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The NEURONS and NEURAL SYSTEM: a 21St CENTURY PARADIGM The NEURONS and NEURAL SYSTEM: a 21st CENTURY PARADIGM This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. A few citations have yet to be defined and are indicated by “xxx.” James T. Fulton Neural Concepts [email protected] July 19, 2015 Copyright 2011 James T. Fulton 2 Neurons & the Nervous System 4 The Architectures of Neural Systems1 [xxx review cogn computation paper and incorporate into this chapter ] [xxx expand section 4.4.4 as a key area of importance ] [xxx Text and semantics needs a lot of work ] Don’t believe everything you think. Anonymous bumper sticker You must not fool yourself, and you are the easiest person to fool Richard Feynman I am never content until I have constructed a model of what I am studying. If I succeed in making one, I understand; otherwise, I do not. William Thomson (Lord Kelvin) It is the models that tell us whether we understand a process and where the uncertainties remain. Bridgeman, 2000 4.1 Background [xxx chapter is a hodge-podge at this time 29 Aug 11 ] The animal kingdom shares a common neurological architecture that is ramified in a specific species in accordance with its station in the phylogenic tree and the ecological domain. This ramification includes not only replication of existing features but further augmentation of the system using new and/or modified features. Very significant differences are seen between the neural systems of the major animal kingdoms, Arthropoda, Mollusca and Chordata. One of the most important is the introduction of the elements of what will be described as stage 3, signal projection (Chapter 9). This new feature results in the formation of the spinal chord and the adoption of the name Chordata for the phylum exhibiting this feature. It also introduces the phenomenon of the “action potential,” a signaling waveform of very specific properties. It is proposed, and the literature asserts, that the action potential phenomenon and signal projection technique are only found in Chordata. (a.k.a. vertebrata). While this work will exhibit a homocentric tendency, it is important to note Homo sapiens is not superior to other species in a variety of respects. The muscles used to move the ears (and the associated neural elements) have largely atrophied in humans. The visual spectrum of Homo sapiens, like that of other large mammals, is constrained by the transmission of the lens of the eye and is deficient in the ultraviolet compared to the smaller mammals (Section 8.2.xxx). The hearing modality of Homo sapiens is inferior to that of other species in several significant aspects (Section 8.3.xxx). This work will also develop the fact that the CNS of chimpanzee is not as sophisticated as that of homo sapiens, and arguably that of the orangutan (genus Pongo) or the bottlenose dolphin (genus Triosops truncatus) The ability of the dolphins to image sound reflections provides them a capability beyond that of the humans (except potentially on a limited scale in a few individuals blind since very early childhood). In developing the architecture of the neural system, it is useful to compare the circuitry to that of man-made microcircuit technology. Man started in the 1960's using what was called resistor-transistor logic (RTL). This technology was soon replaced by the more power efficient diode-transistor logic (DTL). Within a few years this technology was in turn replaced by a still more efficient transistor-transistor logic (TTL). The reader should immediately realize that the biological neural system is analogous to this highest level of man’s technology. Adopting a similar framework, the neurons, synapses and 1Released: July 19, 2015 Architectures 4- 3 Nodes of Ranvier of the biological neural system are used to implement Activa-Activa logic (AAL). To be complete, it is noteworthy the biological system does not use the man-made technology known as MOS or CMOS. Neither does it employ digital circuit technology. The biological system is an analog circuit based system and employs only a few binary (two-stable state) circuits. The second introductory quotation describes the position of Lord Kelvin at the time the Scientific Method was being defined. While the quotation is pedagogically important, Lord Kelvin developed multiple conceptual models that failed miserably (in some cases catastrophically from a financial perspective), most significantly by ignoring the discoveries of Maxwell relative to acoustic, thermal and electro-magnetic propagation. In presenting the third quotation, Bridgeman characterized the current state of research into the neural system. He said, “The neuroanatomical analyses can tell us where to look for a particular mechanism, and neurophysiology can tell us when the critical information is being processed, but current techniques leave the ‘how’ of the process tantalizingly