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Duke Elder - Anatomy 2: Structure of the eye > Optic nerve > Flashcards

Optic nerve Flashcards

1
Q

Why is the optic nerve unique anatomically?

A
  • it is the only tract in the CNS to leave the cranial cavity
  • only central nervous system tract that can be visualised clinically
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2
Q

How is the optic nerve subdivided?

A

into fascicles, by connective tissue and glial septae, and is surrounded by cerebrospinal fluid

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3
Q

What surrounds the optic nerve?

A

CSF

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4
Q

What is shown in the image?

A

histological section of the optic nerve head

LC = lamina cribrosa

A and V = central retinal artery and vein

SAS = subarachnoid space

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5
Q

What is shown in the image (right)?

A

transverse section of the orbital portion of the optic nerve revealing the arrangement of the myelinated nerve fascicles (darkly stained) separated by pial septae (PS) which penetrate as far as the central retinal artery (CRA)

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6
Q

What is shown in the image?

A

cross-section (trichrome stain) of an entire optic nerve and surrounding meninges posterior to the entry of the central retinal artery

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7
Q

What is shown in the image?

A

Blood supply of the optic nerve. The four sources of vessels supplying the optic nerve include:

  1. branches from the central retinal artery or its branches
  2. branches from the circle of Zinn-Haller
  3. choroidal branches
  4. pial branches.
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8
Q

What are the 4 sources of blood vessels supplying the optic nerve?

A
  1. branches from the central retinal artery or its branches
  2. branches from the circle of Zinn-Haller
  3. choroidal branches
  4. pial branches
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9
Q

What is shown in the image?

A

Scanning electron microscopy of the lamina cribrosa (LC).

A and Vn = Apertures for the central artery and vein

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10
Q

What forms the optic nerve at its most distal point (optic disc)?

A

convergence of ganglion cell axons at the optic disc

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11
Q

What proportion of axons leaving the eye are foveal/macula fibres?

A

90%

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12
Q

What is the name of the structure formed by the foveal/macula fibres leaving the eye?

A

maculopapillary bundle

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13
Q

What is the path of axons of nerve fibres leaving the eye, from the optic disc?

A

extend along the optic nerve through the orbit to traverse the optic canal in the sphenoid bone

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14
Q

What are the 4 main portions into which the optic nerve can be divided?

A
  1. intraocular (1mm)
  2. orbital (25-30mm)
  3. intracanalicular (4-10mm)
  4. intracranial (10mm)
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15
Q

How long is the intraocular portion of the optic nerve?

A

1mm

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16
Q

How long is the orbital portion of the optic nerve?

A

25-30mm

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17
Q

How long is the intracanalicular portion of the optic nerve?

A

4-10mm

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18
Q

How long is the intracranial portion of the optic nerve?

A

10mm

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19
Q

What are the limits of the intraocular portion of the optic nerve?

A

extends from the surface of the optic disc to the posterior margins of the sclera

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20
Q

Are the nerve fibres myelinated in the intraocular portion of the optic nerve?

A

not myelinated

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21
Q

What are the three regions of the intraocular portion of the optic nerve?

A
  1. retinal (pars retinalis)
  2. choroidal (pars choroidalis)
  3. sceral (pars scleralis)
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22
Q

At what point does myelination of the optic nerve commence?

A

level with the termination of the subarachnoid space at the posterior limits of the lamina cribrosa

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23
Q

What type of cell concentration changes as the fascicles of the optic nerve pass posteriorly from the optic disc into the intraocular portion?

A

glial cells become more common

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24
Q

In which portion of the intraocular optic nerve are columns of glial cell nuclei especially prominent and what proportion of tissue mass do they account for here?

A

scleral portion

40% of tissue mass

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25
Q

What is the diameter of the optic disc?

A

1.7 - 2.8mm (varies depending on measurement)

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26
Q

What factors can cause the diameter of the optic disc to vary and what disease is this linked to?

A

within a population and between races

linked to susceptibility to glaucoma

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27
Q

How do the layers of the retina and choroid terminate at the edge of the optic disc?

A

as specialised regions of glial tissue, the intermediary tissue (of Kuhnt) and marginal border tissue (of Elschnig)

28
Q

What explains the blind spot in the visual field?

A

the absence of retinal tissue in the region of the optic disc

29
Q

What happens in the area surrounding the optic disc to the ganglion cells?

A

the 1.2 million ganglion cell axons in the nerve fibre layer become crowded towards the disc. create raised area or papilla

30
Q

In which aspect of the papilla, the raised area due to crowded ganglion cell axons, thickest and why?

A

on the lateral aspect owing to the large number of fibres in the maculopapillary bundle

31
Q

What structure does the raised margin of the optic disc surround?

A

an indentation called the physiological cup

32
Q

What happens to the fibres passing posteriorly from the papilla?

A

they pierce the sieve-like connective tissue mesh, the lamina cribrosa, which fills the posterior foramen

33
Q

What is the lamina cribrosa?

A

sieve-like connective tissue mesh which fills the posterior scleral foramen, formed of irregular collagen fibre bundles continuous with the sclera

34
Q

What type of tissue is the lamina cribrosa made of and what is the arrangement of this?

A

irregular collagen fibre bundles continuous with the sclera, arranged in the form of circles or a figure of eight

35
Q

What tissue is continuous with the adventitia surrounding the central retinal artery and vein?

