Basic Organizing Principles Of The Nervous System

Breakthroughs have occurred in our understanding of these principles. Visual sensory processing, the task of the visual system, is a good example. Like other sensory analyses, it is far more complex than once surmised. At their first way station in the brain, the lateral geniculate nucleus (LGN), optic tract fibers end on neurons in one of two neuronal populations: a large-celled group in the lower two layers of the LGN and a more numerous small-celled group making up the upper four layers. When thalamic projections of these two types of cells reach the primary visual cortex in the occipital lobe, visual processing diverges into two distinct, only partly connected streams. The parvocellular system streams into inferior temporal cortex for analysis of color and form and the magnocellular system into posterior parietal cortex for analysis of location and movement. In brief, these ventral and dorsal streams are the "What?" and "Where?" systems. Taken together, they comprise (at last count) 32 distinctive cortical areas interconnected by association fibers in a complex, often nonlinear, reciprocal manner, affording divergence and convergence modes of data analysis.

Visual analysis draws upon other sensory systems and systems distant from visual processing: the motor system, core integrative systems (reticular formation, thalamus), the affect and drive-related limbic system, and the decision-making and memory-related frontal cortex. Perceptual meaning emerges and responses drafted as impulses diverge and converge in the network in domains of alertness, attention, comparison, affect, cognition, and conation. By feature analysis, integration, pattern synthesis, and data storage and retrieval, the nervous system may oversee activity inside and outside it, maintain homeostasis, and generate and monitor responses, all in a timely way, but not necessarily. We now consider eight attributes of the nervous system. They provide a context for its design and details.

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