General Concept of the Nervous System Structural Plan of Neural Centers in the Animal Series
The nervous system represents the ultimate boundary in the evolution of living matter, and the most complicated machinery of noblest activities that Nature has to offer. As soon as this system appears, the unity of the living being is accentuated, its resources to procure food and its defenses against the attacks of the external world multiply, acquiring also greater precision, efficiency and congruency. And, in the highest levels of the animal series, so admirable phenomena as sensation, thought and will, emerge to perfect these defensive systems. It is true, however, that plants and simpler invertebrates, in spite of lacking a nervous system, have irritability, that is the capacity of every living cell to react in response to external stimuli. But there is no reason to believe that this property comes together with a conscious representation, nor that responses to external stimuli have the efficacy and congruency present in animals provided with a nervous system. In any case, such sensory manifestations require the existence of a conductive and motor substratum within the protoplasm of protozoans that must be represented by the spongioplasm or cellular reticule. The membrane would have the role of reception of sensations and their transmission to the motor apparatus. The network-like communications established between the membrane and the nucleus on the one hand, and all the filaments of the reticule on the other, assure the functional unity of the various cell components.
KeywordsMotor Neuron Sensory Neuron External World Sensory Cell Cerebral Ganglion
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- 1.My brother has made numerous attempts of impregnation of the actinia nervous system with silver chromate as well as with methylene blue with no success. Apparently, also Retzius has met with failure in this respect.Google Scholar
- 2.The hypothesis of the conscious activity of lower neural centers, supported by Pflüger for the spinal cord and extended by Durand de Gros and Forel for all nerve cells, is both an ingenious and daring concept. Although it is rather difficult to adopt, it has the advantage of filling the dynamic abyss that appears to exist between the cerebral ganglion and the sympathetic and spinal cord neural centers. In any event, it is an arduous question that will surely elicit serious controversies. Tanzi should be mentioned among the authors that fought it resolutely, and that considered the brain as the only site of conscious psychic activity. See his recent and well thought dissertation: I limiti della Psicologia. Discorso inaug. dell’anno accad. (1896, 1897).Google Scholar
- 3.[For a long time, Th. Meynert attributed also the functional diversity of nerve cells to the difference in their peripheral connections, and used this principle to explain why regions of the cerebral cortex, apparently of identical structure, mediate very different activities.]Google Scholar
- 4.[The passage from an imperfect sensory mechanism to an apparatus of the same order but more perfect appeared as a very embarrassing difficulty to the theory of natural selection. Let us take, for example, the panoramic vision of fish, reptiles and amphibians, that is associated with the complete decussation of the optic nerves fibers. In higher mammals, this vision becomes binocular and of a single field, and the optic nerves cross only partially, one portion remaining uncrossed. Consequently, it could be assumed that this arrangement would cause diplopia, a kind of vision more imperfect than in lower vertebrates; this is not the case, however, and the contrary is true.] [This and similar arguments do not make us reject the principle of natural selection. We have mentioned them only to demonstrate the need to consider other, as yet unknown, factors of progressive evolution.]Google Scholar
- 5.The causes for this progressive concentration of sensory cells are difficult to clarify. The first cause that comes to mind is that sensory neurons can acquire a greater protection against noxious influences of the environment by abandoning the epidermis and concentrating in successively deeper mesodermic planes. This migration would be even more advantageous if it were proven that the cutaneous sensory neurons are incapable of regeneration. The law of economy of matter is also applicable to this case, although not so efficiently as in other structural arrangements of the centers. The migration of cell somata has been able to economize nerve cells, since the peripheral processes of sensory cells lengthen and ramify, innervating larger and larger expansions of skin as we advance in the animal series. More neurons would be required if they resided in the skin and emited only one or a small number of short branches than if their peripheral arborization reached a great extension and originated in deeply seated somata. Perhaps the progressive expansion of the peripheral arborization has been a mere consequence of the increasing amplitude of the cutaneous surface in the phylogenetic scale. These causes assume the role played by variation, as well as natural selection, and other means of similar action, in fixating and exaggerating newly appeared useful arrangements.Google Scholar
- a.This is the first hint of the existence of an inhibitory mechanism.Google Scholar
- b.At this stage, Cajal’s ignores the thalamus as an integral part of the sensory pathways to the cortex. See annotationi in Chapter XIX.Google Scholar
- c.It is indeed amazing the depth of Cajal’s thoughts in predicting, already a century ago, the present view of storage of long-term memory in the cerebral cortex, and its disruption in processes such as Alzheimer’s disease, where there is alteration particularly of neurons establishing cortico-cortical connections [Hof, Morrison (1994) in: Terry, Katzman, Bick (eds) Alzheimer Disease. Raven Press, New York, pp 197-229]Google Scholar