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 1 (949) 759-0630 [email protected] October 21, 2014 Copyright 2011 James T. Fulton [xxx table of contents numbering still screwed up ] [xxx Broca & wernicke are minor foci in long signaling chains, see Lieberman, Eve Spoke page 100 ] [xxx consolidate 10.2 and 10.8 ] 10 The Morphology of the Neural system 1 Form follows function, and the available real estate 10.1 Introduction Anatomy has played a major role in the past development of the neurological system. However, as science has advanced to the histological and cytological level, the ability to describe operations from a morphological perspective, particularly an intuitive morphological perspective, has become less than useful. This chapter will review the gross morphology of the nervous system but will follow a different path at the more precise levels. At those levels, electrophysiology provides much more useful and precise information. The morphologist has used the term functional in a very nebulous manner, to describe the participation of an engine, circuit, or neuron in the signal processing or manipulation related to a high level modality. This work will use functional in a narrower sense related to the specific operational significance of a single cytological neuron or its equivalent electrical circuit of physiology. When discussing the morphology of the nervous system, and particularly the central nervous system (CNS), the work of Blinkov & Glezer should not be overlooked2. It is by far the definitive reference on all morphological and histological elements of the CNS. Young has made a series of very significant observations in a 2000 paper3. They will be developed throughout this chapter. However, some of them deserve early attention. Under the title, “Inconvenient results for vision-as-analysis,” he makes the following observations. “. it is widely assumed that a large proportion of the synapses in primary visual cortex come from neurons in the LGN, the principal relay for signals from the eye to the cortex in primates. But this is not the case.” He goes on to justify his position with a number of citations. With regard to vision, he notes, “Both types of data indicate that V1 is situated nearest the visual periphery in connectional terms, and that other visual stations are successively more central, culminating in anterior IT and other rostral parts of the temporal lobe, . .” This work suggests it is the thalamic reticular nucleus (TRN) of the thalamus that is the culminating point prior to transfer of the percept signals to the saliency map associated with area 7. Young concludes, “The old certainties, among them that one can employ a bar or grating in a denuded visual scene and hope to understand normal vision, may be beginning to give way.” To understand the operation of the CNS, it is very important to understand the functional flexibility 1Released: 21 October 2014 2Blinkov, S. & Glezer, I. (1968) The Human Brain in Figures and Tables. NY: Basic Books 3Young, M. (2000) The architecture of visual cortex and inferential processes in vision Spatial Vision vol 13, pp 137-146 2 Neurons & the Nervous System inherent in the neuron as an electrolytic-semiconductor-based circuit (Chapter xxx). The neurons of the retina have been shown to be relatively simple in their internal structure and interconnection architecture4. An important specific feature was that the nucleus and soma played minor roles in the operation of the retinal neurons. It was the active electrolytic device, the Activa, within the soma that was the cornerstone of neural operation. Further analysis showed that many Activa were present in the photoreceptor cells of the retina outside of the soma. When exploring the auditory modality, it was found that the Activa also appeared at what must be described as Nodes of Ranvier along the neurites of the neuron and outside of the soma5. Exploration of the CNS has uncovered another unexpected feature (Section xxx). It appears the branching of axons at Nodes of Ranvier can result in the formation of analog-oriented axon segments as well as phasic-oriented axon segments emanating from the same neuron. Thus, an analog signal can be converted to a phasic signal at a Node of Ranvier in addition to within the hillock of a soma. Appreciating this fact leads to a better appreciation of the physiological architecture of the nervous system. 