Embryology of nervous system

1. EMBRYOLOGY OF NERVOUS SYSTEM Dr NAVEEN THOTA SVIMS

2. I ntroduction Formation of neurons and neuroglial cells Neural tube and its sub divisions Spinal cord Medulla oblongata Pons Midbrain Cerebral hemisphere Corpus striatum Cerebral cortex Autonomic nervous system

3. Formation of neurons and neuroglial cells Neural tube first lined by single layer of cells Proliferate to form several layers Matrix layer Mantle layer Marginal layer

4. Contd.. Stages in the formation of a nerve cell are: Apolar neuroblast Bipolar neuroblast Unipolar neuroblast Multipolar neuroblast Axon and dendrites

5. Neuroglial cells are also formed from germinal cells of the ependymal layer Glioblasts migrate in to mantle and marginal layer as medulloblasts They differentiate either into astroblasts or oligodendroblasts Microglial cells are mesodermal in origin

6. Myelination of fibers Nerve fibers which remain with in the brain and spinal cord receives support from and are ensheathed by neuroglial cells Peripheral nerves special sheath called the neurolemma derived from schwann cells Myelin of the CNS derived from oligodendrocytes

7. Neural tube and its subdivisions Whole of the nervous system is derived from ectoderm except blood vessels and neuroglial elements Ectoderm situated on the dorsal aspect of embryonic disc forms the neural plate Neural groove Neural tube Enlarges cranial part and caudal tubular part

8. Contd.. The cavity of the brain shows three dilatations Prosencephalon , mesencephalon , rhombencephalon Prosencephalon Diencephalon Telencephalon Rhombencephalon Metencephalon Myelencephalon

9. Contd.. The relative position is altered by number of flexures Cervical flexure Mesencephalic flexure Pontine flexure Telencephalic flexure

10. Neural crest Cells between the neural plate and the rest of the ectoderm form primordia of the neural crest DRG Sensory ganglia of 5, 7, 9, 10 Neurons and satellite cells of sympathetic ganglia Parasymathetic ganglia Schwann cells

11. Contd.. Specific cells adrenal medulla Chromaffin tissue melanoblasts

12. Spinal cord Devolops from caudal cylindrical part of neural tube Cavity of the tube bounded by thick lateral wall, thin roof and floor Tube subdivides in to three layers matrix layer, mantle layer and marginal layer Ventral layer of the mantle layer grows faster than dorsal layer Line seperating the compressed ventral part from the dorsal part is called sulcus limtans

13. Contd.. Dorsal or alar lamina Ventral or basal lamina Posteriomedeian fissure Anteriomedian fissure Nerve cells that devolop in mantle zone of the basal lamina becomes the neuron of anterior grey column and from the alar lamina the neuron of the posterior grey column

14. Contd.. Dorsal nerve root ganglia are formed by axons of the cells that develop from neural crest Axons from the post grey column enter the marginal layer to form the ascending tract Descending tracts are formed by the axons projecting from the brain Grey columns divide the white matter in to anterior, posterior and lateral columns

15. POSITIONAL CHANGES OF THE CORD In the third month of development the spinal cord extends the entire length of the embryo, and spinal nerves pass through the intervertebral foramina at their level of origin . With increasing age, the vertebral column and dura lengthen more rapidly than the neural tube, and the terminal end of the spinal cord gradually shifts to a higher level. At birth, this end is at the level of the third lumbar vertebra . As a result of this disproportionate growth, spinal nerves run obliquely from their segment of origin in the spinal cord to the corresponding level of the vertebral column. The dura remains attached to the vertebral column at the coccygeal level . In the adult, the spinal cord terminates at the level of L2 to L3, The dural sac and subarachnoid space extend to S2. Below L2 to L3, a threadlike extension of the pia mater forms the filum terminale , which is attached to the periosteum of the first coccygeal vertebra and which marks the tract of regression of the spinal cord. Nerve fibers below the terminal end of the cord collectively constitute the cauda equina .

