Abstract
[A first question is whether all parts of the neuron, namely the soma, dendrites and axon conduct nerve impulses.]
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Footnotes
[According to Golgi, the passage of the excitation in the olfactory glomerulus would occur through a neural net. This authors’s insistence remained in full force even after the publication of our observations, and those of His, Edinger, Lenhossék, Retzius, Van Gehuchten, Calleja, P. Ramón, etc. Golgi’s as well as Monti’s (1895a) assertions could not be confirmed by Kölliker (1896) or Blanes (1898), who has dedicated a well documented investigation to refute these errors.]
Recently, K. Schaffer (1897a) has resurrected Golgi’s theory on the exclusively nutrient role of dendrites. According to this author, only the axon and its collaterals have a conductive capacity, with the distinction that collaterals would function as devices for the reception of nerve impulses, and the axon as an apparatus for the emission of said impulses. To accept Schaffer’s opinion, it would be necessary to disregard almost all structural facts discovered in neural centers and sense organs during the last ten years. The data on which Schaffer attempts to support his concept are either totally false or capriciously interpreted to support the hypothesis. We shall mention here some which Schaffer considers as more important: 1st. the recurrent course of the axon, that according to this author would be the rule, is actually an exception (recall the numerous non-recurrent collaterals of axons of the spinal cord, cerebral pyramidal cells, granules of the dentate gyrus, etc.); 2nd. the collateral character unwarrantedly assigned to the peripheral process of spinal ganglia sensory cells (assertion that is against all phylogenetic and ontogenetic data and that, in addition, designates as collateral the thick branch, and as axon the thin branch of a bifurcation); 3rd, the lack of myelin in dendrites, which according to Schaffer is contrary to the axonal nature of these processes, as if there were not an infinite number of axons devoid of myelin and capable however of conducting (fibers of the olfactory nerve, optic fibers in the retina, sympathetic axons, axons of cerebellar granules, axons of invertebrates, etc.). To realize the weakness of Schaffer’s argument, it suffices to state that it has not taken into account the following important facts which demolish the theory of this author: 1st., retinal ganglion cells, cerebellar granules, bipolar acoustic and olfactory cells, sympathetic cells, etc., lack axon collaterals; which would be the receptive apparatus in these cases?; 2nd., why do mitral cells dendrites ramify in olfactory glomeruli, not only in mammals, but also in amphibians, reptiles and fish where such regions lack capillaries or have them in extremely small numbers?; 3rd., how could it be possible to exclude the conductive role of dendritic processes in retinal ganglion cells?; 4th., what significance shall we give to the numerous examples of cells with somata or dendrites appearing surrounded by axonal terminal baskets? Shall we forget all of these important patterns, as Schaffer does, only because they prove the conductive capacity of the dendritic apparatus? [Attempts, such as Schaffer’s, should not be feared now that we know about the uniform neuro-fibrillar structure of the dendrites and all other parts of the nerve cell.]
“The nerve impulse,” says this scientist,” can march from the cell to the terminal axonal arborization, as well as in the opposite direction. The motor excitation marches only from the cell to the axonal arborization, whereas the sensory one goes in one or the other direction”. It is evident that Waldeyer was hesitant in adopting a single formula, even when considering the concrete issue of the direction of axonal transmission.
“This role of collecting the impulses”, we said while speaking about dendritic processes of central cells, “appears unquestionable in two examples: in olfactory glomeruli (where the olfactory fine fibers enter in relation with the thick dendritic processes of mitral cells), and in Purkinje cells, where dendritic branches come in contact with parallel fibers”. As shown in other passages, however, our thought fluctuated between the idea of polarization and that of indifferent conduction. The structure of the gray matter was not yet sufficiently known to allow the formulation of a general theory on the dynamics of nerve cells.
This description of the transmission in the auditory pathways is based on the works of Retzius, Lenhossék and Van Gehuchten regarding the terminations in the inner ear, and those of Held, Kölliker and ours on the nuclei and central pathways.
The doctrine that follows has been taken from our article: Laws of morphology and dynamics of nerve cells (Cajal, 1897b). A portion of the ideas in that article was developed in our lecture at the Atheneum on February 6, 1897, and elegantly summarized in Gaceta Médica de Granada of February 15, 1897 by our dear friend and scholar anthropologist Dr. Olóriz.
