Optic nerve

anatomy of optic nerve

1. OPTIC NERVE Anatomy - Pravda C haturvedi

2. EMBRYOLOGY Optic nerve head is formed in late embryonic period as optic stalk encloses the hyloid artery. (8 th week) Physiological cupping develops at 15 th week. Optic nerve develops from embryonic optic stalk (4 th week ) and connects optic vesicle to fore brain. As the stalks lengthens, nerve becomes thinner and is progressively occupied by axons growing from ganglion cell layer. Myelination begins centrally, progresses in centrifugal direction. Myelination is completed shortly after birth.

3. anatomy Second cranial nerve. Starts from optic disc, extends upto chiasma . Backward continuation of nerve fibre layer of retina. Also contains afferent fibres for light reflex.

4. COMPARABLE TO SENSORY TRACT OF BRAIN :- An out growth of brain. Not covered by neurilemma , so doesn’t regenerate. Surrounded by meninges . Fiber diameter 2-10 microns. Sensory nerve fiber diameter ~20 microns. Both primary and secondary neurons in retina.

5. PARTS OF OPTIC NERVE Total length 47-50mm Divided into four parts:- intra ocular part- 1mm intra orbital part- 30mm intra canalicular part- 6-9mm intra cranial part- 10mm

6. 1. Intraocular part Passes through sclera , choroid and appears as optic disc. 1.5 mm vertical diameter. Itself divided into: Surface Nerve Fiber Layer Prelaminar layer Lamina cribosa Retrolaminar region

7. 2. Intraorbital part Extends from back of the eyeball to the optic foramina. Tortuous course to give room for ocular movements. Covered by dura , arachnoid and pia . Pial sheath contains capillaries and send septa to divide nerve into fasciculi . Near optic foramen, optic nerve is closely surrounded by annulus of zinn .

8. relations: The long and short cilliary nerve and arteries surround the nerve. b/w optic nerve and lateral rectus muscle are situated cilliary ganglion, divisions of occulomotor , sympathetic and abducent nerve. The ophthalmic artery, sup ophth vein and nasocilliary nerve cross the optic nerve superiorly from lat to med side.

9. 3. Intracanalicular part Closely related to ophthalmic artery. Leaves the sheath at the orbital end of canal. Sphenoid and post ethmoidal sinuses lie medial to it and are separated by thin bony lamina.

10. 4. Intracranial part About 1cm in length. Lies above cavernous sinus and converges with the fellow nerve to form optic chiasma over diaphragma sellae . Internal carotid runs first below and then lateral to it. The ant perforated substance, m ed root of olfactory nerve and ant cerebral artery lie above it.

11. Arrangement of nerve fibers in optic nerve In optic nerve head: Same as in retina. In distal region: Temporal fibers situated in temporal half. Upper and lower fibers separated by pappilomacular bundle. Nasal fibers situated on nasal side. In proximal region: macular fibers placed centrally.

12. Blood supply Intraocular: .SNFL- by anastomoses b/w CRA, choriocapillaries and (if present) cilioretinal arteries. . Prelaminar and laminar region- from short post cilliary arteries. . Retrolaminar region- pial blood capillaries.

13. 2. I ntraorbital part : supplied by two systems: axial and periaxial . Periaxial : 6 branches of ICA i.e. ophthalmic, LPCA,SPCA, Lacrimal and central. Axial: intaneural branches of CRA.

14. 3. I ntracanalicular : ophthalmic artery is sole supply. 4. Intracranial part: perichiasmal artery ( a branch of sup hypophyseal branch of ICA)

15. Venous drainage Choroid smaller proportion. Mainly by CRV and to alesser extent by pial venous system. Both systems drain into ophthalmic venous system and ultimaely into cavernous sinus.

16. Blood brain barrier at optic nerve Capillaries have non fenestrated endothelial linings with tight junction. It is incomplete due to continuity b/w extracellular space in choroid and optic nerve. There is no barrier present at the level of choroid.