Nervous systemThe 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.

DEVELOPMENT OF NERVOUS SYSTEM

• Nervous system is ectodermal in origin

• Nervous system starts developing in the 3rd week of intrauterine life

Central nervous system

• CNS starts developing in the 3rd week of intrauterine life

• CNS appears above the notochord

17-day embryo

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.

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.

Formation of neural plate

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

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.

Formation of neural groove

Formation of neural trenchAs 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.

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.

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.

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.

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.

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.

• 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.

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.

Development of Development of neural crest cellsneural 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.

Development of neural crest cellsDevelopment 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.

Development of neural crest cellsDevelopment 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.

Development of neural crest cellsDevelopment 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.

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

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

• 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.

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.

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.

Secondary brain vesicles During fifth week five secondary brain vesicles

develop. Prosencephalon divides into two secondary

vesicles.1. Telencephalon or endbrain vesicle2. Diencephalons3. Mesencephalon or midbrain vesicle does not

divide. Rhombencephalon divides into two secondary

vesicles.4. Metencephalon 5. Myelencephalon

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.

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.