Morphology of the Nerve Cell (Continuation)

  • Santiago Ramón y Cajal


Mode of termination of dendritic processes.—In the last chapter we described the dendritic processes as relatively thick appendages with rough surface, bifurcated several times and terminated in the same gray focus where the cell body of origin resides. But, how is this termination realized?


Methylene Blue Purkinje Cell Nerve Cell Dendritic Process Axonal Process 
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  1. 1.
    See several figures of cerebral pyramidal cells in Dejerine and Dejerine-Klumke (1895a), which were drawn and prepared by Dr. Azoulay.Google Scholar
  2. 2.
    A recent publication (Stefanowska, 1897b) confirms the existence of spines, named as pyriform corpuscles. This author considers them also as an arrangement to increase the receptive surface of the protoplasm. In addition, such pyriform small bodies could modify their form and dimension during life, loosening their contacts with nerve fibers to a variable degree. This latter opinion seems to us rather risky, because the described varicose alterations of the spines, appear to be artifacts due to delayed fixation.Google Scholar
  3. 3.
    The demonstration of free endings of nerve fibers, both of collateral and terminal branches, appears mentioned first in our publication on the “Structure of nervous centers in birds” (1888b) and in another which appeared soon after on the “Structure of the cerebellum” (1888a). In this article, we also indicated the need to explain the passage of the nerve impulse from one cell to another, in favor of a contact established between the soma and the axonal branchlets, or by means of some sort of inductive influence similar to that occurring in electric coils. Actually, the observations on which these inferences are based, were made in 1886 and 1887; but in order to reach absolute certainty we delayed to publish the fruit of our labor until 1888.Google Scholar
  4. 4.
    [We shall have the chance to demonstrate the weakness of the neurogenetic arguments of Dohrn, Beard, Büngner, Bethe, Apathy, Fragnito, Modena, etc. against the neuron doctrine, when dealing with the histogenesis of the spinal cord. Readers who want to form an opinion on this over-ripened question should refer to our recent publications (Cajal, 1906a, c).]Google Scholar
  5. 5.
    [The arborization on Purkinje cells adopts its climbing character only in the adult nervous system; in the fetus, it starts as purely perisomatic, surrounding mostly the lower half of the cell body, and then the upper half. Later, it turns perisomatic and peridendritic, and finally, after some time, becomes exclusively peridendritic]Google Scholar
  6. 6.
    We must offer some clarifications to avoid uncertainties about the concept of morphologic and functional progress. In our opinion, each animal has a perfect nervous system for its class, because no other could serve better, without structural redundancies, for the special requirements of that organism. When we state that neurons of a mammal are more perfect than those of a fish, we only attempt to express the idea that mammalian neurons are capable of developing more complex acts, and serving as a substratum for a much higher number of neural reactions. There is the same relation between nervous systems of worms and vertebrates as that between a pocket watch that keeps only the hours, and a tower clock which keeps and sounds the hours, days, weeks and months. Both are perfect and useful for their respective purposes, but the mechanism of one must be much more complicated than that of the other. Each organism has the nervous system that it needs because Nature rejects the superfluous as well as the inadequate. What would be the usefulness of a bird retina to a worm or a mollusk, or a mammalian brain to a fish? For this reason, we consider that the unipolar cells of invertebrates fulfill their mission in these animals much better than the multipolar cells of mammals. Probably, the displacement of dendrites of invertebrate neurons (which, as it is known, emerge from the axon) represents some economic arrangement of matter or conduction time, or serve other presently unknown useful arrangements. In summary: the most absolute correlation exists in the organic systems of each animal; and the neural centers, which have the role of associating and transforming actions assigned to various tissues, only complicate their mechanisms in time with the expansions and improvements taking place in the skin, viscera and muscles. The complications of the neural apparatus represent perhaps not the cause, but the effect of improvements initiated in the other tissues by adaptation.Google Scholar


  1. a.
    Spines have been reported to arise also from other neuronal regions, such as the axon initial segment [Westrum (1970) J Comp Neurol 139: 337-356] as well as from the soma [Pasik, Pasik, DiFiglia (1976) Res Publ Assoc Res Nerv Ment Dis 55: 57-89].Google Scholar
  2. b.
    The existence of varicosities in both dendrites and axons of some neurons are by now well established facts, as shown by electron microscopy of gold-toned Golgi impregnated normal tissue [Di Figlia, Pasik, Pasik (1980) J Neurocytol 9: 471-492].Google Scholar
  3. c.
    Cajal uses the term ventral acoustic ganglion for the ventral cochlear nucleus.Google Scholar
  4. d.
    The existence of pericellular axonal arborizations in spinal and cranial ganglia has been the subject of a long debate. The very detailed drawings of Ehrlich, Cajal, and Dogiel are indeed convincing. However, the advent of electron microscopy has provided evidence for the reinterpretation of the light microscopic picture. Apparently, the interdigitation of neuronal somatic protrusions and infoldings of satellite cells membranes, which sometimes occur in several layers and are also present around the initial glomerulus, may account for such an appearance [Matsuda, Uedara (1984) Cell Tissue Res 235: 13-18]. In any event, no profile with axonal charateristics, i.e. presence of synaptic vesicles, has ever been observed [Pineda, Maxwell, Kruger (1967) Am J Anat 121: 461-488]. This is a clear example of what Cajal himself cautioned against, namely the overinterpretation of light microscopic images.Google Scholar
  5. e.
    Fig. 18.—a, perpendicular approach; b, oblique approach of a fine fiber providing two boutons.Google Scholar
  6. f.
    Histologie reads in error a instead of A; a, thread ball endings.Google Scholar
  7. g.
    Is Cajal’s consideration of the possibility of existence of interdendritic and interaxonal bridges a prelude of the discovery of gap junctions?Google Scholar
  8. h.
    Cajal’s view on the rarity of interdendritic contacts has been superseded by the demonstration of dendrodendritic synapses, the existence of which gives rise to complex synaptic arrangements of the triadic, serial and reciprocal types as observed in electron micrographs, rather frequently, in thalamic nuclei, olfactory bulb, retina, etc.Google Scholar
  9. i.
    This cruciate articulation between a Purkinje cell dendrite and a parallel fiber has been designated as the crossing-over synapse [Hámori, Szentágothai (1964) Acta Biol Acad Sci Hung 15: 95-117].Google Scholar

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© Springer-Verlag Wien 1999

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  • Santiago Ramón y Cajal

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