Optic nerve Flashcards
Why is the optic nerve unique anatomically?
- it is the only tract in the CNS to leave the cranial cavity
- only central nervous system tract that can be visualised clinically
How is the optic nerve subdivided?
into fascicles, by connective tissue and glial septae, and is surrounded by cerebrospinal fluid
What surrounds the optic nerve?
CSF
What is shown in the image?
histological section of the optic nerve head
LC = lamina cribrosa
A and V = central retinal artery and vein
SAS = subarachnoid space
What is shown in the image (right)?
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)
What is shown in the image?
cross-section (trichrome stain) of an entire optic nerve and surrounding meninges posterior to the entry of the central retinal artery
What is shown in the image?
Blood supply of the optic nerve. The four sources of vessels supplying the optic nerve include:
- branches from the central retinal artery or its branches
- branches from the circle of Zinn-Haller
- choroidal branches
- pial branches.
What are the 4 sources of blood vessels supplying the optic nerve?
- branches from the central retinal artery or its branches
- branches from the circle of Zinn-Haller
- choroidal branches
- pial branches
What is shown in the image?
Scanning electron microscopy of the lamina cribrosa (LC).
A and Vn = Apertures for the central artery and vein
What forms the optic nerve at its most distal point (optic disc)?
convergence of ganglion cell axons at the optic disc
What proportion of axons leaving the eye are foveal/macula fibres?
90%
What is the name of the structure formed by the foveal/macula fibres leaving the eye?
maculopapillary bundle
What is the path of axons of nerve fibres leaving the eye, from the optic disc?
extend along the optic nerve through the orbit to traverse the optic canal in the sphenoid bone
What are the 4 main portions into which the optic nerve can be divided?
- intraocular (1mm)
- orbital (25-30mm)
- intracanalicular (4-10mm)
- intracranial (10mm)
How long is the intraocular portion of the optic nerve?
1mm
How long is the orbital portion of the optic nerve?
25-30mm
How long is the intracanalicular portion of the optic nerve?
4-10mm
How long is the intracranial portion of the optic nerve?
10mm
What are the limits of the intraocular portion of the optic nerve?
extends from the surface of the optic disc to the posterior margins of the sclera
Are the nerve fibres myelinated in the intraocular portion of the optic nerve?
not myelinated
What are the three regions of the intraocular portion of the optic nerve?
- retinal (pars retinalis)
- choroidal (pars choroidalis)
- sceral (pars scleralis)
At what point does myelination of the optic nerve commence?
level with the termination of the subarachnoid space at the posterior limits of the lamina cribrosa
What type of cell concentration changes as the fascicles of the optic nerve pass posteriorly from the optic disc into the intraocular portion?
glial cells become more common
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?
scleral portion
40% of tissue mass
What is the diameter of the optic disc?
1.7 - 2.8mm (varies depending on measurement)
What factors can cause the diameter of the optic disc to vary and what disease is this linked to?
within a population and between races
linked to susceptibility to glaucoma