Development of the Nervous System

Dr. Wegdan Mohammed Mustafa

Neurulation

Neurulation is the formation of the neural tube.

At the third week of development, the notochordand prechordal mesoderm induces the overlyingectoderm to thicken and form the neural plate.

The lateral edges of the neural plate elevate to formneural folds, and the depressed mid-region formsthe neural groove.

Gradually, the neural folds approach each other inthe midline, where they fuse to form the neuraltube.

The open ends of the neural tube form the cranial and caudal neuropores.

Closure of the cranial neuropore occurs at day25, whereas the caudal neuropore closes atday 27.

The rostral part of the NT becomes the adult brain.

The caudal part of the NT becomes the spinal cord.

Brain Vesicles

The cephalic end of the neural tube shows threedilations, the primary brain vesicles, which developduring week 4.

Prosencephalon (forebrain)

Mesencephalon (midbrain)

Rhombencephalon (hindbrain)

Prosencephalon (forebrain), which consists oftelencephalon and diencephalon.

Mesencephalon (midbrain)

Rhombencephalon (hindbrain), which consists ofmetencephalon (pons & cerebellum) and myelencephalon(medulla oblongata)

Brain Flexures

Cephalic flexure (midbrain flexure): between theprosencephalon and the rhombencephalon.

Cervical flexure: between the rhombencephalon &spinal cord

Pontine flexure: marks the junction between themetencephalon and the myelencephalon

Ventricular System

The lumen of the neural tube gives rise to theventricular system.

lumen of the:

Telencephalon: lateral ventricles

Diencephalon: third ventricle

Rhombencephalon: fourth ventricle

Mesencephalon: cerebral aqueduct , whichconnects the third and fourth ventricles.

Spinal cord: central canal

Neural Crest Cells

Develop from the lateral edge of the neural plate.

Cranial neural crest cells differentiate into:-

Meninges

Sensory ganglia

Schwann cells

Sympathetic chain

Spinal ganglia

Cranial nerve ganglia

Neuroepithelial

The wall of a recently closed neural tube consistsof neuroepithelial cells.

During the neural groove stage and immediatelyafter closure of the tube, neuroepithelial cellsdivide rapidly, producing neuroepithelial layer.

Once the neural tube closes, neuroepithelial cellsbegin to give rise to another cell type calledneuroblasts.

Neuroepithelial cells of the neural tube give rise to:

A: Neuroblasts: form all neurons found in the CNS.

B: Glioblasts: are formed by neuroepithelial cells afterproduction of neuroblasts ceases.

Development of the Spinal Cord

The spinal cord develops from the caudal part of the neuraltube.

Neuroblasts form the mantle layer, which forms the graymatter of the spinal cord.

The continuous addition of neuroblasts to the mantle layer,result in formation of a ventral and a dorsal thickening on eachside of the neural tube.

The basal plate is a ventral thickening, which form ventralhorn of the spinal cord & contains motor neurons .

The alar plate is a dorsoal thickening, which form the dorsal horn of the spinal cord & contains sensory neurons.

Neural tube

The marginal layer, contains nerve fibers emergingfrom neuroblasts in the mantle layer.

As a result of myelination of nerve fibers, this layertakes on a white appearance and therefore is call thewhite matter of the spinal cord.

Development of the Meninges

Mesenchyme surrounding the neural tubecondenses to form primordial meninges.

The external layer of this membrane form thedura mater.

The internal layer form pia mater and arachnoidmater, which is derived from neural crest cells.

MyelinationMyelination is formation of myelin sheath around

the nerve fibers.

Oligodendrocytes form myelination in the CNS.

Schwann cells form myelination in the PNS.

Myelination of the spinal cord begins during month 4in the ventral roots.

Myelination of the corticospinal tracts is notcompleted until the end of 2 years of age

Myelination of the association neocortex extends to30 years of age.

Spina Bifida

Spina bifida is a general term for neural tubedefects (NTDs) affecting the spinal region.

It consists of a splitting of the vertebralarches and may or may not involveunderlying neural tissue.

Spina bifida occulta is a defect in thevertebral arches that is covered by skin andusually does not involve underlying neuraltissue.

It occurs in the lumbosacral region (L4 toS1) and is usually marked by a patch of hairoverlying the affected region.

Spina bifida cystica is a severe NTD inwhich neural tissue and/or meningesprotrude through a defect in the vertebralarches and skin to form a cystlike sac.

In some cases only fluid-filled meningesprotrude through the defect (spina bifidawith meningocele)

In others neural tissue is included in the sac(spina bifida with meningomyelocele).