16. Medulla oblongata Myelencephalon Early devolopment simillar to spinal cord sulcus limitans divides in to alar and basal lamina Roof plate becomes greatly widened resuting which alar plate comes dorsolateral to basal plate Alar plate forms olivary nuclei and cranial nerve nuclei

17. Contd..

18. Pons Ventral part of metencephalon Contribution alar lamina of the myelencephalon and gives rise to pontine nuclei axons arising from them forms MCP Lateral part of alar lamina becomes rhombic lips to form the cerebellum Nuclei arising from basal and alar plate lie in dorsal or tegmental part of pons

19. Contd.. Ventral part of pons constitutes: Middle cerebellar peduncle Corticospinal , corticobulbar and corticopontine fibers

20. Mid brain Nuclei of the basal lamina Occulomotor nuclei Trochlear nuclei Edinger Westphal nuclei Alar lamina gives rise to Colliculi Red nucleus Substantia nigra

21. Contd.. Marginal layer ventral part of mesencephalon invaded by down growing fibers of CST, corticobulbar and cortico pontine pathway and forms crus cerebri

22. Cerebellum Devolops from dorsolateral part of alar lamina of the metencephalon Rhombic lips on either side grows and fuses medially to form cerebellum Consists of usual matrix, mantle and marginal layer Cells of the mantle layer migrating in to marginal layer forms cortex and those don’t forms dentate, emboliform , fastigial and globose nuclei

23. Contd.. Superior cerebellar peduncle Middle cerebellar peduncle Inferior cerebellar peduncle

24. Cerebral hemispheres Median diencephalon and lateral telencephalon Telancephalon Cerebral cortex and corpus striatum Diencephalon Epithalamus , thalamus and hypothalamus Telencephalic vesicles are small initially rapidly grows upwards, forwards and backwards completely covers the diencephalon

25. Contd.. Cavity of diencephalon form third ventricle and of telencephalic vesicles form lateral ventricles Each lateral is at first a small spherical space Forward and backward growth elongates anterioposteriorly Grows downwards and forwards to form the temporal lobe and horn Backward growth to form occipital pole and posterior horn

26. Thalamus and hypothalamus Devolops from diencephalon Lateral wall of diencephalon becomes thickened Divided by epithalamic and hypothalamic sulci Epithalamus represented by habenular and pineal body

27. Corpus striatum Derivative of telencephalon Telencephalic vesicle can be subdivided in to basal part which is thick and superior part is thin Some cells migrate in to marginal layer and forms cortex and remaining cells form copus striatum Corpus striatum subdivided into medial and lateral divisions

28. Contd.. Fibers descending from cerebral cortex passes through medial and lateral parts constitutes internal capsule Medial or deeper part form caudate nucleus and superficial or lateral part forms lentiform nucleus

29. Cerebral cortex Formed by migration of cells from the mantle layer in to overlying marginal layer Region of insula relatively slow in growth and gradually overgrown by adjacent area forms the opercula Cortex divided in to Hippocampal Pyriform neocortex

30. Contd.. Pyriform cortex that recives olfactory sensation constitutes uncus , parahippocampal gyrus and anterior perforated substance Telencephalon has a medial wall opposing each other gives rise to hippocampal cortex Pyriform cortex arises from marginal layer superficial to corpus striatum With formation of inferior horn hippocampus follows the curve and assumes aring shaped configuration

31. Contd.. Superior part of the hippocampus becomes ridimentary and forms indesium gresium White matter of the cerebral cortex is formed by projection, association, commisural and ascending fibers

32. Cerebral commisures Part of the neural tube that closes the cranial end of prosencephalon is lamina terminalis Anterior commisure Hippocampal commisure Corpus collosum Optic chiasma , habenular commisure , posterior commisure .

33. Autonomic nervous system Sympathetic nervous system Parasympathetic nervous system

34. Sympathetic nervous system Preganglionic neurons devolop from mantle layer of thoracolumbar region of the spinal cord Located in lateral horn of the cord After short course in the spinal nervethey leave them and grows towards the postganglionic neurons

35. Contd.. Postganglionic neurons are of two types Sympathetic trunk Visceral ganglia

36. Parasympathetic nervous system Cranial parasympathetic out flow Derived GVE nuclear column of the brain stem Edinger westphal nucleus Salivatory nucleus Lacrimatory nucleus Dorsal nucleus of vagus Sacral parasympathetic out flow Preganglionic cells are formed in the mantle layer of the spinal cord near the sulcus limitans S2-S4

37. Contd.. Postganglionic neurons lies near the viscera

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