Porth's Essentials of Pathophysiology, 4e

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Nervous System

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axial skeleton, skeletal muscles, and sensory organs such as the eyes and ears. Throughout life, the organization of the nervous system retains many patterns that were established during embryonic life. During the 2nd week of development, embry- onic tissue consists of two layers, the endoderm and the ectoderm. At the beginning of week 3, the ecto- derm begins to invaginate and migrates between the two layers, forming a third layer called the mesoderm (Fig. 34-5). Mesoderm along the entire midline of the embryo forms a specialized rod of embryonic tissue called the notochord . The notochord and adjacent mesoderm provide the necessary induction signal for the overlying neuroectoderm to differentiate and form a thickened structure called the neural plate . Within the neural plate a groove develops and sinks into the underlying mesoderm. Its walls fuse across the top, forming an ectodermal tube called the neural tube . The neuroectoderm of the neural plate gives rise to the brain and spinal cord of the CNS, while the notochord becomes the foundation around which the vertebral column develops. The surface ectoderm separates from the neural tube and fuses over the top to become the outer layer of skin. This process involved in the formation of the neural plate and neural folds and closure of the folds begins at the cervical and high thoracic levels and zippers both rostrally and caudally. Complete closure occurs at the rostral-most end of the brain around day 25 and at about day 27 in the lumbosacral region. Most congeni- tal defects, such as spina bifida, result from failure of fusion of one or more neural arches of the developing vertebral column during the fourth week of embryonic development. As the neural tube closes, ectodermal cells called neural crest cells migrate away from the dorsal surface of the neural tube to become the progenitors or parent cells of the neurons and neuroglial cells of the PNS. Some of these cells gather into clusters to form the dor- sal root ganglia at the sides of each spinal cord segment and the cranial ganglia that are present in most brain segments. Neurons of these ganglia become the affer- ent or sensory neurons of the PNS. Other neural crest cells become the pigment cells of the skin or contribute to the formation of many structures of the face, cer- tain cells of the autonomic nervous system, and other structures. During development, the more rostral (toward the head) portions of the embryonic neural tube—approxi- mately 10 segments—undergo extensive modifica- tion and enlargement to form the brain (Fig. 34-6). In the early embryo, 3 swellings, or primary vesicles, develop, subdividing these 10 segments into the fore- brain, containing the first 2 segments; the midbrain, which develops from segment 3; and the hindbrain, which develops from segments 4 to 10. In the prosen- cephalon or forebrain, two pairs of lateral outpouchings develop: the optic cup, which becomes the optic nerve and retina, and the telencephalic vesicles, which become the cerebral hemispheres. Within the prosencephalon, the hollow central canal expands to become enlarged

SUMMARY CONCEPTS (continued)

Developmental Organization of the Nervous System The organization of the nervous system can be under- stood in terms of its embryonic development, in which newer functions and greater complexity result from the modification and enlargement of earlier developed structures. Thus, the rostral or front end of the CNS, which is the last to develop, is more specialized in its functions than the caudal or tail end structures, namely the brain stem and spinal cord, which are the first to develop. The dominance of the rostral end of the CNS is reflected in what has been termed a hierarchy of con- trol, with the forebrain having control over the brain stem and the brain stem having control over the spi- nal cord. In the developmental process, newer functions were added to the surface of earlier developed systems. As newer functions were added to the surface of earlier developed systems and became concentrated at the ros- tral end of the CNS, they also became more vulnerable to injury. Nothing exemplifies this principle better than the persistent vegetative state (discussed in Chapter 37) that occurs when severe brain injury causes irreversible damage to higher cortical centers, while lower brain stem centers such as those that control breathing remain functional. Embryonic Development The nervous system appears very early in embryonic development (22 to 23 days). This early development is essential because it influences the development and organization of many other body systems, including the ■■ Neurotransmitters can produce either excitatory or inhibitory effects and can be broadly categorized into three groups based on their chemical structure: (1) amino acids (e.g., glutamic acid, glycine, and GABA) that serve as neurotransmitters at most CNS synapses; (2) peptide neurotransmitters (e.g., endorphins and enkephalins) that are involved in pain perception and sensation; and (3) monoamines (e.g., epinephrine and norepinephrine) that serve as neurotransmitters for the ANS. ■■ Neuromodulators bring about long-term changes that subtly enhance or depress the action of target receptors. Neurotrophic factors are polypeptides that influence the proliferation, differentiation, and survival of neuronal and nonneuronal cells.They are secreted by axon terminals.