Anatomy of optic nerve

complete anatomy of optic nerve

1. ANATOMY OF OPTIC NERVE Dr.M.Sudheer Kumar Post graduate Guntur Medical College ,GUNTUR

2. Optic nerve is a tract of the brain, an outgrowth of the cerebral vesicle, whose fibres possess no neurolemma [ so , doesn’t regenerate when cut ] and which is surrounded by meninges , unlike any peripheral nerve. Primary and secondary sensory neurons of the visual pathway are in the retina.

3. Ensheathed in pia , the optic nerve extends posteromedially from the globe at the optic nerve head to the chiasma in the cranial cavity. As it passes to the optic foramen, the optic nerve takes a sinuous course which allows for ocular movements

4. Cross section of optic nerve Within the orbit  round In the canal  oval nerve leaves the canal  piriform

5. The total length of the nerve is about 47 to 50 mm ,and divided into 4 parts 1)Intraocular 1 mm 2) Intraorbital 30 mm 3) Canalicular 6 -9 mm 4)Intracranial 10mm

6. INTRA OCULAR NERVE HEAD K/A (OPTIC PAPILLA / OPTIC DISC) Extends from its anterior surface ,in contact with the vitreous to a plane which is level with that of the posterior scleral surface (about 1 mm ) The choroid ends abruptly here, as do all elements of the retina except its axons. These axons bend at a right-angle into the nerve head and pass posteriorly through the scleral canal

7. Zones of nerve head 1) Superficial nerve fibre layer 2) Prelaminar zone 3) Lamina cribrosa 4) Retrolaminar

8. Superficial nerve fibre layer This is covered by the inner limiting membrane of Elschnig which is composed of astrocytes and is in continuity with the inner limiting membrane of retina Glial cells and interaxonal processes are relatively sparse here but increase progressively towards the retrolaminar nerve.

9. The Muller cells of nerve fiber layer are in continuity with the , astrocytes where the retina terminates at the optic disc edge. The Muller cells form the internal limiting membrane of Elschnig In some specimens, Elschnig's membrane is thickened in the central part of the disc to form the central meniscus of Kuhnt . A glial lining called the Intermediary tissue of Kuhnt at the termination of the retina.

10. HORIZONTAL SECTION OF OPTIC NERVE HEAD

11. Prelaminar zone The prelaminar region contains bundles of axons lying within astrocytic channels. The astrocytic processes are largely circumferential. Wolter described glial processes around axonal bundles and between individual axons in prelaminar region On the nasal side choroidal stroma is directly adjacent to the astrocytes surrounding the nerve. This collection of astrocytes surrounding the canal is known as the border tissue of Jacoby .

12. This loose glial tissue does not bind the axon bundles together as do the Muller cells of the retina and therefore fibres here are more easily separated… This may explain why the disc swells so easily in papilloedema while the adjacent retina does not.

13. Lamina cribrosa The lamina cribrosa forms a band of dense compact connective tissue across the scleral foramen. Its sieve-like arrangement (the cribriform plate) transmits the axon bundles of the nerve and the central retinal vessels, through a series of round or oval apertures embraced by strong trabeculae

14. Each trabecula results from the ingrowth of a branch of the short ciliary arteries or circle of Zinn , accompanied by glia and scleral connective tissue. So ,Each trabecula , therefore, has a vessel in it surrounded by collagen bundles and elastic fibres. External to this are glial cells. Lamina cribrosa gets its rich blood supply from circlemof zinn

15. Retrolaminar portion The retrolaminar nerve axons are myelinated , unlike those of the retina and nerve head proper. Myelination accounts almost entirely for the doubling of nerve diameter from 1.5 to 3.0 mm after traversing the lamina cribrosa . However, glial cell numbers also increase in a centripetal direction.