Recently, Lugaro defended Van Gehuchten’s conjecture regarding the problem created for the theory of dynamic polarization by the occurrence of a common trunk in unipolar spinal ganglion cells. According to Van Gehuchten, this trunk is formed actually by two parallel bundles of fibers, a cellulipetal one which is a continuation of the peripheral process, and another cellulifugal, prolongation of the central process. To support this concept, Lugaro states that, in preparations fixed and hardened in potassium dichromate and stained with hematoxylin, he could not see the direct continuation of the fibrils from the peripheral branch with those of the central process at the site of bifurcation, as it should occur if our hypothesis of axipetal polarization were true. The two fascicles would form an angle at the bifurcation to pass through the trunk to the soma. [Michotte, a student of Van Gehuchten, shares this view, based on his observations on sections treated with reduced silver nitrate.] But even assuming that this pattern is a pre-existing condition, how does Lugaro know that the fibrils, [or neurofibrils as they are known today], appearing in dichromate preparations (fibrils which have also been attributed to coagulations) represent the path of the nerve impulse? Could we not, on equal or perhaps firmer ground, attribute this conductive function to the interfibrillar fluid, which fills the mesh of colorless spongioplasm? Could the special cyanophilic substance that appears to soak uniformly cellular processes and is characterized by its affinity to méthylene blue in the Ehrlich method, play also such a role? In view of Lugaro’s [and Michotte’s] assertions we have made new observations on the concrete issue of the bifurcation of the trunk of unipolar spinal ganglion cells, which do not favor the hypothesis of the Italian scientist. Here are some of these findings. 1st. During the embryonic stage, the trunk is not formed by the apposition of two polar branches, but by the displacement of the nucleus toward the periphery, a phenomenon which results in the stretching of the spongioplasm between the sites of origin of the processes and the nuclear region of the soma, the position and direction of the central and peripheral fibrils remaining invariable. 2nd. In all developmental phases of these elements, the Golgi method does not show a crevice at the level of the bifurcation, but a straight or angled profile, without discontinuity between the central and peripheral processes. [3rd. We were not able to observe the double bundle of fibrils described by Lugaro in bifurcations stained with the Ehrlich method and examined with a Zeiss 1.40 objective; instead, there was a granular blue material extending continuously from the peripheral to the central branch. 4th. The fibrils that Lugaro observed in preparations fixed with potassium dichromate are not confirmed in those fixed with alcohol, in which the 1.40 objective shows only a grid or spongioplasm of very fine colorless trabecules at the level of the bifurcation, and never bundles of independent filaments. 5th. In sections of embryos treated with reduced silver nitrate, we often observed a bridge of neurofibrils joining the peripheral dendritic process to the axon in cells of mammalian spinal ganglia, and arcuate cells of the optic lobe of birds. Furthermore, bundles of neurofibrils in the common trunk are frequently joined to each other by fine oblique secondary neurofibrils. 6th. Finally, the ingenious experiments of Bethe in invertebrates, demonstrated the possibility of the direct passage of impulses from sensory axonal arborizations to accessory processes of motor axons. (See footnote 9)]. In summary: the pattern indicated by Lugaro appears to us as an unusual phenomenon, or the mere result of coagulations caused by potassium dichromate. In any event, even if the fibrillar arrangement could be demonstrated in living cells it would not warrant the assumption that this is the only intracellular path of the nerve impulse (see Lugaro, 1897a). Moreover, the fibrillar arrangement is negated by some authors, and we consider it as rather improbable because other methods such as Nissl’s never reveal independent fibrils at the level of the axon and dendritic processes, but a continuous reticule. Moreover, the fibrillar theory losses ground every day. The two authors who have dealt with this issue recently, Lenhossék and Held, are against it, making it clear that with adequate magnification, the neural protoplasm shows a spongy texture analogous to that indicated by Bütchli in all cells. For Held, even this alveolar mesh would be the result of the action of fixatives (vacuolization due to the penetration of fluids), because it should be remembered that in the living state, both dendrites and the axon appear completely homogeneous or very finely granulated (see. Held, 1897). The preconception of considering the soma (which in a final analysis is no more than a piece of conductor) as an obligatory path of all impulses arriving through the dendrites is not based on any positive finding. Physiology and pathological anatomy only teach us that the cell body, or more precisely the nucleus, exert a trophic influence on the axon and dendritic processes. But what reasons are there to assume that this influence extends to the actual act of transmission? Even after separating a nerve from its cell of origin, it still retains its excitability and is able to elicit muscle contractions. The impulse itself may exit from the peripheral ending of a sensory conductor, before passing through a soma. Does not all of these appear to indicate that the nucleus is not relevant, at least immediately, to the phenomenon of conduction?
