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Neurons, the basic cells of the nervous system, are highly specialized cells designed for the processing and transmission of electrical and chemical signals. The nervous system can be divided into two anatomical divisions, each with their own subdivisions:

  • The central nervous system (CNS)
    • Brain. The brain, contained within the skull (cranium) begins its embryonic development as the cephalic end of the neural tube, before rapidly growing and differentiating into three distinct swellings: the prosencephalon, the mesencephalon, and the rhombencephalon (Table 2-1).
    • Spinal cord. The spinal cord has an external segmental organization (Figure 2-1). Each of the 31 pairs of spinal nerves that arise from the spinal cord has a ventral root and a dorsal root, with each root made up of one to eight rootlets, and each root consisting of bundles of nerve fibers.1 In the dorsal root of a typical spinal nerve, lies a dorsal root ganglion, a swelling that contains nerve cell bodies.1
  • Peripheral nervous system (PNS)
    • Cranial nerves (CN; with the exception of the second CN). The brain stem is literally the stalk of the brain. The brain stem gives rise to 10 of the 12 pairs of CNs.
    • The spinal nerve roots.
    • The dorsal root ganglia (or spinal ganglia).
    • The peripheral nerve trunks and their terminal branches.
    • The peripheral autonomic nervous system.

Table 2-1 Derivation and Functions of the Major Brain Structures
Figure 2-1

Schematic illustration of the spinal cord. (Reproduced with permission from Waxman SG. Correlative Neuroanatomy. 24th ed. New York, NY: McGraw-Hill; 2000:70.)


The spinal cord provides a conduit for the two-way transmission of messages between the brain and the body. These messages may descend, or ascend along pathways, or tracts, which are fiber bundles of similar groups of neurons, each with specific functions (Boxes 2-1, 2-2, and 8650385). The central gray matter of the spinal cord, which roughly resembles the letter H, contains two anterior (ventral) ...

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