10.1.1 Establishing the discussion framework While a general equivalence is recognized between the brains of the higher primates and the human, it must be realized that at the current state of research, many significant differences are being documented that differentiate the human brain from all other (at least primate) brains6. In 2007, Preuss provided an even broader discussion of the homology, analogy and differences between the brains of various species, both primate and non-primate7. His report is indicative of a turning point among the anatomical, morphological and histological communities toward more reliance on the traffic flow patterns among the engines of the CNS. He focuses on the major role of the superior colliculus, but does not quite come to recognize the separation between the SC and the uniquely positioned engine (the perigeniculate nucleus (PGN) located along the Brachia of the Superior Colliculus. It is the PGN that focuses on the sensory functions previously traced to the SC with the remaining portions of the SC focused on the motor activity. The combination of the PGN, the remainder of the SC and the TRN constitute a powerful system for controlling the sensing, reaching and grabbing activity so critical to higher mammal and primate activity. His paper is an important one. It will be cited repeatedly in this chapter. In the 2007 paper, Preuss notes the lack of uniform attention to the many functional systems employed among the mammals, and the resultant difficulty in drawing precise conclusions in many areas. Two potential phylogenic trees of the primates are described in Section 1.1.3. They differ primarily because of the limited research concerning one of the human’s closest relatives, the orangutans (genus Pongo). It is proposed here that the orangutans and humans share a single family, Hominidae with the chimpanzees (Pan) and gorillas (Gorilla) grouped in the family, Panidae. The monkeys are in a more primitive suborder, anthropoidea. While the chimpanzee has been widely used in behavioral research and claimed by many to be a model for the human neural system, it is clear it is less than a complete model. The members of Macacus (particularly the Rhesus monkeys) are even more limited. It has been widely reported the Rhesus monkey does not present a V4 analogous to that of humans. The least studied but the most similar of the primates to the human is proposed to be the orangutan. It exhibits a variety of characteristics similar to those of humans (including the only other primate to copulate face-to-face.). Their intelligence is well noted but poorly quantified. Interest in the abilities of the orangutan has increased recently. Oxnard has performed a large number of multivariate analyses across the broader spectrum of the primates that includes many results suggesting a closer relationship 4Fulton, J. (2004) Biological Vision: A 21st Century Tutorial. Vancouver, BC, Canada: Trafford 5Fulton, J. (2006) Biological Hearing: A 21st Century Tutorial. Vancouver, BC, Canada: Trafford 6Preuss, T. (2004) What is it like to be a human? In Gazzaniga, M. ed. (2004) The Cognitive Neurosciences, 3rd Ed. Cambridge, MA: MIT Press pp 5-22 7Preuss, T. (2007) Evolutionary specializations of primate brain systems In Ravosa, M. & Dagosto, M. eds. Primate Origins: Adaptations and Evolution. NY: Springer Chapter 18 System Morphology 10- 3 between the orangutan and homo than previously documented8. Pages 246-247 show alternate primate cladograms depending on his criteria. Hopefully, his material will encourage other researchers to include the orangutan in their studies. The cited work from 1984 includes citations to his earlier work. The limited capability of his multivariate morphometric analysis compared to the later multi-dimensional scaling combined with the associated stress test (is important. Table 3.1 of the 1984 book shows a wide variation in the rank order of the primates based on the number of dimensions employed (ranging from 3 to 17). In keeping with his philosophy of introducing his multivariate analysis without providing details, the criteria for establishing these rank orders were not provided. All of the work in the 1984 book was based on the fossil skeletal record. The human exhibits a perigeniculate nucleus/pulvinar couple that is unique to the human species (unless shared with the orangutan and possibly members of Cetacea). It occurs in two variants the visual and auditory couples. The (vision) perigeniculate nucleus/pulvinar couple is associated with the foveola of the retina, a 1.2 degree diameter disk centered on the line of fixation. This couple is key to the ability of the human to perceive fine spatial detail and to read. The (auditory) perigeniculate nucleus/pulvinar couple is associated with the ability of humans to perceive the fine structure of complex sound patterns, and to appreciate music. No animal is known to compete with the human in fine spatial vision. Therefore, there is no known animal model for this pre-eminent portion of the visual neural system. There is no known animal model for the more sophisticated aspects of the auditory neural system, although Cetacea may equal the performance of the human (and exceed it in specific capabilities).