[Actually, there is no direct proof that the cell body has other properties as a conductor than those of dendrites, or that the latter are subordinated to the point of directing impulses always toward the soma with no detours. The presence of the nucleus within the soma, the main argument of those who adhere to this thesis, is not in our view a sufficient reason to believe that the cell body is the first to undergo or sustain more directly the trophic influence exerted by the nucleus upon all parts of the neuron, having, therefore, some role in determining the direction and march of impulses. To demonstrate that, at least in some cases, the nucleus is foreign to the phenomenon of conduction, let us recall that a nerve separated from its cell of origin may still discharge. A sectioned motor nerve can contract the muscles if the peripheral segment is excited; a sensory nerve, cut within the ganglion, may evoke pain on excitation of the central segment, that is the dorsal root itself.]
[Alfred Bethe (1897, 1898b), with no knowledge of our work, has confirmed remarkably our ideas on the subject. Separating the axons of certain motor nerves from their cell bodies very close to their origin in an invertebrate (Carcinus moenas), Bethe observed no major alterations in reflexes. This would indicate that impulses pass from sensory nerves to accessory appendages or dendrites of motor axons, then to these very axons, and finally to the muscles, always in a cellulifugal direction, and with no influence of the absence of the cell body on conduction. The reflexes were certainly weaker after a few days and finally disappeared on the operated side. Bethe explains this disappearance by attributing a nutritive role to the cell body. The soma would be therefore a trophic center for its processes, and in no way indispensable for the function of conduction. We believe that similar results could be obtained in mammals, if operations of the type of Bethe’s were performed, as for instance the decortication of spinal ganglia.]
[Regarding final causes, we must declare that the terms goals, designs, improvements, etc. employed by us, are only expressions coined by usage. Indeed, according to us, there is no intentional direction, no preconceived plan in the evolution of Nature; only variations and adaptations which have prevailed because of their usefulness for survival. Therefore, the economy laws discussed in this chapter indicate simply modes or directions followed by variations useful to the animal during its phylogenetic evolution.]
With this experiment, Sherrington claims in vain to contest the polarization theory. Actually, his arguments harm only Van Gehuchten’s hypothesis on the intrinsic impossibility of retrograde conduction. Aside from this, the illustrious physiologist of Liverpool admits that, in the normal state, the sensory impulse marches from the axon toward its branches.
Physiologists consider the nerve impulse as an oscillatory movement with a wavelength of about 18 mm, estimated by the extent of the negative variation during the electric excitation of a nerve, the propagation velocity being 28 meters per second. For more details consult among other works of Physiology, those of Forster & Sherrington (1897) and Landois (1893).
We call unit of sensation the simple excitation collected during the action of a stimulus on a retinal cone, a hair cell of the organ of Corti, or an olfactory or sensory dendritic expansion.
Annotations
Textura reads in error invertebrates instead of vertebrates.
Cajal considers the nucleus of the mesencephalic root of the trigeminal nerve, as the origin of the masticatory nerve. (See annotation f in Chapter III).
Cajal refers here to axon collaterals of deep seated stellate cells, or basket cells. This subject shall be discussed in more detail in Volume II, Chapter XVI.
At the turn of the 19th century, the difference between graded (electronic) potentials characteristic of dendritic and somatic conduction, and action potentials prevalent in axons was unknown.
The hypothesis on the dendritic nature of peripheral processes of sensory cells was based purely on the direction of conduction. A later consideration is annotated in j.
Fig. 24.—a, tufted cell; b, terminal arborization of mitral cell apical dendrite; c, granule cell dendrites; e, collateral of mitral cell axon; h, supporting cell.
Fig. 25.—e, Textura and Histologie read in error ganglion cells instead of ganglion cells axons; G, S, unidentified.
Rods and cones are considered by some as neuroepithelial cells, and not as bipolar neurons. However, they do have neuronal features such as clear nuclei with concentrated nucleolus, central processes with typical organelles of axonal endings, a cilium from which the outer segment derives. For discussion see: Polyak (1957) The vertebrate visual system. Univ Chicago Press, Chicago, p 220.
Textura omits mentioning in the text the equivalent of present Fig. 28.
Here, Cajal’s concept of the nerve impulse propagating along the peripheral process and continuing through the central process with no major invasion of the cell body, is supported by contemporary findings. In fact, when such invasion was recorded, it occurred considerably later than the arrival of the nerve impulse at central terminals [Darian-Smith, Mutton, Proctor (1965) J Neurophysiol 28: 682-694].
See annotation d in Chapter IV for discussion of pericellular arborizations in sensory ganglia.
Histologie omits mentioning the equivalent of Fig. 33 in the text.
Fig. 33.—H, perinuclear neurofibrillar net continuing in the single process; a, neurofibrillar net focused at a more superficial plane.
Fig. 37.—A, cell soma.
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y Cajal, S.R. (1999). Physiologic Inferences from the Morphology and Connectivity of Neurons. In: Texture of the Nervous System of Man and the Vertebrates. Springer, Vienna. https://doi.org/10.1007/978-3-7091-6435-8_5
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