Rhombencephalon (Hindbrain)

Rhombencephalon (hindbrain) Myelencephalon  medulla oblongata  Most caudal of the brain vesicles.  Lateral walls are everted, with the sulcus limitans clearly distinguished and alar plates lateral to basal plates. Therefore, motor nuclei develop medial to sensory nuclei.   Basal plate contains motor nuclei, which are divided into three groups: o Medial somatic efferent group: CN12 that supplies tongue musculature o Intermediate special visceral efferent group: CN9 and 10 that supplies pharynx. o Lateral general visceral efferent group: CN9 and 10 that supplies involuntary muscles of the respiratory tract, intestinal tract and heart. Alar plate contains sensory nuclei, which are divided into four groups: o General visceral afferent group: CN10 for viscera connections with GIT and heart. o Special visceral afferent group: CN9 for taste with tongue and epiglottis. o General somatic afferent group: CN9 connections for sensations of pain, temperature and touch from the pharynx. o Special somatic afferent group: CN9 connections with middle hear for hearing and balance.    Some neuroblasts from alar plate migrate ventrally and form neurons in olivary nuclei. The roof of myelencephalon consists of a layer of ependymal cells, covered by vascular mesenchyme and pia. The two form tele choroidea. When projected into the 4th ventricle, it is known as choroid plexus. Metencephalon  cerebellum and pons  Similarly characterised to myelencephalon in terms of alar and basal plate arrangement.  Two new components form the: o Cerebellum: a coordination centre for posture and movements. o Pons: pathway for nerve fibres between the spinal cord and the cerebral and cerebellar cortices.  Each basal plate contains three groups of motor neurons: o Medial somatic efferent group: nuclei of abducens nerve. o Special visceral efferent group: nuclei of trigeminal and facial nerves. o General visceral efferent group: axons that supply the submandibular and sublingual glands.  Marginal layer of the basal plates expands as it makes a bridge for nerve fibres connecting the spinal cord; the area being the pons.  The pons contain the pontine nuclei, which originate in the alar plates of the metencephalon and myelencephalon.  Alar plate contains three groups of sensory neurons: o Lateral somatic afferent group: neurons of the trigeminal nerve. o Special visceral afferent group. o General visceral afferent group. Cerebellum  Dorsal lateral part of alar plates bend medially to form rhombic lips.   These lips thicken at the 6th week to form the cerebellar plate. Plate separated into cranial and caudal portion by a T groove. o Caudal portion (archicerebellum) = flocculondodular lobe (most primitive)  connect with vestibular apparatus. o Cranial portion (paleocerebellum) = vermis and anterior cerebellar hemispheres  limb sensory.  Cerebellar plates consists of 3 layers; the neuroepithelial, mantle and marginal.       Cells formed by the neuroepithelium migrate to the surface of the cerebellum to form the external granular layer. o Cells retain their mitotic property and form a proliferative zone on the surface of the cerebellum. In the 3rd month, neuroepithelium proliferates from ventricular zone to form internal granular layer: o Deep cerebellar nuclei (e.g. Dentate). o Purjinke cells. o Golgi cells. In the 6th month of development, the external granular layer gives rise to a number of different cells that migrate to differentiating Purkinje cells: o Granule cells. o Basket cells. o Stellate cells. Cortex reaches size after birth. Deep nuclei reaches final position after birth. Both internal layer and external layer give rise to astrocytes, Bergman cells and oligodendrocytes.