Metencephalon

The metencephalon, similar to the myelencephalon, is characterized by basal and alar plates (Fig. 19.19). Two new components form: (a) the cerebellum, a coordination center for posture and movement (Fig. 19.20), and (b) the pons, the pathway for nerve fibers between the spinal cord and the cerebral and cerebellar cortices.

Each basal plate of the metencephalon (Fig. 19.19) contains three groups of motor neurons: (a) the medial somatic efferent group, which gives rise to

Cerebellum Roof Metencephalon

Figure 19.20 A. Dorsal view of the mesencephalon and rhombencephalon in an 8-week embryo. The roof of the fourth ventricle has been removed, allowing a view of its floor. B. Similar view in a 4-month embryo. Note the choroidal fissure and the lateral and medial apertures in the roof of the fourth ventricle. C. Scanning electron micrograph of a mouse embryo at a slightly younger stage than in A, showing the cerebellar primordium (arrow) extending into the fourth ventricle (V). M, mesencephalon. D. High magnification of the cerebellar region in C. Choroid plexus (arrow) in the roof of the fourth ventricle (V).

Figure 19.20 A. Dorsal view of the mesencephalon and rhombencephalon in an 8-week embryo. The roof of the fourth ventricle has been removed, allowing a view of its floor. B. Similar view in a 4-month embryo. Note the choroidal fissure and the lateral and medial apertures in the roof of the fourth ventricle. C. Scanning electron micrograph of a mouse embryo at a slightly younger stage than in A, showing the cerebellar primordium (arrow) extending into the fourth ventricle (V). M, mesencephalon. D. High magnification of the cerebellar region in C. Choroid plexus (arrow) in the roof of the fourth ventricle (V).

the nucleus of the abducens nerve; (b) the special visceral efferent group, containing nuclei of the trigeminal and facial nerves, which innervate the musculature of the first and second pharyngeal arches; and (c) the general visceral efferent group, whose axons supply the submandibular and sublingual glands.

The marginal layer of the basal plates of the metencephalon expands as it makes a bridge for nerve fibers connecting the cerebral cortex and cerebellar cortex with the spinal cord. Hence this portion of the metencephalon is known as the pons (bridge). In addition to nerve fibers, the pons contains the pontine nuclei, which originate in the alar plates of the metencephalon and myelencephalon (arrows, Fig. 19.19).

The alar plates of the metencephalon contain three groups of sensory nuclei: (a) a lateral somatic afferent group, which contains neurons of the trigeminal nerve and a small portion of the vestibulocochlear complex, (b) the special visceral afferent group, and (c) the general visceral afferent group (Fig. 19.19).

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