Origin of the nervous system

2. Origin of the nervous system • The nervous system develops from the neural plate. • The neural plate is a thickened area of the embryonic ectoderm • The neural plate then differentiates to form the neural tube, neural folds and neural crest. • The neural tube differentiate into the CNS (brain + spinal cord) • The neural crest will give rise to cells that form the peripheral and autonomic nervous system

3. Origin of the nervous system 2 • Neurulation is the formation of the neural tube • Begins in the region of the 4th-6th somites • The cranial 2/3 of the neural plate  the future brain • The caudal 1/3 of the neural plate  spinal cord • Neural canal “the lumen of the neural tube comunicates with the amniotic fluid • the walls of the neural tube thickens to form the brain and the spinal cord • The neural canal is converted into the ventricular system of the brain and the central canal of the spinal cord

4. P 426 “Before we are born”

5. Development of the spinal cord • The lateral walls of the neural tube thickens gradually  reduce the size of the neural canal  central canal of the spinal cord is formed • The wall of the neural tube is composed of the neuroepithelium  constitute the ventricular zone  gives rise to all neurons and microglial cells in the spinal cord.

6. Pg 429 “Before we are born”

7. Development of the spinal cord • Proliferation and differentiation of the neuroeithelial cells produce thick walls and thin roof and floor plates • Differential thickening of the lateral walls of the spinal cord  shallow groove on each side  sulcus limitans • This groove separate the alar plate “ the dorsal side” from the basal plate “ the ventral part” • Cell bodies in the alar plates forms the dorsal gray columns • Cell bodies in the basal plate form the ventral and lateral gray horns

8. Pg 428 “Before we are born”

9. Development of spinal ganglia • The axons of cells in the spinal ganglia are at first biploar  the 2 processes unite in a T-shape fashion unipolar • The unipolar neurons in the spinal ganglia “dorsal root ganglia” are derived from neural crest cells • The peripheral processes of spinal ganglion cells pass in the spinal cord nerves to sensory endings in somatic or visceral structures • The central processes enter the spinal cord  dorsal roots of spinal nerves

10. Positional changes of spinal cord • In the embryo: the spinal cord extends the entire length of the vertebral canal. • Because the vertebral column and dura mater grow more rapidly than the spinal cord  the caudal end of the spinal cord gradually comes to lie at relatively higher levels. • The spinal cord in a newborn terminates at L2 or L3 • The spinal cord in the adult terminates at the inferior border of L1.

11. Myelination of nerve fibers • The myelin sheaths surrounding nerve fibers within the spinal cord are formed by oligodendrocytes. • The myelin sheaths around the axons of peripheral nerve fibers are formed by the plasma membranes of neurolemmal (Schwann) cells.

12. Congenital anomalies of spinal cord

13. Development of brain • Fusion of the neural folds in the cranial region forms 3 primary brain vesicles • Forebrain “prosencephalon” • Midbrain “mesencephalon” • Hindbrain “rhombencephalon” • During development, • the forebrain divides into  telencephalon and diencephalon • the hindbrain divides into  metencephalon and myelencephalon • The midbrain does not divide • Consequently, there are 5 secondary brain vesicles

14. Pg 438 “Before we are born”

15. Brain flexures • During development, the embryonic brain grows rapidly and bends ventrally with the head fold  this produces the • Midbrain flexure in the midbrain • Cervical flexure at the junction of the hindbrain and spinal cord • Later, unequal growth of the brain between these flexures produces the pontine flexure. • This flexure results in thinning of the roof of the hindbrain

16. Hindbrain • The cervical flexure demarcates the hindbrain from the spinal cord • Later this junction will be defined as the level of the superior rootlet of the cervical nerve. • The pontine flexure divides the hindbrain into • Myelencephalon “caudal”  develops into medulla oblongata • Metencephalon “rostral-toward the front”  develops into the pons and cerebellum. • The cavity of the hindbrain becomes the 4th ventricle and the central canal in the caudal part of the medulla

17. Myelencephalon1 • Neuroblasts from the alar plates in the mylencephalon migrate into the marginal zone and form isolated areas of the grey matter  gracile nuclei“medial” and cuneate nuclei“laterally • The ventral area of the medulla contains the pyramids ( pair of fiber bundles) • During development, as the walls of the medulla move laterally, the alar plates lie lateral to the basal plates  motor nuclei medial to sensory nuclei

18. Myelencephalon2 • Neuroblasts from the basal plate form nuclei: • General somatic efferent: neurons of hypoglossal nerve • Special visceral efferent: neurons innervating muscles derived from pharyngeal arches • General visceral efferent: neurons of the vagus and glossopharyngeal nerves • Neuroblasts of the alar plate form nuclei: • General visceral efferent: receive impulses from viscera • Special visceral afferent: receive taste fibers • General somatic afferent: receive impulses from the surface of the head • Special somatic afferent: receiving impulses from ear • Olivary nuceli

19. cerebellum Pg 439 “Before we are born”

20. Metencephalon • The cerebellum develops from thickenings of dorsal parts of the alar plates • Neuroblasts in the intermediate zoon of the alar plates migrate and differentiate into the neurons of the cerebellar cortex • Cells from the alar plate give rise to the dentate nucleus “largest nucleus”, pontine nuclei, the cochlear & vestibular nuclei and the sensory nuclei of the trigeminal nerve. • Bands of nerve fibers cross the median plane and from a bulky ridge  pons

21. Pg 440 “Before we are born”

22. Midbrain • The neural canal that passes through the midbrain narrows  cerebral aqueduct “ a canal that connect the 3rd and the 4th ventricles” • Neuroblasts migrate from the alar plates of the midbrain into the tectum “roof” and aggregate to form  superior and inferior collicluli  concerned with the visual and auditory reflexes • Neuroblasts from the basal plates give rise to  neurons in the tegmentum"red nuclei, 3rd and 4th cranial nerve nuclei, and the reticular nuclei” and the substantia nigra • Cerebral peduncles “fibers from the cerebrum” pass through the midbrain  brain stem  spinal cord

23. Pg 441 “Before we are born”

24. Forebrain • Optic vesicles are 2 lateral outgrowths that appear on each side of the forbrain  primordia of the retinae and optic nerves • Cerebral vesicles is the second pair of outhgrowths  primordia of cerebral hemispheres and lateral ventricles • Telencephalon“anterior part of the forebrain” and diencephalon“posterior part of the midbrain” contribute to the formation of 3rd ventricle.

25. Diencephalon • Swellings develop in the lateral wall of the 3rd ventricle  epithalamus, thalamus and hypothalamus. • The thalamus develops rapidly and bulges into the cavity of the 3rd ventricle • The hypothalamus arises by proliferation of neuroblasts in the intermediate zone of the diencephalon • The epithalamus develops form the roof and dorsal portion of the lateral walls of the diencephalon

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27. Telencephalon • The telencephalon consist of • Cerebral vesicles:”2 lateral diverticula” primordia of the cerebral hemispheres • The median portion of telencephalon forms the anterior part of3rd ventricle • At first, the cerebral vesicles are in communication with cavity of 3rd ventricle through the interventricular foramina as the cerebral hemispheres expand, they meet in the midline. • The mesenchyme trapped between them gives rise to the falx cerebri “fold of dura mater” • Later, corpus striatum develops as a swelling in the floor of each cerebral hemisphere

28. Congenital anomalies of the brain AnencephalyEncephalocele