Chapter 151. The Brainstem: Mesencephalon (third Vesicle) - Review of Medical Embryology Book

The mesencephalon is morphologically the most primitive of the brain vesicles. Its basal and alar plates, separated by the sulcus limitans (as seen in transverse section), are easily identified

A medial somatic efferent group is represented by the oculomotor (III) and trochlear (IV) cranial nerves, which innervate the preoptic (eye) muscles
A small general visceral efferent group is represented by the Edinger-Westphal nucleus, also associated with the oculomotor (III) nerve, and innervates the sphincter pupillary muscle

The marginal layer of each basal plate enlarges and forms the basis pedunculi (crus cerebri or cerebral peduncles)

The crura serve as pathways for the nerve fibers descending from the cerebral cortex to the lower centers in the pons and spinal cord
In the adult, the crura contain the corticospinal, the corticorubral, and the corticopontine tracts

The alar plates initially appear as 2 longitudinal elevations separated by a shallow midline depression

With development, a transverse groove divides each longitudinal elevation into an anterior (superior) and a posterior (inferior) colliculus

The nuclei of the posterior colliculus serve as synaptic relay stations for the auditory reflexes
The nuclei of the anterior colliculus serve as correlation and reflex centers for the visual impulses
The colliculi are formed by waves of neuroblasts produced by the neuroepithelial cells that migrate into the overlying marginal zone and become arranged in stratified layers
In higher vertebrates that have a visual and auditory neocortex, the colliculi tend to regress somewhat and become only oculo- and auditory motor reflex centers, independent of conscious perception. In lower vertebrates, the colliculi are true sensory "brains"

Some feel that the cells of the alar plate also give rise to the tegmental nuclei, the nucleus ruber (red nucleus), and the substantia nigr Others, however, feel they develop in situ from the basal plates

These nuclei are considered to be suprasegmental structures and are part of the extrapyramidal motor pathways. The red nuclei have a dual structure consisting of large phylogenetic ancient cells and many new small cells, also suggesting a dual origin

THE CAVITY OF THE MESENCEPHALON grows smaller, as a result of the growth of its walls, and eventually forms the so-called aqueduct of Sylvius which connects the third and fourth ventricles
THE CEREBRAL PEDUNCLES become more prominent with development as more descending groups of fibers pass through the developing midbrain on their way to the brainstem and spinal cord