Break

Development of the Myelencephalon

The myelencephalon is a brain vesicle thatgives rise to the medulla oblongata.

It has basal and alar plates.

The basal plate contains motor nuclei.

These nuclei are divided into three groups:

Somatic efferent

Special visceral efferent

General visceral efferent

• The somatic efferent group continues rostrallyinto the mesencephalon, it is called thesomatic efferent motor column.

• In the myelencephalon it includes neurons ofthe hypoglossal nerve that supply the tonguemusculature.

• The general visceral efferent group containsmotor neurons that supply involuntarymusculature of the respiratory tract, intestinaltract, and heart.

• The special visceral efferent group extends intothe metencephalon, forming the special visceralefferent motor column.

• Its motor neurons supply striated muscles of thepharyngeal arches.

• In the myelencephalon the column is representedby neurons of the accessory, vagus, andglossopharyngeal nerves.

The alar plate contains three groups ofsensory relay nuclei.

Somatic afferent

Special visceral afferent

General visceral afferent

Somatic afferent group, receives impulsesfrom the ear and surface of the head by wayof the vestibulocochlear & trigeminalnerves.

Special visceral afferent, group receivesimpulses from taste buds of the tongue.

General visceral afferent, group receivesinteroceptive information from thegastrointestinal tract and heart.

Metencephalon

• The metencephalon form cerebellum & pons• The metencephalon, similar to the myelencephalon, is

characterized by basal and alar plates.• Each basal plate of the metencephalon contains three

groups of motor neurons:• Somatic efferent group, which contains nucleus of the

abducent nerve.• Special visceral efferent group, containing nuclei of the

trigeminal and facial nerves, which innervate themusculature of the first and second pharyngeal arches.

• General visceral efferent group, whose axons supply thesubmandibular and sublingual glands.

The alar plates of the metencephaloncontain three groups of sensory nuclei:

Somatic afferent group, which containsneurons of the trigeminal nerve and asmall portion of the vestibulocochlearcomplex.

Special visceral afferent group.

General visceral afferent group.

Mesencephalon

The mesencephalon gives rise to the midbrain.

In the mesencephalon each basal plate containstwo groups of motor nuclei:

Somatic efferent group, represented by theoculomotor and trochlear nerves, whichinnervate the eye musculature.

General visceral efferent group, represented bythe nucleus of Edinger-Westphal, whichinnervates the sphincter pupillary muscle.

• Alar plates of the mesencephalon appear as fourelevations, superior and inferior colliculus.

superior and inferior colliculi are concerned with visual and auditory reflexes, repetively.

Cerebellum

The cerebellum is formed from the rhombic lips,which are dorsolateral part of the alar plates.

The rhombic lips compress cephalocaudally and formthe cerebellar plate.

In a 12 week embryo, cerebellar plate shows a smallmidline portion, the vermis, and two lateral portions,the hemispheres.

A transverse fissure separates the nodule from thevermis and the lateral flocculus from thehemispheres.

This flocculonodular lobe is phylogenetically themost primitive part of the cerebellum.

Development of the Telencephalon

Cerebral hemispheres arise at the beginning of the fifthweek

Develop as bilateral evaginations of the lateral wall ofthe prosencephalon.

the cerebral hemispheres grow in the anterior, dorsal,and inferior directions results in the formation offrontal, temporal, and occipital lobes, respectively.

In the final part of fetal life the surface of the cerebralhemispheres grow so rapidly to form many convolutions(gyri) separated by fissures and sulci appear on itssurface.

Meningocele, Meningoencephalocele, and Meningohydroencephalocele

• They are all caused by an ossification defect in the bones ofthe skull.

• The most frequently affected bone is the squamous part ofthe occipital bone.

• Meningocele consisting of a protrusion of the cranialmeninges.

• Meningoencephalocele consisting of a protrusion of partof the cerebellum that is covered by meninges and skin.

• Meningohydroencephalocele consisting of a protrusionof part of the occipital lobe that contains part of theposterior horn of a lateral ventricle.

Anencephaly

The vault of the skull does not form, leaving themalformed brain exposed.

it’s due to the failure of the cephalic part of theneural tube to close.

Hydrocephalus

• It is characterized by an abnormal accumulation ofcerebrospinal fluid within the ventricular system.

• In most cases, hydrocephalus in the newborn isdue to an obstruction of the cerebral aqueduct.

Microcephaly

• It describes a cranial vault that is smaller thannormal