Updated: Oct 15, 2011
The nervous system is “segmented”, a critical property of vertebrate bodies that allowed the building of modular designs in different parts of the body, in a kind of “conservation” of developmental processes. This method of building a vertebrate body conserved successful designs and promoted small changes in that design to cause major adaptations in vertebrates to different habitats. Once scientists realized that segmentation occurred as a basic property of vertebrate design, its discovery in different systems (skeletal, muscular, urinary, reproductive, and digestive) allows us to study evolution more efficiently. The modular building of segments in the brain and spinal cord mimics the modules formed from bone, muscle, and parts of the body. Thus, keeping segmentation in mind, we can understand how certain basic principles of form and function are kept in the spinal cord and brain during development, even if the final form of the brain appears to have changed so drastically.
The positioning of the future nervous system to be close to the layer that gives rise to all muscles, bones, ligaments, tendons, and cartilage of our skeleton means that its property of segmentation will mirror the segmentation found in our skeletal system. Segmentation refers to the tendency of the body to be broken up into pieces, each of which carries properties similar to the previous piece, and generally in the same order. For instance, each vertebra is a segment, as is each of the parts of an arm which form the hand, forearm, and upper arm. Although the segments of the arm look different in the adult, they looked exactly alike at one time in the embryo.
Segmentation in the spinal cord is fairly obvious, even in the adult. Each segment looks very similar to all the other ones there, with differences in the shape and size of the gray matter and white matter reflecting the demands of the territory controlled by that segment. The spinal cord will have 8 cervical segments, 12 thoracic segments, 5 lumbar segments, and 5 sacral segments. I think there are 3 coccygeal segments that have become incorporated into S3-S5 during human evolution, for theoretical reasons (to be explained under Nervous System category at some point).
The brain, since it starts out as the anterior-most three segments of the neural tube which then split into five segments, will have centers and functions assigned to each that mirror the anatomy and physiology of the following segments, including those in the spinal cord. The brain segments become specialized very quickly and maintain only some of the layout seen in the spinal cord segments. The three main segments, forebrain, midbrain, and hindbrain do not stay that way for long. The forebrain splits up into the telencephalon and diencephalon. The midbrain remains very much the same in form and name, the mesencephalon. The hindbrain splits up into metencephalon and myelencephalon. These last two parts form the anterior and posterior medulla. The telencephalon sends out swellings from the middle of the segment on the lateral side that rapidly balloon and form infoldings to greatly increase its surface area to form the cerebral hemispheres. The diencephalon stays deeply enveloped by the cerebral hemispheres to form the thalamus, epithalamus (pineal body and habenula) and hypothalamus (pituitary, suprachiasmatic nuclei, metabolic centers, to name a few structures).
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