The central nervous system develops from the neural plate and neural tube. The neural tube forms three primary brain vesicles - the prosencephalon, mesencephalon, and rhombencephalon. The brain vesicles differentiate into the structures of the forebrain, midbrain, and hindbrain. The spinal cord develops from the caudal end of the neural tube. Neuroblasts form the gray matter while myelination leads to the white matter. Spinal nerves develop from ventral and dorsal nerve roots. The CNS continues developing and structures such as the cerebellum, cerebral hemispheres, and ventricles emerge.

1. CNS System
Themba Hospital FCOG(SA) Part 1 Tutorials
By Dr N.E Manana

2. CNS
• The central nervous system (CNS) appears at the beginning of the
third week as a slipper shaped plate of thickened ectoderm, the
neural plate, in front of the primitive node
• Its lateral edges soon elevate to form the neural folds
• The neural folds continue to elevate, approach each other in the
midline, and fuse, forming the neural tube
• Fusion begins in the cervical region and proceeds in cephalic and
caudal directions (neuropores)

4. CNS
• The cephalic end of the neural tube shows three dilations, the
primary brain vesicles:
(1) The prosencephalon, or forebrain
(2) The mesencephalon, or midbrain
(3) The rhombencephalon, or hindbrain
• Simultaneously, it forms two flexures: (1) the cervical flexure at the
junction of the hindbrain and the spinal cord and (2) the cephalic
flexure in the midbrain region

5. CNS
• When the embryo is 5 weeks old, the prosencephalon consists of two
parts:
(1) The telencephalon, the primitive cerebral hemispheres;
(2) The diencephalon, characterized by outgrowth of the optic vesicles
• Rhombencephalic isthmus, separates the mesencephalon from the
rhombencephalon
• The rhombencephalon also consists of two parts:
• (1) The metencephalon (the pons and cerebellum)
• (2) The myelencephalon
• The boundary between these two portions is marked by the pontine
flexure
• The lumen of the spinal cord, is continuous with that of the brain vesicles,
forming ventricles

6. • 18.5

7. Spinal cord
• The wall of a recently closed neural tube consists of neuroepithelial cells.
• Once the neural tube closes, neuroepithelial cells begin to give rise to
another cell , primitive nerve cells, or neuroblasts
• They form the mantle layer, a zone around the neuroepithelial layer
• The mantle layer later forms the gray matter of the spinal cord.
• Myelination of nerve fibers in the outermost layer takes on a white
appearance and therefore is called the white matter of the spinal cord

9. Spinal cord
• As a result of continuous addition of neuroblasts to the mantle layer, each
side of the neural tube shows a ventral and a dorsal thickening
• The ventral thickenings, the basal plates, which contain ventral motor horn
cells, the dorsal thickenings, the alar plates, form the sensory areas
• A longitudinal groove, the sulcus limitans, marks the boundary between
the two
• Roof and floor plates, do not contain neuroblasts; they serve primarily as
pathways for nerve fibers crossing from one side to the other
• In addition to ventral motor horn and the dorsal sensory horn, a group of
neurons accumulates between the two areas and forms a small
intermediate horn (sympathetic portion of the autonomic nervous system)

10. Neuroblasts

13. Neural Crest Cells
• During elevation of the neural plate, a group of cells appears along each
edge (the crest) of the neural folds
• Crest cells migrate laterally and give rise to sensory ganglia (dorsal root
ganglia) of the spinal nerves and other cell types
• During further development, neuroblasts of the sensory ganglia form two
processes
• In the spinal cord, they either end in the dorsal horn or ascend through the
marginal layer to one of the higher brain centers
• The peripherally growing processes join fibers of the ventral motor roots
and thus participate in formation of the trunk of the spinal nerve.

15. Spinal Nerves
• Motor nerve fibers begin to appear in the fourth week, arising from
nerve cells in the basal plates (ventral horns) of the spinal cord
(ventral nerve root)
• Dorsal nerve roots form as collections of fibers originating from cells
in dorsal root ganglia (spinal ganglia)
• Central processes from these ganglia form bundles that grow into the
spinal cord opposite the dorsal horns.
• Distal processes join the ventral nerve roots to form a spinal nerve
• Almost immediately, spinal nerves divide into dorsal and ventral
primary rami.

