149. The Brainstem: Metencephalon (fourth Vesicle)

  1. The metencephalon develops from the anterior part of the rhombencephalon and extends from the pontine flexure to the rhombencephalic isthmus. It differs from the myelencephalon in forming 2 specialized components
    1. THE DORSAL PORTION OR ROOF PLATE forms the cerebellum, which functions as a coordination center for posture and movement
    2. THE VENTRAL PORTION OR FLOOR PLATE, which becomes the pons, functions as the pathway for nerve fibers between the cord and the cerebral and cerebellar cortices
  2. The basal or floor plate and pons: the major morphologic features of the metencephalon do not change even though the lateral walls reapproach each other, and the basal motor and sensory alar plates are seen easily. Each basal plate contains 3 groups of motor neurons
    1. THE MEDIAL SOMATIC EFFERENT GROUP gives rise to the abducens (VI) cranial nerve
    2. THE SPECIAL VISCERAL EFFERENT GROUP contains the nuclei of the trigeminal (V) and facial (VII) cranial nerves which innervate the muscles of branchial arches I and I
      1. The nuclei of nerves V and VII pass across the level of both the metencephalon and myelencephalon
    3. THE GENERAL VISCERAL EFFERENT GROUP contains the superior salivatory nucleus, axons of which grow out into the facial nerve to supply the submandibular and sublingual glands as well as the nasal and lacrimal glands
    4. THE MARGINAL LAYER OFTHE BASAL PLATES expands and thickens to serve as a bridge for the nerve fibers connecting the cerebral cortex and cerebellar cortex with the spinal cord and is known as the pons
      1. In addition to nerve fibers, the pons contains pontine nuclei which originate in the sensory alar plates of the metencephalon and myelencephalon
        1. These nuclei serve as relay stations in the extrapyramidal pathways that connect the cortex of the telecephalon with the cerebellum
      2. The axons of the pontine nuclei grow toward the cerebellum and form the middle cerebellar peduncles
  3. The alar plate and rhombic lip: development is very complicated
    1. THE VENTROMEDIAL PORTION of the alar plate contains 3 groups of sensory nuclei
      1. The lateral somatic afferent group contains neurons of the pontine portion of the trigeminal (V) nerve and a small part of the vestibulocochlear complex (VIII)
      2. The special visceral afferent group is represented by the cranial portion of the nucleus of the solitary tract
      3. The general visceral afferent group is represented by the most cranial part of the dorsal sensory nucleus of the vagus (X) nerve
    2. THE DORSOLATERAL PARTS of the alar plates bend medially and form the rhombic lips which project partly into the lumen of the fourth ventricle and partly above the attachment of the roof plate (extraventricular portion) to give rise to the cerebellum (see Section 150 next for development of the cerebellum)

the brainstem: metencephalon  (fourth vesicle): image #1