Conclusions

The human nervous system is similar in cellular elements, structure, function, and basic plan to the nervous systems of all vertebrates, especially those of its fellow mammals. This article has highlighted many organizing principles of the human nervous system: anatomical pervasiveness, physical and functional coherence, centralized organization, structural specialization, use-designed components, phyletic uniformity with versatile adaptability, inherent plasticity, and recourse to chemical messengers (diversified neuroac-tive substances that perform short- and long-range tasks). These generalizations afford perspective and context for more complex principles and bewildering details.

In documenting these, this article has overviewed the three major divisions of the human nervous system, its seven principal CNS regions, its major neuronal and neuroglial subpopulations, and its diversified cellular, subcellular, and molecular features.

As is apparent, the structure and organization of the human CNS are multifaceted and complex. Those concerned with functional circuitry seek to discern the fine details of the pathways serving specific functions and unite these with the neuroanatomy described earlier. Certainly, the major pathways are defined at an anatomical level, but what remains is the integration of anatomy with function. In this task, we must consider models of brain organization and bring these together with real structures and real connections.

First, the linear model. Stimuli are processed in a pathway-specific manner: like a road map. This is classical neuroanatomy. Whatever model pre-vails, this one will always be valuable, in the clinic and in the grand history of neuroscience before anyone recognized this supreme discipline or called it that.

Second, the multiple pathway concept, the lens model, of neural processing, converging on a single endpoint. Particular stimuli can select many pathways and still arrive at common endpoints. These features embody the concept of focused output, where the pathways are several but the output convergence points obligatory, e.g., as in recovery of function after brain damage and probably in several neurodegenera-tive diseases.

Third is the network model. It is the most versatile, addressing functional neuroanatomy in terms of probabilities and multiple cross-communicating pathways that develop a consensus output but nonetheless activate many outputs and options. A particular stimulus may be processed over many pathways and arrive at several outcomes, depending on variables not well-understood.

All of these models, and perhaps others, are valid. All apply, both to discrete stimuli and to output transformations. Neuroscientist Carl Cotman observes, "Functional neuroanatomy posits we know the basic circuits. From these we elucidate pathways serving function, covering a range from normal sensory activity to the neuronal plasticity operational in brain damage and disease. It is unlikely that one model will be involved in or satisfy all computational transactions of the human nervous system, nor adequately explain them.''

From what we know of the seamless interplay of neurons, neuroglia, the central and peripheral regions, functional subsystems, chemical substances, and nonneural components of the human nervous system (notably its autoregulatory vasculature), it may be that many models of the widest sort will come together, if and when we have an explanation of the human mind.

Illustration by Cheryl A. Cotraan

Light breaks on secret lots,

On tips of thought where thoughts smell in the ruin .. .

— "Light Breaks Where No Sun Shines" Dylan Thomas, 1933, published 1934

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