Neuroblasts, or primitive nerve cells, arise exclusively by division of the neuroepithelial cells. Initially they have a central process extending to the lumen (transient dendrite), but when they migrate into the mantle layer, this process disappears, and neuroblasts are temporarily round and apolar (Fig. 19.9 A). With further differentiation, two new cytoplasmic processes appear on opposite sides of the cell body, forming a bipolar neuroblast (Fig. 19.9 B). The process at one end of the cell elongates rapidly to form the primitive axon, and the process at the other end shows a number of cytoplasmic arborizations, the primitive dendrites (Fig. 19.9C). The cell is then known as a multipolar neuroblast and with further development becomes the adult nerve cell or neuron. Once neuroblasts form, they lose their ability to divide. Axons of neurons in the basal plate break through the marginal zone and become visible on the ventral aspect of the cord. Known collectively as the ventral motor root of the spinal nerve, they conduct motor impulses from the spinal cord to the muscles (Fig. 19.10). Axons of neurons in the dorsal sensory
horn (alar plate) behave differently from those in the ventral horn. They penetrate into the marginal layer of the cord, where they ascend to either higher or lower levels to form association neurons.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.