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Principles of Brain Organization

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  1. Introduction
  2. Classification of neurons
  3. Architecture Neurons and their processes
  4. Connections
  5. Distinctiveness of the human brain
  6. Conclusion
  7. Suggested readings

The human brain contains approximately 1011 nerve cells or neurons. In general, neurons are composed of four morphologically identified regions: (1) the cell body or soma, which contains the nucleus and can be considered the metabolic center of the neuron; (2) the dendrites, processes that arise from the cell body, branch extensively, and serve as the major recipient zones of input from other neurons; (3) the axon, a single process that arises from a specialized portion of the cell body (the axon hillock) and conveys information to other neurons; and (4) the axon terminals, fine branches near the end of the axon that form contacts (synapses) generally with the dendrites or the cell bodies of other neurons, release neurotransmitters, and thereby provide a mechanism for interneuronal communication.

[...] The locus ceruleus, a small group of norepinephrine-containing neurons in the brainstem that sends axonal projections to the entire cerebral cortex and other brain regions, is an example of a divergent system. In contrast, the output of multiple brain regions may be directed toward a single area, forming a convergent system. Third, the connections among regions may be organized in a hierarchical or parallel fashion or both. For example, visual input is conveyed in a serial or hierarchical fashion through several populations of neurons in the retina to the lateral geniculate nucleus, to the primary visual cortex, and then progressively to the multiple visual association areas of the cerebral cortex. [...]

[...] The growth of an axon along distinct pathways is guided by molecular cues from its environment and eventually leads to the formation of synapses with specific target neurons. Although the projection of axons is quite precise, some axons initially produce an excessive number of axon branches or collaterals and thus contact a broader set of targets than are present in the adult brain. During later development the connections of particular neurons are focused by the pruning or elimination of axonal projections to inappropriate targets. Within the adult brain the connections among neurons or neural circuits follow several important principles of organization. [...]

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