16. Myelination
• Schwann cells myelinate the peripheral nerves with each cell
myelinating only a single axon
• These cells originate from neural crest, migrate peripherally, and wrap
themselves around axons, forming the neurilemma sheath
• Beginning at the fourth month of fetal life, many nerve fibers take on
a whitish appearance as a result of deposition of myelin, which is
formed by repeated coiling of the Schwann cell membrane around
the axon
• The myelin sheath surrounding nerve fibers in the spinal cord has a
completely different origin, the oligodendroglial cells

17. Positional Changes of the Cord

18. Rhombencephalon: Hindbrain
• Consists of the myelencephalon, the most caudal of the brain vesicles, and
the metencephalon which extends from the pontine flexure to the
rhombencephalic isthmus
• The myelencephalon is a brain vesicle that gives rise to the medulla
oblongata, It differs from the spinal cord in that its lateral walls are everted
• The basal plate, similar to that of the spinal cord, contains motor nuclei.
These nuclei are divided into three groups: (1) medial somatic efferent (2)
intermediate special visceral efferent (3) lateral general visceral efferent
• In the metencephalon and the mesencephalon, the column contains
neurons of the abducens (VI) , trochlear (IV), and oculomotor(III) nerves,
respectively.

19. Rhombencephalon: Hindbrain
• In the myelencephalon, the column is represented by neurons of the
accessory (XI), vagus (X), and glossopharyngeal (IX) nerves
• The general visceral efferent group contains motor neurons that
supply involuntary musculature of the respiratory tract, intestinal
tract, and heart
• The alar plate contains three groups of sensory relay nuclei, the
somatic afferent (general sensory) , the intermediate, or special
afferent, group receives impulses from taste buds and The medial, or
general visceral afferent, group receives interoceptive information
from the gastrointestinal tract and heart

22. Metencephalon
• The metencephalon, similar to the myelencephalon, is characterized
by basal and alar plates
• Two new components form (1) the cerebellum, and (2) the pons
• Each basal plate of the metencephalon contains three groups of
motor neurons: (1) the medial somatic efferent (2) the special visceral
efferent (3) the general visceral efferent group
• The alar plates of the metencephalon contain three groups of sensory
nuclei: (1) a lateral somatic afferent , which contains neurons of the
trigeminal nerve; (2) the special afferent (3) the general visceral
afferent group

23. Cerebellum
• The dorsolateral parts of the alar plates bend medially and form the
rhombic lips
• As a result of a further deepening of the pontine flexure, the rhombic lips
compress cephalocaudally and form the cerebellar plate
• In a 12-week embryo, this plate shows a small midline portion, the vermis,
and two lateral portions, the hemispheres
• A transverse fissure soon separates the nodule from the vermis and the
lateral flocculus from the hemispheres
• During further development, a number of cells formed by the
neuroepithelium migrate to the surface of the cerebellum to form the
external granular layer
• In the sixth month of development, the external granular layer gives rise to
various cell types.

26. • 18.21

27. Mesencephalon: Midbrain
• Each basal plate contains two groups of motor nuclei: (1) a medial
somatic efferent , represented by the oculomotor and trochlear
nerves (2) a small general visceral efferent group,
• The marginal layer of each basal plate enlarges and forms the crus
cerebri.
• These crura serve as pathways for nerve fibers descending from the
cerebral cortex to lower centers in the pons and spinal cord.
• With further development, a transverse groove divides each elevation
into an anterior (superior) and a posterior (inferior) colliculus

28. • 18.23

29. Prosencephalon: Forebrain
• The prosencephalon consists of the telencephalon, which forms the
cerebral hemispheres and the diencephalon, which forms the optic
cup and stalk, pituitary, thalamus, hypothalamus, and epiphysis
• The diencephalon, which develops from the median portion of the
prosencephalon
• The roof plate of the diencephalon consists of a single layer of
ependymal cells covered by vascular mesenchyme. Together, these
layers give rise to the choroid plexus of the third ventricle
• The most caudal part of the roof plate develops into the pineal body,
or epiphysis.

30. Prosencephalon: Forebrain
• The alar plates form the lateral walls of the diencephalon.
• A groove, the hypothalamic sulcus, divides the plate into a dorsal and
a ventral region, the thalamus and hypothalamus, respectively
• The hypothalamus, forming the lower portion of the alar plate
• Differentiates into a number of nuclear areas that regulate the
visceral functions, including sleep, digestion, body temperature, and
emotional behaviour.

