As a neuron theory or neuron theory in the strict sense, a more or less “atomistic” perspective is seen in neuroanatomy and neurophysiology . According to a generally shared opinion today, neurons or nerve cells are the smallest specific units of the nervous system and are therefore also referred to as its “building blocks”. (a) However, this should not lead to an absolutization of opposing viewpoints and perspectives ( reductionist-atomist versus holistic ) or the application of a machine paradigm. (a)
Six main clauses
Santiago Ramón y Cajal (1852–1934) established six main theorems in his classic formulation, which was last published in 1935, which are considered the basis of neuron theory:
- Every neuron is an anatomical unit . It consists of a nerve cell body with all of its extensions (dendrites and neurites). The neurons are connected to one another through contact points ( synapses ). At these points of contact there is no substantial transition of the neurons, but a membrane with a kind of cement substance is present.
- Each neuron is also a genetic unit . It is derived directly from a single embryonic cell, a neuroblast .
- Each neuron represents a functional unit of the nervous system. Outside of the neurons there is no substance that is capable of the services of the nervous substance (conduction of excitation, etc.).
- Each neuron is a trophic unit . This results, for example, after cutting through the neurite in the degeneration of the cut portion, in the chromatolysis in the associated cell body and in the regeneration through outgrowth of the central stump at the severing site. The Schwann cells are only involved in nourishing the axillary sprouts. There is no such thing as "autogenous regeneration". Once a cell has died, it can no longer regenerate.
- The neuron reacts as a nosological unit in that it is also involved in other pathological processes - cf. Zf. 4 - independent, d. H. independently of the other neurons, at least in the first phase of such processes, it responds to damaging influences.
- The neuron represents an electrophysiological unit . The nervous excitation is polarized, i. that is, it propagates from the dendrites towards the axis cylinder.
Different points of view emerged in the assessment of the performance principles. While researchers like Camillo Golgi (1843–1926) and others viewed the association of conduction pathways as a product of a “ syncytium ”, advocates of neuron theory demanded the functional independence of neurons. This latter view by Cajal has established itself as being anatomically justifiable and therefore correct. The synapses of the neuron chains do indeed reveal an intercellular space delimited by membranes. There is therefore no syncytial association. (b) (b)
The representatives of neuron theory besides Cajal are Heinrich Wilhelm Waldeyer (1836–1921), Wilhelm His (1831–1904), Albert von Kölliker (1817–1905) and Gustaf Retzius (1842–1919). In 1891, Waldeyer gave the nerve cell the succinct name "Neuron" . The advocates of this doctrine followed the principles of reductionism .
In addition to Golgi, the main representatives of the neural network association and the theory of continuity are Franz Nissl (1860–1919), Stephan Apáthy (1863–1922), Hans Held (1866–1942) and Albrecht Bethe (1872–1954) on a functional level . The historically older conception of the network association is also known as the reticulum theory. The advocates of this doctrine followed the principles of wholeness .
The advocates of the idea of functional networking also have successes against the adherents of a strict neuron theory. The theory of continuity seemed to be supported by the fact that during embryonic development there were connections between the primitive nerves formed by neuroblasts and the embryonic dispositions of the successive organs. Due to known developmental relationships between neuronal centers and periphery, concerns arose against a strict interpretation of neuron theory. Waldeyer's claim that every neuron is also genetically a unit, cf. the above-mentioned main clause No. 2 could not prevail in this way. The question of the ability of the nervous system to adapt to new environmental conditions ( neuroplasticity ) and the well-known facts about the self-organization of nerve tissue are arguments for the network concept to this day. Machines do not have such possibilities to the same extent. (c) (c)
Despite the above-mentioned fundamental difference between machines and living substances, attempts are being made to compare nerve cells with technical devices, especially in view of the success of artificial neural networks . These comparisons also serve in the heuristic sense of finding model ideas as an aid to neurological research. For example, the illustrative image of a switching device can be used, such as that used in telephone exchanges. The nerve fiber can be compared to the wire ( white matter = control unit), the nerve cell to the switch ( gray matter = control unit). However, such comparisons do not explain the complex structure and the corresponding function of the cerebral cortex, for example, with different types of nerve cells and switching connections. Also ontogenetic questions of development of this blueprint should not be understood by the fact that we know the structure of the components. Rather, the question arises of a meaningful connection of these components of the nervous system to a functioning whole. The following quote tries to answer this question:
“In spite of the significant advances made with the help of a refined histological method, there is such a large gap between our present-day morphological knowledge and the functional performance of the nervous system that we often only have hypothetical ideas about the nervous processes in physiological and pathological terms are able to do. "
Even if technical model ideas have proven to be very fruitful and not only for anatomy and physiology , but z. B. also play a role in psychosomatics ( cybernetics ), technology and living tissue must not be equated. The gap between the hypothesis and the established knowledge must not be overlooked.