A

elastic tissue from the choroid and Bruch’s membrane; is continnuous with and ‘anchored’ to the adventitia

36
Q

What separates the collagenous bundles around the central retinal artery and vein in the lamina cribrosa from the axons?

A

a covering of glial tissue which may protect the nerve fibres as they pierce the irregular openings

37
Q

How long is the scleral canal (through which optic nerve passes) and what is its shape?

A
  • 0.5mm long
  • may vary in shape from cone-like (narrowest portion nearest the disc) to double cone or funnel-like
38
Q

What happens to the optic nerve fibres posterior to the pars scleralis in terms of their covering?

A

they become myelinated by oligodendrocytes causing a doubling of the thickess of the optic nerve

39
Q

Which part of the optic nerve is damaged in glaucoma?

A

intraocular portion only

40
Q

What may cause the axonal damage in the optic nerve in glaucoma?

A

may be a consequence of either:

  • interference with blood flow
  • interruption of axonal transport

raised IOP is significant risk factor

41
Q

Is there a conclusion on why specific regions of the optic nerve are more likely to be damaged than others?

A

no - no single hypothesis has been proposed

42
Q

What is the clinical result of parts of the optic nerve being more likely to be damaged than others?

A

visual field defects or scotomas

43
Q

What factor may influence central retinal venous pressure as it traverses the lamina cribrosa in glaucoma?

A

the translaminar pressure gradient (TLPG): differential pressure gradient across the lamina cribrosa between the cerebrospinal fluid pressure and the intraocular pressure (IOP)

44
Q

What process may the TLPG (translaminar pressure gradient) in glaucoma cause in the central retinal venous pressure?

A

arteriolisation of the vessel wall and endothelial lining

45
Q

What are 2 factors that may influence the TLPG and what process may these be responsible for?

A
  1. buffering effects of orbital tissue, pia mater
  2. conformation of the lamina intself

–> may be responsive for outward bowing of lamina in the optic cup

46
Q

What is thought to be the overall role of increased TLPF and its relationship to IOP?

A

progression of optic nerve damage and glaucoma

47
Q

What is papilloedema?

A

swelling of the papillary fibres, which appears as a raised white disc margin, and is partly the result of the lack of Müller cells in this region

48
Q

The absence of which cells partly contributes to the raised white disc margin in papilloedema?

A

Müller cells

49
Q

What is the function of Müller cells in the remainder of the retina (absent at the papilla)?

A

serve to bind the nerve fibres together

50
Q

What is papilloedema a sign of?

A

raised intracranial pressure transmitted to the subarachnoid space, which envelops the optic nerve as far anteriorly as the sclera surrounding the optic nerve

51
Q

What are the limits of the orbital portion of the optic nerve?

A

extends backwards and medially from the back of the eye to the optic canal in the sphenoid at the apex of the orbit

52
Q

Which layers cover the orbital portion of the optic nerve?

A

the meninges: pia, arachnoid and dura

53
Q

How do the dura mater and arachnoid mater over the orbital portion of the optic nerve blend with tissues at the back of the eye?

A

the dura and arachnoid blend with the sclera, and the subarachnoid space (SAS) around the nerve terminates at the posterior surface of the sclera in the form of a fluid-filled ring

54
Q

What are the central retinal vessels vulnerable to due to crossing the subarachnoid space?

A

vulnerable in cases of increased intracranial pressure (particularly the vein)

55
Q

What are the diameters of the axons in the orbital portion of the optic nerve?

A

majority 1 μm in diameter and approximately 10% are between 2 and 10 μm

56
Q

What happens to the glial septae between fascicles in the optic nerve in the intraocular portion once it reaches the orbital portion?

A

they extend into the orbital portion but become less distinct as the orbital apex is approached

57
Q

What is the shape of the optic nerve in the orbital portion and why?

A

slight S-shaped bend - allows full range of ocular movement without stretching the nerve

58
Q

What happens to the tissue surrounding the optic nerve as it approaches the orbital apex?

A

it is surrounded by the tendinous annulus, which is the origin of the rectus muscles

59
Q

Where is the intracanalicular portion of the optic nerve?

A

passes through the optic canal (foramen) accompanied by the opthalmic artery and sympathetic nerves

60
Q

Which 2 structures travel with the optic nerve in the optic canal (foramen)?

A
  1. ophthalmic artery
  2. sympathetic nerves
61
Q

What happens to the dura surrounding the optic nerve at the optic canal?

A

it splits at the orbital opening, the majority continuing as the dural sheath of the nerve inside the canal and a thinner portion blending with the periorbital fascia

62
Q

Why has the blood supply of the optic nerve been extensively investigated?

A

due to its importance in the pathogenesis of glaucoma

63
Q

What are the 4 sources of branches of arterial supply to the intraocular portion of the optic nerve?

A
  1. central retinal vessels and their branches
  2. scleral vessels (circle of Zinn-Haller)
  3. choroidal vessels
  4. pial vessels
64
Q

What source are the central retinal vessels, scleral vessels and choroidal vessels derived from?

A

ophthalmic or central retinal artery

65
Q

What source are the pial vessels supplying the intraocular portion of the optic nerve derived from?

A

adjacent branches of the internal carotid artery

66
Q

How do the majority of capillaries supplying the optic nerve reach it?

A

pierce the nerve and course longitudinally within it via the glial septae

Duke Elder - Anatomy 2: Structure of the eye (14 decks)

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