16. Relations of nerve head To neighbouring retina The layers of the retina, except the stratum opticum (Retinal nerve fiber layer) are separated from the optic nerve by the Intermediary tissue of Kuhnt These glial cells lack tight Junctions and therefore no blood-brain barrier exist in between the peripapillary choriocapillaris and the optic nerve head at this level,, ,

17. This probably explains the late fluorescence of the nerve head , which occurs with fundus fluorescein angiography (FFA)

18. Relation to neighbouring choroid The two layers of the basal lamina of Bruch's membrane end almost together, at the entry of the nerve. i.e Choroid ends abruptly at optic nerve head.

19. Relation to neighbouring sclera marginal tissue of Elschnig ,is an annular region of neuroglia between choroid, sclera and optic nerve fibres.

20. Ophthalmoscopic features of the optic nerve head That part of the nerve head visible with the ophthalmoscope is termed the optic disc. Its intrapapillary parts are the optic cup and the neuroretinal rim ,,, Optic disc colour The pink colour of the disc is due to the rich capillary supply to its rim, which can be demonstrated by fluorescein angiography.

21. This is most noticeable inferotemporally where the rim is widest.. The white appearance of the cup is due to scattering of light by the lamina cribrosa and the sparse vascularity of this region. Nerve fibre loss in chronic glaucoma leads to increasing exposure of the lamina as axons are lost so that its pores become more visible as the cup enlarges.

22. Disc shape The optic disc may be round, but is usually oval in shape, its vertical diameter being on average 9% longer than its horizontal. The cup is 8% wider in the horizontal, so that the rim is wider above and below

23. Disc size Disc size Normal disc area ranges widely, from 0.86 mm2 to 5.54 mm2 with a mean of 2.69 ± 0.7 mm2 Ophthalmoscopically , the disc corresponds in size to the internal opening of the scleral canal.

24. Optic cup . optic disc is excavated by a funnel-shaped depression . Cup area correlates with disc area and, hence, is large in large discs and small in small discs.

25. Normal disc with small shallow cup. A hyperpigmented alpha zone is present on the nas since the cup is horizontally oval, the neuroretinal rim is widest below , then above and then nasally..

26. Zone alpha is the more peripheral zone and is an irregular hypo or hyperpigmented region associated histologically with irregularities of the retinal pigment epithelium and parapapillary choiroid . Peripherally, it is adjacent to the retina and centrally, to zone beta. This zone corresponds to the choroidal crescent, in which it was envisaged that the pigment epithelium failed to extend to the disc margin

27. Zone beta is related to the disc centrally and to the retina It corresponds to the scleral crescent Always closer to the optic disc than zone alpha In the normal nerve head.

28. Neuroretinal rim The tissue outside the cup is termed the neuroretinal rim. Contains the retinal nerve axons as they enter the nerve head. Rim area ranges from 0.8-4.66 mm2 and correlates with disc area. It is broadest in the lower segment of the disc, then above, then nasally and then temporally. It is narrowest in the temporal horizontal disc region in 99.2% of all discs.

29. This typical rim configuration correlates with the diameter of the retinal artery and vein, which is larger below and temporal than above and temporal.. nerve fibre visibility more detectable in the inferior temporal arcade than the superior. This implies that there is a greater retinal axonal mass and vascularity in the inferotemporal sector.

30. Clinical significance In primary open angle and other forms of chronic glaucoma a progressive loss of retinal ganglion cells occurs. This leads to a characteristic pattern of axonal loss at the neuroretinal rim, with enlargement of the cup, particularly at the upper and lower poles of the disc. The vertically-oval cup of chronic glaucoma contrasts with the horizontontally -oval cup of the normal disc.

31. Characteristic and early sign of glaucomatous optic nerve damage is the occurrence of flame-shaped haemorrhages on the rim ,usually at the inferior or superior temporal margin.

32. Cup/disc ratio The cup/disc ratio is the ratio of cup and disc width, measured in the same meridian ,usually the vertical or horizontal. Because the disc is vertically oval and the cup horizontally oval , the cup/disc ratio is normally lower in the vertical meridan in the majority of individuals . It increases in chronic glaucoma.