As can be seen from the example of neuron theory, the cognitive process is favored by the contrast between theories based on an elementary approach (neuron theory) on the one hand and holistic ideas about the functioning of the entire nervous system (neuron association) on the other. Karl Jaspers (1883–1969) demonstrated this process using the example of the basic neurological scheme (reflex activity). (a)
Although the nerve cell is to be regarded as the smallest biological unit of the nervous system, from a functional point of view the interaction of neural cell groups has an essential task in dealing with specialized tasks. Here it is not individual nerve cells that are capable of performing higher or less complex tasks, but rather different types of networks that serve very different requirements. The individual nerve cell adapts to the requirements of such networked tasks by adjusting or changing the synapse weight according to the respective conditions. This ensures that incoming signals are either passed on or not. The interaction of nerve cells is gaining in importance not only in terms of the conceptual definition of neuronal centers and the understanding of their anatomical and topical nature and structure, but is particularly important for the understanding of functional relationships in neuropsychological syndromes . In this respect, for example, the opposing standpoints of association psychology (in the reductionist sense) and gestalt psychology (in the holistic sense) can be viewed as objects of fruitful discussion and supplementation. (d) (b) The self-organizing functioning of neural networks cannot be completely replaced by ideas of a machine paradigm alone.
- Alfred Erich Hoche : The neuron theory and its opponents. A. Hirschwald, Berlin 1899
- Franz Nissl : The theory of neurons and their followers. A contribution to solving the problem of the relationships between nerve cell, fiber and gray. Fischer, Jena 1903.
- Axel Karenberg : Neuron theory. In: Werner E. Gerabek , Bernhard D. Haage, Gundolf Keil , Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte. Walter de Gruyter, Berlin and New York 2005, ISBN 3-11-015714-4 , p. 1044.
- Olaf Breidbach : Nerve cells or nerve networks? About the development of the neuron concept. In: E. Florey, O. Breidbach (Ed.): The brain - organ of the soul? Berlin 1993, pp. 81-126.
- ↑ Robert F. Schmidt (Ed.): Outline of Neurophysiology . 3rd edition, Springer, Berlin 1979, ISBN 3-540-07827-4 ; P. 1 on tax authority "Neuron".
↑ a b c d Manfred Spitzer : Spirit in the net , models for learning, thinking and acting. Spectrum Akademischer Verlag Heidelberg 1996, ISBN 3-8274-0109-7 :
(a) p. 3 ff. On stw. “Neuron Theory”;
(b) p. 3, 347 on head. “neuronal syncytium”;
(c) pp. 52, 95, 103 ff., 155, 247 on stw. “Self-organization”;
(d) p. 234 ff. on head. "Association psychology" and p. 139 on head. "Gestalt psychology".
^ A b c Alfred Benninghoff and Kurt Goerttler : Textbook of Human Anatomy. Shown with preference given to functional relationships. 3rd volume nervous system, skin and sensory organs. 7th edition, Urban & Schwarzenberg, Munich 1964:
(a) p. 110 on Stw. "Machine paradigm";
(b) p. 109 ff. on tax. “Neuron theory and continuity theory”;
(c) p. 109 ff. as (b).
- ↑ a b c Helmut Ferner : Anatomy of the nervous system and the human sensory organs. 2nd edition, Reinhardt, Munich 1964; P. 34 ff. On taxation "Neuron theory".
- ^ A b Hermann Voss and Robert Herrlinger : Taschenbuch der Anatomie. Nervous system, sensory system, skin system, incremental system, Volume III. 12th edition, VEB-Gustav-Fischer, Jena 1964; P. 4 f. to Stw. "Neuron as a morphological, functional and genetic unit".
- ^ Thure von Uexküll : Basic questions of psychosomatic medicine. Rowohlt Taschenbuch, Reinbek near Hamburg 1963; Pp. 243 ff., 257 ff. On tax. "Communication engineering, model ideas".
^ A b Karl Jaspers : General Psychopathology . 9th edition, Springer, Berlin 1973, ISBN 3-540-03340-8 :
(a) p. 130 - on stw. “Neurological basic scheme”;
(b) p. 133 to district “Antagonism basic schemes”; P. 137 f. to Stw. "Sequence of Wholes".
- ↑ Ludwig J. Pongratz : Problem history of psychology , Bern, Munich 1967, ISBN 3-7720-1717-7 .