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Basal And Alar Plates

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Figure 19.23 A and B. Position and differentiation of the basal and alar plates in the mesencephalon at various stages of development. Arrows in A indicate the path followed by cells of the alar plate to form the nucleus ruber and substantia nigra. Note the various motor nuclei in the basal plate.

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Figure 19.23 A and B. Position and differentiation of the basal and alar plates in the mesencephalon at various stages of development. Arrows in A indicate the path followed by cells of the alar plate to form the nucleus ruber and substantia nigra. Note the various motor nuclei in the basal plate.

oculomotor and trochlear nerves, which innervate the eye musculature, and (b) a small general visceral efferent group, represented by the nucleus of Edinger-Westphal, which innervates the sphincter pupillary muscle (Fig. 19.23B). The marginal layer of each basal plate enlarges and forms the crus cerebri. These crura serve as pathways for nerve fibers descending from the cerebral cortex to lower centers in the pons and spinal cord. Initially the alar plates of the mesencephalon appear as two longitudinal elevations separated by a shallow midline depression (Fig. 19.23). With further development, a transverse groove divides each elevation into an anterior (superior) and a posterior (inferior) colliculus (Fig. 19.23B). The posterior colliculi serve as synaptic relay stations for auditory reflexes; the anterior colliculi function as correlation and reflex centers for visual impulses. The colliculi are formed by waves of neuroblasts migrating into the overlying marginal zone. Here they are arranged in layers (Fig. 19.23 B).

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