DEVELOPMENT OF CRANIAL NERVES & AUTONOMIC NERVOUS SYSTEM Dr Samina Anjum

CRANIAL NERVES 4th week- All cranial nerve nuclei are present III –XII CN arise from brainstem Only III CN arise outside the region of hind brain

SEGMENTATION IN HINDBRAIN In the hind brain, Proliferation centers in the neuroepithelium establish eight distinct segments – Rhombomeres which give rise to motor nuclei of CNs

CRANIAL NERVES Motor neurons for Cranial nuclei are within the brain stem while sensory ganglia are outside the brain. Organization of CNs is homologous to the spinal nerves, but all CN don't contain both sensory & motor fibers.

CRANIAL NERVE - SENSORY GANGLIA Originate from: Neural crest cells Ectodermal placodes: Otic Nasal 4Epibranchial

EPIBRANCHIAL PLACODES Represented by ectodermal thickenings dorsal to pharyngeal arches Contribute to ganglia for the nerves of the pharyngeal arches.

PARASYMPATHETIC GANGLIA Visceral efferent ganglia are derived from neural crest cells, & their fibers are carried by CN III, VII, IX & X

AUTONOMIC NERVOUS SYSTEM Functionally, the autonomic nervous system can be divided into two parts: Sympathetic portion in the thoracolumber region Parasympathetic portion in the cephalic and sacral region

SYMPATHETIC NERVOUS SYSTEM In the 5th 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.

NEURAL CREST CELLS

SYMPATHETIC GANGLIA Neural crest cells form a bilateral chain of segmentally arranged sympathetic ganglia. interconnected by longitudinal nerve fibers. Together, they form the sympathetic trunks on each side of the vertebral column.

EXTENT OF SYMPATHETIC CHAIN From there position in the thorax, neuroblasts migrate toward the cervical & lumbosacral regions, extending the sympathetic trunk to their full length.

FUSION OF GANGLIA Initially the ganglia are arranged segmentally, later this arrangement is obscured, in the cervical region, by fusion of the ganglia.

PREAORTIC GANGLIA Some sympathetic neuroblasts migrate in front of the aorta to form preaortic ganglia, such as CELIAC SUPERIOR MESENTERIC INFERIOR MESENTERIC

SYMPATHETIC ORGAN PLEXUSES Other sympathetic cells migrate to the heart, lungs, and gastrointestinal tract, where they give rise to sympathetic organ plexuses.

VISCEROEFFERENT COLUMN & PREGANGLIONIC FIBERS Once sympathetic trunks have been established, nerve fibers originating in the visceroefferent column (intermediate horn) of the thoracolumber segments (T1-L2,L3) of the spinal cord penetrate the ganglia of the trunks (paravertebral ganglia). They are known as preganglionic fibers (Myelinated, form white communicating rami) Hence, white rami are found only from L1-L2 or L3 segments of the spinal cord.

POSTGANGLIONIC FIBERS Axons of the sympathetic ganglion cells, the postganglionic fibers, have no myelin sheath. They either pass to other levels of sympathetic trunks or extend to the heart, lungs, and intestinal tract. Other fibers, the gray rami communicantes, pass from the sympathetic trunk to spinal nerves & from there to peripheral blood vessels, hair, & sweat glands. Gray rami are found at all levels of the spinal cord.

Fate of cephalic fibers: Synapse with ganglia in cervical region Fibers pass without synapsing, as preganglionic fibers of greater(T5-9), lesser(T10-11) and least splancnic nerves(T12) to prevertebral ganglia. A few fibers from Gr.Spl N end directly in adrenal medulla ACh Fate of caudal fibers: Synapse with ganglia in lower lumber & sacral regions region Post ganglionic fibers to Smooth muscle and glands of viscera

SUPRARENAL GLAND The suprarenal gland (adrenal gland) develops from 2 components: MESODERMAL /Mesenchyme ECTODERMAL/ NCcells

FORMATION OF CORTEX OF THE SUPRARENAL GLAND During the 5th week of development, mesothelial cells between the root of the mesentery & the developing gonad begin to proliferate & penetrate the underlying mesenchyme, differentiate into large acidophillic organ which form fetal cortex, or primitive cortex.

Cont…. Shortly after, a 2nd wave of cells from the mesothelium penetrate the mesenchyme and surrounds the original acidophilic mass definitive cortex of the gland. These cells smaller than those of first wave, later forms the definitive cortex of gland.

Cont… While fetal cortex is being formed, cells originating in the sympathetic system (NC cells) invade its medial aspect, where they are arranged in cords and clusters. These cells give rise to medulla of suprarenal gland. They stain yellow brown with chrome salts, hence are called chromaffin cells. During embryonic life chromaffin cells are widely distributed through out the embryo, but in adult they are limited to adrenal medulla only.

FORMATION OF MEDULLA OF THE SUPRARENAL GLAND

Cont… After birth fetal cortex regresses rapidly except for its outer most layer, which differentiate into reticular zone. but zona reticulosa is recognizable until the end of 3rd year. Recent studies show that a transitional zone is located b/w fetal cortex and adult cortex. It has been suggested that zona fasciculata is derived from this zone. The zona glomerulosa and zona fasciculata are present at birth Adult structure of cortex is achieved at puberty.

Cont… Suprarenal glands of human fetus are 10-20X larger than in adult due to extensive size of fetal cortex , which produces steroid precursors that are used by placenta for the synthesis of estrogen.

PARASYMPATHETIC NERVOUS SYSTEM Neurons in the brainstem & the sacral region (S2-4) of the spinal cord give rise to preganglionic parasympathetic fibers. Fibers travel via the III, VII, IX, & X nerves. Post ganglionic fibers arise from ganglia derived from neural crest cells and pass to structures they innervate (pupil of eye, salivary glands, viscera)

Red- Sympathetic NS Blue : Parasympathetic nervous system

HIRSCHSPRUNG DISEASE Congenital Megacolon Failure of parasympathetic ganglia to form in the wall of part or all of the colon & rectum because the neural crest cells fail to migrate. The colon is dilated above the affected region, which has a small diameter because of tonic contraction of innervated musculature.

RADIOGRAPH OF COLON AFTER BARIUM ENEMA

Cranium Bifidum (bony defect in cranium) Meningocele Meningoencephalocele Meningohydroencephalocele

Holoprosencephaly: Loss of midline structures

Schizencephaly: large clefts in CH, loss of brain tissue

Anencephaly

Hydrocephalus

Arnold Chiari Malformation

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OCULOMOTOR NERVE

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GLOSSOPHARYNGEAL NERVE

VAGUS NERVE