33. The ratio has a median value of 0.3. an asymmetry of greater than 0.2 has been taken to signify enlargement and to be of diagnostic importance in glaucoma. Since small discs commonly have no cup, the presence of a cup/disc ratio of 0.2-0.3 in a small disc may in fact indicate early glaucomatous nerve damage.. whereas in a primary macrodisc , cup/disc ratio of 0.8 may be entirely normal

34. RELATIONS OF THE OPTIC NERVE IN THE ORBIT As the nerve traverses the annular tendon at the optic foramen, the attachments of the superior and medial recti are adherent to its dural sheath… This may explain the pain in extreme movement and characteristic of retrobulbar neuritis Between the nerve and lateral rectus are the oculomotor , nasociliary , sympathetic and abducent nerves and sometimes the ophthalmic vein

35. Anteriorly ,nerve is seperated from extra ocular muscles by orbital fat. Posteriorly , near optic foramena , optic nerve closely surrounded by annulus of zin and origin of 4 recti muscles The ciliary ganglion lies between the nerve and lateral rectus.

36. RELATIONS OF THE OPTIC NERVE IN THE OPTIC CANAL The pial sheath is adherent to the nerve. The dura , lining the canal as periosteum , splits at its orbital end to become the periorbita and the dural sheath of the optic nerve.. Intra canalicualr part of optic nerve closely related to ophthalmic artery. Sphenoid and post ethemoid sinuses lies medial to it

37. INTRACRANIAL RELATIONS OF THE OPTIC NERVE The nerve is superior to the diaphragma sellae , & anterior part of the cavernous sinus. Between the nerves and anterior to the chiasma is a triangular space covered by the diaphragma , overlying part of the hypophysis cerebri . Above rs the anterior perforated substance, medial root of the olfactory tract.

38. SHEATHS OF THE OPTIC NERVE In the cranial cavity the optic nerve is surrounded only by pia , but in the optic canal arachnoid and dura are present. At the optic foramen the cranial dura splits into periosteum ( periorbita ) and the dural covering of the optic nerve…. Thus in the canal and in the orbit the nerve is surrounded by sheaths of all three meninges .

39. BLOOD SUPPLY OF THE OPTIC NERVE Intra ocular part of optic nerve Surface nerve fiber layer , supplied by capillaries derived from retinal arterioles , which anastamose with vessels of pre laminar region Pre laminar region supplied by vessels of ciliary ganglion

40. Lamina cribrosa region..supplied by ciliary vessels , derived from short post ciliary arteries , and arterial circle of zinn-haller . Retro laminar region…by both ciliary and retinal circulation..

41. Intra orbital part blood supply By 2 systems… 1) Peri axial system of vessels 2)axial system of vessels - Peri axial system of vessels , derived from 6 branches of ICA , i.e 1)ophthalmic artrey 2) long post ciliary arteries 3) short post ciliary arteries

42. 4) lacrimal artery 5)central artery of retina AXIAL SYSTEM OF VESSELS.. .Intra neural branches of central retinal artery .central collateral arteries which come from central retinal artery , before it pierces the nerve . Central artery of optic nerve

43. Blood supply of Intra canalicular part By peri axail system of vessels… Pial plexus in this part supplied by branches of ophthalmic artery. Blood supply of intra cranial part.. .supplied exclusively by periaxial system of vessels

44. Venous drainage Optic nerve head is primarily by CENTRAL RETINAL VEIN ORBITAL PART…peripheral pial plexus and central retinal vein distally Intra cranial part…. pial plexus

45. Signs of optic nerve dysfunction Reduced visual acuity for distance and near is common, but is non-specific; acuity may be relatively preserved in some conditions. Relative afferent pupillary defect (RAPD) Dyschromatopsia is impairment of colour vision, which in the context of optic nerve disease mainly affects red and green.

46. Diminished light brightness sensitivity Diminished contrast sensitivity Visual field defects

47. Ref.. WOLF anatomy of the eye and orbit Anatomy and Physiology of eye by Khurana THAN Q