Popular Science Monthly/Volume 11/June 1877/Transmission of Excitations in Sensory Nerves
By PAUL BERT.
PHYSIOLOGISTS are as yet by no means agreed whether the nerves which, from their special functions, have been termed motor and sensor nerves, are in their essential properties identical or different; in other words, whether a sensor nerve can transmit excitations whose result is motion, and vice versa. We do not even know whether an excitation produced midway in the length of a nerve is simultaneously propagated in both directions, centrifugally and centripetally. The admirable experiments made by Philipeaux and Vulpian to determine these knotty questions are, as Vulpian was the first to observe, susceptible of a different interpretation from that unanimously put upon them by the world of science.
Such being the state of the case, I thought it advisable to take up again an experiment I had made in 1863, but which I had neglected to prosecute, in view of the apparently far more conclusive and far more general results obtained by the able experimenters just named. This experiment I have now completed, and strengthened against any objection that might have been raised against it.
If we pinch a sensor nerve at any point of its length, the pain that is felt clearly shows that the excitation is propagated in the centripetal direction; but of a centrifugal propagation we know nothing, for the very simple reason that at the terminal extremity of the nerve there is no nervous apparatus of perception. But if we can bring this extremity into relation with the perceptive centre, the brain we shall see whether there is sensation, which would imply centrifugal propagation.
The means employed in bringing about this condition of things is very simple indeed. I remove, for the length of two or three centimetres, the skin from the tip of a young rat's tail, and insert the flayed part into the subcutaneous cellular tissue through an opening made in the skin of the animal's back. A few stitches suffice to hold the parts in place, and soon they adhere firmly, the rat's tail then having the ansate form.
Fig. 1.—Rat with Tip of Tail inserted into the Dorsal Tissues.
Eight months later I cut the tail in two, thus leaving two caudal stubs. Immediately after section, the dorsal stub was manifestly sensitive; and when it was pinched vigorously, the rat would squeal and run off. Hence it plainly appears that, in this fragment of a tail, the excitation of the sensor nerves is propagated from the thick to the slender end, or in a direction inverse to what is held to be the normal course. The process was about as follows: the sensor nerves distributed to the extremity of the tail, on being wounded by the removal of the skin, united with the sensor nerves of the dorsal region, which in like manner had been cut into. In due course of time the nerve-cicatrix became able to transmit the shocks (whatever their nature) which an excitation produces in a nerve. When, now, we pinch the extremity of the dorsal stub, the shock is transmitted along the excited caudal nerve, passes through the cicatrix, and follows the dorso-cutaneous nerve into the spinal marrow, which carries it to the brain, where it results in a sensation of pain.
The history of the case will be readily understood on examining the following diagram, in which, for simplicity's sake, I have represented only one of the nerve-filaments of the tail (N C) and one of the nerve-filaments of the back (D N).
But this sensibility of the dorsal fragment begins to grow less on the second day after section, and soon it is gone. If, some days later, we examine the nerves of this fragment with a microscope, we shall find that they have undergone the changes usual in nerves that have been cut off from their trophic centres; and this is true no less of the portion concealed under the skin than of the free (and certainly living) portion hanging from the animal's back: not a single sound nerve-
Fig. 2.—M E, spinal marrow. N C, nerve-fllament distributed to the end of the tail. G, its trophic ganglion. D N, a, nerve-filament distributed to the skin of the back, lacerated in the operation. G, its trophic ganglion. C. the cicatrix which united the two nerves, and which is now permeable to nervous shocks. S, point where the tail was cut in two; a, b, arrows showing the two directions in which sensory excitations are transmitted.
tube is to be seen in it. On the contrary, the stump of the tail, which has retained its natural position, has every nerve sound, with not a single diseased tube.
Thus, then, physiological facts are in agreement with histological observations, and they both prove conclusively that the sensor nerves which transmitted the centrifugal excitation were the normal nerves of the dorsal stub, and that here we have neither new-formed nerves nor nerve-fibres with ansate terminations. It is further demonstrated—and this is a fact not without interest—that the relations with the nerve-centres of perception, from which results sensation, are more easily established than those with the trophic ganglionic centres, which nourish the sensor nerves. Who knows but that, had I waited longer before I cut the tail in two, the influence of the new trophic centres would have become sufficient to maintain the nerves of the dorsal fragment in their integrity, and that sensibility would have persisted after section? Some months after the dorsal stub had become insensible, it again regained sensibility, the diseased nerves having been regenerated. At first, just as I had observed in 1863, the animal refers the impression it receives to the region of the back where the nerve-cicatrix is: this is the reverse of the illusion observed in cases of amputation. By little and little the rat is educated, and at last recognizes the exact point that is excited; thus showing that our so-called innate knowledge of the place occupied in space by every point of our bodies is, like all our knowledge, merely the result of repeated experiences.
To conclude, the experiment I have just recounted shows that excitation of a sensor nerve at any point of its length is propagated in two directions, both centrifugally and centripetally. The same is doubtless true of a motor nerve. Consequently, it is highly probable that, as Vulpian has held, the nerves are simple conductors, differentiated only by their functions, which latter depend upon the kind of apparatus at their two extremities: for motor nerves there is a motor nerve-cell and muscular fibre, and for sensor nerves a receiving nerve-cell and an impressionable termination.—La Nature.