The nervous system begins developing in the 3rd week of gestation from ectodermal tissue. The central nervous system starts as a neural plate that folds into a neural groove and then fuses to form the neural tube. The neural tube will develop into the brain and spinal cord. Neural crest cells emerge from the neural tube and migrate throughout the body, giving rise to many cell types. By the 5th week, the brain has divided into five vesicles that will form the structures of the adult brain.

2. Nervous system
The nervous system consists of three
parts:
 CNS includes the brain and spinal cord.
 PNS includes nerve cells outside the CNS and
cranial and spinal nerves that connect the brain
and spinal cord with peripheral structures.
 ANS has parts in both CNS and PNS. It consists
of neurons that innervate smooth muscle,
cardiac muscle, or glandular epithelium, or
combinations of these tissues.

3. DEVELOPMENT OF
NERVOUS SYSTEM
• Nervous system is ectodermal in origin
• Nervous system starts developing in the
3rd week of intrauterine life

4. Central nervous system
• CNS starts developing in the 3rd week of
intrauterine life
• CNS appears above the notochord

5. 17-day embryo

6. Formation of the neural tube
 Formation of the neural tube begins in
the region of fourth to sixth pairs of
somites
• It starts developing above notochord.
• It start developing during 3rd week .
• It develops from ectoderm.

7. Formation of neural plate
• The development of the neural plate
begins at the cephalic end and gradually
extends toward primitive streak.
• Neural plate is located in the mid-dorsal
region in front of the primitive pit.
• The cells of the neural plate make up
neuro-ectoderm.
• It is also called neurectoderm.

8. Formation of neural plate
• Under the inductive influence of
notochord and paraxial mesoderm the
ectoderm overlying the notochord
thickens to form the neural plate.

9. Formation of neural groove
Edges of the neural plate become raised and
more elevated to form the neural folds (day-20).
The formation of the paraxial mesoderm helps in
the elevation of the edges of the neural plate.
The central mid-region of neural plate above the
notochord remains in situ. The notochord has
some binding force with this central mid-region.
Thus the neural plate is converted into neural
groove.

10. Formation of neural groove

11. Formation of neural trench
As the paraxial mesoderm thickens, the neural
groove deepens down between the right and
left paraxial mesoderms and converted into
neural trench. This is day-21.

12. Formation of neural tube
 Gradually the neural folds approach each other
and fuse in the midline.
 This fusion begins in the future cervical region
at the level of 4th pair of somites.

13. Parts of neural tube
At this stage,
the cranial two thirds of the developing
neural tube, as far as 4th pair of
somites, represent the future brain,
and the caudal one third distal to the 4th
pair of somites, represents the future
spinal cord.

14. Formation of neural tube
Dorsal views of human embryo at different stages,
describing the development of neural tube.
The amnion has been cut and removed.

15. Formation of neural tube
Dorsal views of human embryo at different stages,
describing the development of neural tube.
The amnion has been cut and removed.

16. Formation of neural tube
• The fusion proceeds in cephalocaudal
direction
• At cephalic and caudal ends, neural tube
temporarily remains open and the lumen
of neural tube communicates with
amniotic cavity by way of anterior and
posterior neuropores.

17. • Anterior neuropore closes at day 25
• Posterior neuropore closes at day 27
• Closure of the neuropores coincides with
the establishment of a blood vascular
circulation for the neural tube.

18. Formation of neural tube
• Walls of neural tube become thickened to
form the brain and spinal cord.
• The lumen of the neural tube is converted
into the ventricular system of the brain
and the central canal of the spinal cord.
• The process of formation of the neural
tube is known as neurulation and the
embryo at this stage may by called
neurula.

19. Development of
neural crest cells
• Neural crest cells are
neuro-ectodermal in
origin.
• They start differentiating
during the formation of
neural tube in the third
week of intrauterine life.
• They appear at the
elevated edges of neural
folds in the region of
mesencephalon.
• Later on they extend
crainally and caudally.

20. Development of neural crest cells
• Gradually the neural
folds approach and
fuse each other in the
midline.
• As the neural folds
elevate and fuse,
neural crest cells at
the lateral border or
crest of neurectoderm
dissociate from the
neural tube.

21. Development of neural crest cells
• Initially they form an
intermediate zone in the
mesoderm between the
surface ectoderm and
developing neural tube.
• Later on, they divide at
the midline and are
present in the
mesoderm, dorsolateral
to the neural tube.

22. Development of neural crest cells
• These cells undergo an epithelial to
mesenchymal transition as they leave the
neurectoderm by active migration and
displacement to enter the surrounding
mesoderm
• The migration occurs in a cranio-caudal
sequence, beginning at the start of fourth
week.

23. The neural crest gives rise
to cells that form most of the
PNS and ANS,
consisting of
cranial, spinal, and
autonomic ganglia.

24. Neural crest cells originating in the
neuro-ectoderm of forebrain, midbrain
and hind brain regions
migrate into the pharyngeal arches and
facial region
Here they form midfacial and
pharyngeal arch skeletal structures and
other tissues, including cartilage, bone,
dentin, tendon, dermis, leptomeningies,
sensory neurons and glandular stroma.

25. • Lateral view of head and neck region of a
developing embryo showing migration
pathways of cranial neural crest cells into facial
region (green patches) from different parts of
developing brain.

26. Formation of Brain
 The neural tube cranial to the fourth pair
of somites develops into the brain.
 The cephalic end of neural tube shows
three dilatations. They are called primary
brain vesicles.
1. Prosencephalon or Forebrain vesicle.
2. Mesencephalon or Midbrain vesicle.
3. Rhombencephalon or Hindbrain vesicle.

28. Flexures in neural tube
During fourth week folding of the embryo
is also taking place.
So by the end of fourth week two flexures
are obvious in neural tube.
 Cephalic flexure. It located in
mesencephalon region.
 Cervical flexure. It is at the junction of
Rhombencephalon and spinal cord.

30. Secondary brain vesicles
 During fifth week five secondary brain vesicles
develop.
 Prosencephalon divides into two secondary
vesicles.
1. Telencephalon or endbrain vesicle
2. Diencephalons
3. Mesencephalon or midbrain vesicle does not
divide.
 Rhombencephalon divides into two secondary
vesicles.
4. Metencephalon
5. Myelencephalon

32. Ventricles of Brain
• Telencephalon consists of a middle narrow
portion and two lateral dilated portions called
primitive cerebral hemispheres. The cavities of
cerebral hemispheres are called lateral
ventricles.
• Diencephalon is characterized by outgrowth of
optic vesicles. The cavity of diencephalon is third
ventricle.
• The cavity of Rhombencephalon is fourth
ventricle.

33. Communications of Ventricles
• Lateral ventricles communicate with third
ventricle through interventricular foramina
of Monro.
• Third ventricle communicates with fourth
ventricle through aqueduct of Sylvius. This
is the narrow lumen present in midbrain.
• Fourth ventricle is continuous inferiorly
with central canal. Central canal is the
lumen of spinal cord.