Development of the Nervous System

PNS and Spinal Cord

(1) Formation of the Neural Tube (NT) and the Origin of Neurons with cell bodies in CNS

Nervous system originates from outermost ectoderm (Neuroectoderm) Þ neural groove Þ NT Þ neurons and glial cells

Process of development:

(a) Neuroectoderm thickens and rounds up to form the neural tube

neural tube Þ brain and spinal cord (all preganglionic autonomic fibers, and all fibers innervating skeletal muscles); central canal Þ fluid-filled ventricular system

Anencephaly – rostral portion of neural tube remains open

Spinal bifida – caudal portion of neural tube remains open

(b) Nerves grow outward from cavity in the neural tube in a laminated (layered) fashion creating 3 layers:

Ependymal layer – innermost layer, has dividing neuroepithelial cell precursors to neurons and glia

ependymal cells line the future ventricles.

Mantle layer – middle layer, has immature nerve cells (future gray matter)

Marginal layer – outermost layer, has axonal processes of developing neurons (future white matter)

(2) Origin of Neurons with cell bodies outside CNS

Neural tube gives rise to neural crest cells, which give rise to:

(a) Primary sensory neurons of the dorsal root ganglia and cranial nerve ganglia

(b) Autonomic neurons of the sympathetic ganglia and parasympathetic ganglia

(c) Schwann Cells – neurolemmal sheath cells that form myelin in PNS
(d) Leptomeninges (pia-arachnoid cells)
(e) Chromaffin cells of adrenal medulla

Caudal and Intermediate Neural Tube Develops in the Spinal Cord

Mantle Layer – gray matter (cell bodies, dendrites, proximal axons)

Marginal Layer – white matter (bundles of axons)

Organization of gray matter regions of spinal cord: nuclei that directly mediate sensory and motor functions form longitudinally oriented columns of cells.

Alar Plate (dorsal) – sensory information

Basal Plate (ventral) – motor area

Sulcus Limitans – shallow groove separating alar and basal plates.

Four Spinal Cord segments: cervical, thoracic, lumbar, sacral

Position of Spinal Cord:

3^rd month gestation – extends entire length of vertebral canal
Birth – ends at L3 vertebra
Adult – ends at L2 vertebra (so do spinal tap below L2/L3!)

Brain

(1) Introduction

neural tube Þ 3 vesicles Þ forebrain, midbrain, hindbrain (Brain stem = midbrain + hindbrain)

gray matter of brain stem is organized like that of the spinal cord (Dorsal = sensory, Ventral = motor)

hollow center Þ ventricular system

Forebrain (prosencephalon) – completely derived from alar plate

telencephalon (cerebral hemispheres, basal ganglia)

diencephalon (thalamus, hypothalamus and neural retina)

Midbrain (mesencephalon)

mesencephalon (midbrain)

Hindbrain (rhombencephalon)

metencephalon (pons and cerebellum)

meylencephalon (medulla oblongata)

(2) Development

Hindbrain (Central canal widens into Fourth ventricle)

Pons – fourth ventricle pushes alar plate neurons laterally. Alar plate becomes dorsolateral somatic and visceral sensory nuclei. Basal plate becomes ventromedial somatic and visceral motor nuclei.

EXCEPTION: pontine nuclei – formed by outpouch of alar plate!

Cerebellum – forms from a particular portion of the alar plate called the rhombic lip. Dorsolaterally located cells proliferate and eventually fuse at the midline, forming the vermis of the cerebellum.

Medulla – fourth ventricle pushes alar plate neurons laterally. Alar plate becomes dorsolateral somatic and visceral sensory nuclei. Basal plate becomes ventromedial somatic and visceral motor nuclei.

EXCEPTION: inferior olivary nucleus – motor nuclei formed by outpouch of alar plate!!!

Midbrain (Fourth ventricle narrows into the cerebral aqueduct)

Mesencephalon – Alar plate forms tectum (processing information about vision and hearing) located in dorsal region, neurons in basal plate regions are associated with motor activity. Red nucleus derived from basal plate. Fourth ventricle narrows into cerebral aqueduct.

Forebrain – past midbrain there is no basal plate, just Alar plate!!! (Central canal widens into Third ventricle)

Diencephalon – Contains 3^rd ventricle in midline. Surrounded by cerebral hemispheres. Out-pocketing of the diencephalon forms the optic cups (future retinas) and infundibulum (neural pituitary primordium).

Cerebral Cortex – Originates from the dorsal mantle layer of the rostral forebrain. As the cerebral hemispheres expand they overgrow and fuse with the lateral wall of the diencephalon. Axons projecting to and from the cerebral cortex (internal capsule) course through this region. The cerebral cortex expands first rostrally to form the frontal lobes, then posteriorly to form the occipital lobe, and finally inferiorly to form the temporal lobe. As a result of the expansion, structures of the cerebral hemispheres take on a C-shaped configuration (including hippocampus, caudate nucleus, and lateral ventricle). The surfaces of the hemispheres grow rapidly and develop gyri and sulci; they are interconnected by the corpus callosum.

structure of cerebral hemispheres (cerebral cortex, basal ganglia, hippocampus, and amygdala) is markedly transformed during development due to proliferation of cells in the cortex. Cerebral hemispheres arise as out-pockets of the most rostral part of the forebrain that expand in the anterior, dorsal and inferior directions. The diencephalon, midbrain, and rostral hindbrain are eventually covered over by the hemispheres like a fist (diencephalon and midbrain) pushing into a large balloon (cerebral hemispheres)

basal ganglia (striatum Þ putamen, caudate nucleus) are groups of neuronal cell bodies that develop in the basal mantle layer of the forebrain and are important for smooth motor coordination

Ventricular System

Central canal (center of neural tube) Þ cavities of brain vesicles Þ ventricular system of CNS

(1) Cavity in Forebrain Þ two lateral ventricles and the Third Ventricle

Interventricular foramen of Monro interconnects all three ventricles

(2) Cavity in Midbrain Þ narrows to become the cerebral aqueduct of Sylvius; this forms a conduit for flow of CSF to Fourth Ventricle (located dorsal to the pons and medulla)

in the fetus CSF can flow down the open central canal bathing the spinal cord from the inside
in most adults the central canal is closed thus preventing CSF from entering the spinal cord.

Hydrocephalus – obstruction of the cerebral aqueduct during development causes the CSF to compress the cerebral hemispheres, enlarging the cranium