Chapter 146. Introduction to Brainstem Development - Review of Medical Embryology Book

Introduction: the brainstem is formed by the myelencephalon (fifth vesicle), the metencephalon (fourth vesicle), and the mesencephalon (third vesicle). The cerebellum is derived from and is attached to the brainstem

IN THE BRAINSTEM, THE GRAY AND WHITE MATTER are not in the same relationships to each other as they are in the spinal cord, nor does the gray matter have the same potential
CEPHALIZATION, through which these suprasegmental structures are found, begins in the brainstem. Vesicle development reveals how these changes occur
THERE IS ALWAYS A ROOF PLATE derived from the dorsal part of the neural tube and a floor plate from its ventral part. The plates and their lateral boundaries delineate the cavities of the neural tube, the future ventricles, and the interventricular communications
THE MYELENCEPHALIC CAVITY, the fourth ventricle, remains in communication with the spinal canal, at its caudal end, and with the mesencephalic cavity via the metencephalic cavity

Segmental nuclei: similar to the medullary centers and consist of nuclei of cranial nerves as well as autonomic nuclei
Suprasegmental structures: indicating cephalization. These structures are relay or association centers and head up the spinal cor Even though they are integrated into a motor system (the extrapyramidal tract), they are derived in whole or in part from the alar plates which, even in the spinal cord, give rise to the synaptic relay and association centers

Development of the suprasegmental structures is characterized by cellular migrations which are more extensive than those in the spinal cor These migrations increase with the complexity of the organs (, the cerebellum)

Some of these structures, in each vesicle, are rather discrete, such as the olivary nuclei, the nuclei of Goll and Burdach, the nuclei of the pons and cerebellum, the red nuclei and substantia nigra, and the nuclei of the corporaquadrigemina (or colliculi)
Others are diffuse, such as the reticular formation, which occupies the ventral portion of the 3 vesicles. The reticular formation is formed from many small nuclei and is derived from either the alar or basal plates, or both, in variable proportions. Its mesencephalic portion activates the extrapyramidal tracts. Its pontine-medullary portion activates the ascending sensory tracts and inhibits the pyramidal motor tracts

WHITE MATTER is formed from myelinated tracts which thicken the marginal layer ventrally or just pass through the gray nuclei

Segmentary association tracts make the brainstem a functionally homogeneous whole and connect it to its subjacent and suprajacent structures
Cerebromedullary or medullocerebral pathways use the brainstem only as a crossover (, the pyramidal tracts, the spinothalamic pain and heat pathways) or make connections at the level of its suprasegmentary centers (, the extrapyramidal pathways, the spinobulbothalamic pathways of proprioception)
Brainstem pathways, , the geniculate tract, connecting the cerebral cortex to the nuclei of the cranial nerves
Spinocerebellar pathways, , Flechsig's tract of deep unconscious sensory perception