DEVELOPMENT OF THE NERVOUS SYSTEM

1. DEVELOPMENT OF THE NERVOUS SYSTEM Prof. Saeed makarem

2. EARLY DEVELOPMENT By the beginning of the second week, three germ cell layers become established: Ectoderm, Mesoderm and Endoderm. Prof. Saeed makarem

3. Each germ layer gives rise to particular tissues and organs in the adult. Prof. Saeed makarem

4. The ectoderm gives rise to: 1- Skin 2- Nervous system. Prof. Saeed makarem

5. The mesoderm forms: 1-Skeleton, 2-Muscular tissues 3-Connective tissues. Prof. Saeed makarem

6. The endoderm gives rise: to the lining of: 1-The alimentary, 2- The respiratory 3-The genitourinary tracts. Prof. Saeed makarem

7. During the 3rd week, the dorsal midline ectoderm undergoes thickening to form the neural plate. • The lateral margins of the neural plate become elevated, forming neural folds. • In between the two neural folds a longitudinal, midline depression, will form the neural groove. • The neural folds then become apposed and fuse together. • Now the neural groove is transformed into the neural tube. Prof. Saeed makarem

8. Prof. Saeed makarem

9. Some cells from the apices of the neural folds become detached to form groups of cells lying dorsolateral to the neural tube. • These are known as the neural crests. • By the middle of the fourth week the formation of the neural tube is completed. Prof. Saeed makarem

10. DEVELOPMENT OF THE BRAIN & SPINAL CORD • Enormous growth, distortion and cellular differentiation occur in the neural tube. • This is maximal in the rostral part,which develops into the brain, while the caudal part willformsthe spinal cord. Prof. Saeed makarem

11. The central cavity within the neural tube becomes: The ventriclesof the brain & The central canal of the spinal cord. Prof. Saeed makarem

12. The neural crests cells form the sensory gangliaof the spinal & cranial nerves, and also the autonomic ganglia. Prof. Saeed makarem

13. As development continues, a longitudinal groove, called the sulcus limitans,appears on the inner surface of the lateral walls of the embryonic spinal cord and caudal part of the brain. • The dorsal and ventral cell groupings thus forming the alar plateand the basal plate,respectively. Prof. Saeed makarem

14. Nerve cells that develop within the alar plate have predominantly sensory functions, while those in the basal plate are predominantly motor. Prof. Saeed makarem

15. Further development also brings about the differentiation of grey and white matter. • The grey matteris located centrally around the central canal, • While thewhite matterforming an outer coat. Prof. Saeed makarem

16. This basic developmental pattern can still easily be recognized in the adult spinal cord. Prof. Saeed makarem

17. DEVELOPMENT OF THE BRAIN • During development, the rostral portion of the neural tube undergoes massive differentiation and growth to form the brain. By the 5th week, three primary brain vesicles can be identified: • Prosencephalon or (forebrain), • Mesencephalon or (midbrain), • Rhombencephalon or (hindbrain). Prof. Saeed makarem

18. The longitudinal axis of the developing CNS (neuraxis) does not remain straight but is bent by : 1- Cephalic, orMidbrain flexure occurring at the junction of the forebrain and midbrain 2-Cervical flexurebetween the brain and the spinal cord. Prof. Saeed makarem

19. By the 7th week further differentiation distinguishes five secondary brain vesicles produced by: • Division of the prosencephalon into 2 lateral vescicle the telencephalonand a median part thediencephalon. • Also, division of the rhombencephalon into the metencephalonandmyelencephalon. Prof. Saeed makarem

20. Prof. Saeed makarem

21. The junction between the metencephalon and the myelencephalon is marked by an additional bend in the neuraxis, called the pontine flexure. Prof. Saeed makarem

22. Prof. Saeed makarem

23. FURTHER DEVELOPMENT OF THE BRAIN • Of the three basic divisions of the brain, the prosencephalon or forebrain is by far the largest. • It is also referred to as the cerebrum. Prof. Saeed makarem

24. The telencephalonundergoes the greatest further development and gives rise to the two cerebral hemispheres. Prof. Saeed makarem

25. The cerebral hemispheres consist of: • Outer layer of grey matter(the cerebral cortex) • Inner mass of white matter, within which various groups of grey matter nuclei are buried Prof. Saeed makarem

26. The mesencephalon, or midbrain, is relatively undifferentiated, (it still retains a central tube-like cavity surrounded by grey matter). Prof. Saeed makarem

27. The metencephalon develops into thePons anteriorly& cerebellum posteriorly. Prof. Saeed makarem

28. The myelencephalon forms the medulla oblongata (medulla). Prof. Saeed makarem

29. As the brain develops, its central cavity also undergoes considerable changes in size and shape forming a system of ventricles, which contain cerebrospinal fluid (CSF). Prof. Saeed makarem

30. DEVELOPMENTAL ANOMALIES Disorders of development disrupt the normal growth and structural organization of the spinal cord and brain. Because the nervous system is derived from embryonic ectoderm, these developmental anomalies also involve the coverings of the nervous system (skin and bone). Prof. Saeed makarem

31. ANENCEPHALY In anencephaly, the brain and skull are minute and the infant does not usually survive. Prof. Saeed makarem

32. SPINA BIFIDA • In spina bifida, the lower part of the spinal cord & nerve roots are underdeveloped and may lie uncovered by skin or the bony spine on the infant's back Prof. Saeed makarem

33. MENINGIOMYELOCELE • It is Spina Bifida with • the meninges of the spinal cord appear on the back of the infant. • Such infants are left with withered, paralysed and anaesthetic lower limbs together with incontinence of the bowel and bladder.