31. • 18.24

32. Hypophysis or Pituitary Gland
• The hypophysis, or pituitary gland, develops from two completely
different parts:
(1) an ectodermal out pocketing of the stomodeum ,Rathke’s pouch
and
(2) Infundibulum
• By the end of the 2nd month the Rathke’s pouch loses its connection
with the oral cavity and is then in close contact with the infundibulum
• Cells of the anterior wall of the Rathke’s pouch proliferate and form
the Adenohypophysis
• The infundibulum gives rise to the stalk and neurohypophysis

33. • 18.26

34. Telencephalon
• Consist of two lateral outpocketings, the cerebral hemispheres and
the lamina terminals
• The cavities of the hemispheres the lateral ventricles, communicate
with the diencephalon via the foramina of Monro
• The hemispheres arise at the beginning of the fifth week
• The corpus striatum starts to appear
• The Hemisphere wall consist of a single layer of ependymal cells
covered by vascular mesenchyme, and the form the choroid plexus
• In the wall of the choroid fissure the hemisphere forms the
hippocampus

35. Telencephalon
• With further expansion, the hemispheres cover the lateral aspect of the
diencephalon, mesencephalon, and cephalic portion of the metencephalon
• The corpus striatum likewise expands posteriorly and is divided into two
parts: (1) a dorsomedial portion, the caudate nucleus, and (2) a
ventrolateral portion, the lentiform nucleus
• At the same time, the medial wall of the hemisphere and the lateral wall of
the diencephalon fuse, and the caudate nucleus and thalamus come into
close contact
• Continuous growth of the cerebral hemispheres in anterior, dorsal, and
inferior directions results in the formation of frontal, temporal, and
occipital lobes, respectively

36. • 18.27

37. Cortex Development
• The cerebral cortex develops from the pallium , which has two
regions: (1) the paleopallium, (2) the neopallium
• Neuroblasts migrate to a subpial position and then differentiate into
fully mature neurons.
• At birth, the cortex has a stratified appearance due to differentiation
of the cells in layers
• The motor cortex contains a large number of pyramidal cells, and the
sensory areas are characterized by granular cells.
• Neural crest cells and ectoderm of the frontonasal prominence to
form the olfactory placodes

38. • 18.29

39. Commissures
• In the adult, a number of fiber bundles, connect the right and left
halves of the hemispheres.
• The most important fiber bundles make use of the lamina terminalis
• The first of the crossing bundles to appear is the anterior commissure
• The second commissure to appear is the hippocampal commissure, or
fornix commissure.
• The most important commissure is the corpus callosum. It appears by
the 10th week of development

40. • 18.30

41. Cerebrospinal Fluid
• Secreted by the choroid plexuses in the brain ventricles
• These plexuses are modifications of the ependymal layer and produce
approximately 400 to 500 mL of CSF per day
• The fluid circulates through the brain ventricles leaving the lateral
ventricles
• Some CSF enters the spinal canal and some exits the fourth ventricle
through its median and lateral apertures to enter the subarachnoid
space that surrounds the CNS
• CSF is absorbed into the venous system from the subarachnoid space
through arachnoid granulations

42. Cranial nerves
• By the 4th week the nuclei for all the 12 cranial nerves are present
• All except the olfactory and the optic nerves arise from the brain stem, and
of these ,only the oculomotor arises outside the region of the hindbrain
• Neuroepithelium proliferation centers in the hindbrain establish 8
segments the rhombomeres, which give rise to IV, V, VI, VII, IX, XI and XII
• Motor neurons for cranial nuclei are within the brainstem, while sensory
ganglia are outsideof the brain
• Not all cranial nerves contain both motor and sensory fibers
• Sensory ganglia originate from the ectodermal placodes and neural crest
cells

43. • 18.40

44. • Table 18.2

47. Autonomic nervous system
• ANS can be divided into two parts: a sympathetic portion in the
thoracolumbar region and a parasympathetic portion in the cranial and
sacral regions
• In the fifth week, cells originating in the neural crest of the thoracic region
migrate on each side of the spinal cord toward the region immediately
behind the dorsal aorta
• They form a bilateral chain of segmentally arranged sympathetic ganglia
• Some sympathetic neuroblasts migrate in front of the aorta to form
preaortic ganglia, such as the celiac and mesenteric ganglia
• Sympathetic organ plexuses

49. • 18.42

50. Suprarenal Gland
• Develops from two components: (1) a mesodermal portion, which forms
the cortex, and (2) an ectodermal portion, which forms the medulla
• During the fifth week of development, mesothelial cells between the root
of the mesentery and the developing gonad begin to proliferate and
penetrate the underlying mesenchyme
• Here, they differentiate into large acidophilic organs, which form the
primitive cortex
• Cells, smaller than those of the first wave, later form the definitive cortex
• Neural crest cells invade its medial aspect, where they are arranged in
cords and clusters

51. • 18.43

52. Parasympathetic Nervous System
• Neurons in the brainstem and the sacral region (S2–S4) of the spinal
cord give rise to preganglionic parasympathetic fibers
• Fibers from nuclei in the brainstem travel via the: the oculomotor (III),
Facial (VII), Glossopharyngeal (IX),and vagus (X) nerves.
• Postganglionic fibers arise from neurons (ganglia) derived from neural
crest cells and pass to the structures they innervate

53. Thank you