elusive 2.” The first quotation highlights the difficulties of relying upon intuition, or what is sometimes described as introspection. The goal of this Chapter is specific. It is to provide a series of both globally and internally consistent models that can support a wide range of experimental results and lead the way to a broader understanding of the neural systems in animals. Thus the goal of this work is to support the theoretical half of the Scientific Method. [xxx modeling } There are a number of problems associated with modeling the signal processing functions in vision. One of the biggest is the plethora of simplistic block diagrams and circuit diagrams found in the literature. The second is the extremely high impedance level found in the initial stages of signal processing. These impedance levels are to be expected; they relate to the extremely small size of the elements and they contribute to lower overall power consumption. A third is the reluctance of the biology community (at least until recently) to embrace the fact that there are synaptic junctions that involve only the movement of electrons across the boundary. No movement of chemical ions (much less complex organic molecules with the complexity of a protein) is involved. Finally, the fact that 95% of the neurons in any organism (99.9% of the neurons in the retina) operate in the analog (not in the binary pulse) mode puts them in a class separate from much of the neural literature3. More specifically, the vast majority of neurons do not generate or process “action potentials.” It is noteworthy that the field of morphology is involved primarily with static conditions or very slowly changing conditions relative to growth. The field has largely overlooked the subject of neural signaling. One neurobiology text, that is heavily weighted toward morphology lists 14 types of cell-to-cell communications in hierarchal order but does not mention neural signaling based on electrolytics4. It is also noteworthy that the literature contains no detailed description of the signals at different positions along the signal path from the input voltage at a neurite to the output voltage at the axon of a neuron. Some of the material in this Chapter is designed for the actual laboratory researcher or serious analyst. It goes into more depth than required by the typical reader. The chapter is focused on the modeling of the larger parts of the neural system. The modeling of an individual Activa, an individual synapses and various individual circuits within a single neuron have been addressed in Chapter 2. Section 4.1 can be read selectively as needed by the reader. Sections 7.2 through 7.3 encompass the various phases of the detailed modeling of the neural system xxx. Section 4.1.1 is critically important if one is to avoid the many conceptual traps found in the 2Bridgeman, B. (2000) Neuroanatomy and function in two visual systems. Behav. Brain Sci. vol. 23, no. 4, pp 535-536 3Xxx 95% analog 4Shepherd, G. (1994) Neurobiology, 5th ed. NY: Oxford Press. pg. 72 in the 1988 edition 4 Neurons & the Nervous System current literature. Section 4.1.2 provides many detailed definitions that are required to insure consistency and correlation of the material in adjacent chapters. Section 4.1.3 concentrates on the high level architecture of the overall neural system, using the non-visual system as a template for understanding the visual system xxx.. Section 4.1.4 provides an internally consistent set of morphological nomenclature for the neural system and Section 4.1.5 does the same thing for the electrical waveforms normally associated with the neural system. Section 4.1.6 provides a review of graphical techniques of specific value in modeling the visual system xxx. Section 4.1.7 discusses the subtle differences between data and information rates. Section 4.1.8 reviews some of the models used in other presentations on the system aspects of the visual process. Section 4.2 develops the increasing complexity of the neural system as evolution has moved forward. Section 4.3 xxx Section 4.4 xxx and rest of sections. 4.1.1 A review of the Scientific Method and its application This discussion will be brief. A more complete discussion appears in the Introduction to Chapter 7. In the context of defining the neural system, the Scientific Method has not been used to its full advantage. There appear to be a variety of reasons for this. Until recently, there was little cross-discipline training of biological researchers. Second, the laboratory techniques required were often arduous and not easily reproduced by independent investigators. Third, the academic culture in biology has many of the hallmarks of a guild. New graduate students typically work along side more senior investigators, rather than in competitions directed at independently verifying the work of such senior investigators.
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