve the point of union of one of the branches with the main nerve trunk was chosen and this point marked with a bit of carbon from the drum which adhered firmly through the experiments. Since the central point of stimulation was in every case nearer the pedal ganglion it was easily kept the same in the two conditions. Although in each of these 16 slugs the nerve was stretched to about twice the length exhibited in the contracted state, in no case was the nerve stretched sufficiently to give rise to an impulse, and it is therefore probable that the degrees of extension of the nerve were within the range employed by the animal in its normal movements. After a second or third repetition of the stretching, the nerve did not always contract to the length first assumed, although freed as much as possible from the restraining force. This was probably due mainly to the viscid mucus in the body cavity which stuck to the nerve niore or less and by slightly hardening on exposure to the air, offered some resistance to its contraction.
Of the sixteen experiments three typical ones are given in detail, the remainder only in summary. In this summary (table IV), the length of nerve," the transmission time" and the "rate" represent the averages calculated from the individual pairs of records in the stretched and contracted condition of the
Fig. I gives a typical series of the tracings obtained from four successive pairs from alternately extended and contracted condition of the same nerve respectively.
A study of these tables shows that there is practically no difference in the actual rate of the nervous impulse in the stretched and the contracted condition of the nerve; the increase in the latent period of the stretched nerve is caused by the additional length of the nerve. In nine of the experiments (Table IV, Nos. 1, 2, 4, 5, 6, 7, 9 and 12) the average rate in the stretched nerve is slightly less than that in the contracted nerve, but, as will be seen from Table I, in these series, individual pairs of records are found which show the reverse. In