Visual pathway Flashcards
Where is the optic nerve myelinated?
Unmyelinated in eyeball
Myelinated posterior to disc
How many nerve fibres in optic nerve?
1.2 million
What are the divisions of the optic nerve?
Intraocular 1mm
Intraorbital 25-30mm
Intracanalicular 4-10mm
Intracranial 10mm
What are the coating of the optic nerve?
Covered by dura, arachnoid and pia -blend with sclera anteriorly
Myelin produced by oligodendryocytes
Divided into fibrous septa
Which part of the anatomy is responsible for optic disc swelling in raised intracranial pressure?
A swollen lamina cribosa
What are the four divisions of the intraocular optic nerve?
Superficial nerve fibre-> optic disc area
Prelaminar area
Laminar area
Retrolaminar area
What is the lamina cribosa?
10 connective tissue plates
Pores transmit unmyelinated axon cells from the eyeball
Provides scaffolding for optic nerve axons
Provides scaffold for central retinal artery and vein
And reinforces posterior area of the globe
How does diameter of optic nerve change behind lamina cribosa?
Increases to 3mm due to myelination
1.5mm within eyeball
Which vessels pass through optic nerve head?
Central retinal artery and vein
Where is the intraorbital part of the optic nerve?
25mm long s shaped
Runs from lamina cribosa to optic foramen
Lies within annulus of zinn at orbital apex
Anterior: long and short ciliary nerves and arteries
Lateral: ciliary ganglion (parasympathetic)
Posterior: nasociliary nerve and ophthalmic artery
Inferior: CN III +central retinal artery and vein
What travels with the intracanalicular portion of the optic nerve?
Opthalmic artery
Sympathetic nerve
Meninges
Dural sheath fuses with periosteum at this stage
Sphenoid and posterior air cells lie medially
What is the blood supply of the intraocular optic nerve?
Intraocular: anastomotic circle of Zinn from short posterior ciliary arteries
What is the blood supply of the intraorbital and intracanalicular optic nerve?
Pial plexus
What is the blood supply of the intracranial optic nerve?
Pial plexus
and branches of internal carotid
Describe the histology of the optic nerve
Supporting glial cells and blood vessels
Nerve bundles
Surrounding meninges
What are the function of the optic nerve?
transmits all visual information
Contrast visual acuity
Colour perception brightness perception
Conducts visual impulses; light reflex and accommodation reflex
Why do we test red saturation?
Difference in optic nerve colour perception
Decreased saturation = optic neuritis/pathology
Where does the central retinal artery enter the optic nerve?
1.2cm from the globe
Where is the optic chiasm?
Junction of anterior wall and floor of 3rd ventricle
Continuous anteriorly with optic nerve and posteriorly with optic tracts
12mm long, 8mm wide and 3mm thick
What are the relations of the optic chiasm?
Anterior: anterior cerebellar and anterior communicating arteries
Lateral: internal carotid artery ascending from cavernous sinus
Posterior: interpeduncular space with infundibulum below and 3rd ventricle above
Superior: 3rd ventricle cavity and anterior wall
Inferiorly: diaphragma sellae, hypophysis cerebri and cavernous sinus
What is a prefixed/postfixed optic chiasm?
79% overlies dorsum sellae
17% lies more anterior (prefixed)
4% lies more posterior (postfixed)
Describe the retinotopic organisation of the optic chiasm
Nasal fibres from retina cross over to the opposite optic tract
Temporal fibres do not cross
Inferior nasal fibres cross in anterior chiasm
Superior nasal fibres cross in posterior chiasm
Macular fibres occupy central chiasm
What is the blood supply to the optic chiasm
Pial plexus branches of internal carotid
Superior hypophyseal posterior communicating anterior cerebral and anterior communicating arteries
Describe the optic tracts
Pass posteriolaterally between tuber cinereum medially and anterior perforated substance laterally
Winds around cerebral peduncle adherent to midbrain
90% of lateral root fibres go to LGN, 10% to superior colliculus (visual body reflexes) and pretectal nucleus (light reflexes)
What is the blood supply to the optic tracts?
Pial arteries
Branches of anterior choroidal, posterior communicating and middle cerebral
Describe the retinotopic organisation of the optic tracts
Fibres rearranged to correspond with LGN Macular dorsolateral Lower retinal lateral Upper retinal medial Peripheral retina more anterior
What does the posterior pituitary secrete?
Oxytocin
ADH
What does the anterior pituitary secrete?
TSH ACTH FSH LH GH PRL endorphins
Where does the pituitary/hypophyseal gland sit?
Small ovoid structure 8-12mm diameter
Sits in sella turcica on superior sphenoid
roof of fossa formed from dura called diaphgrama sellae
Central perforation for infundibulum connecting hypophysis to floor of 4th ventricle
Circle of willis lies superiorly
What are the visual field defects associated with optic chiasm damage
Anterior: sectoranopia
Body: Bitemporal hemianopia
Posterior: quandrantanopia
How is the cavernous sinus different to other sinuses in the brain?
Traversed by septa which subdivide the blood causing sluggish blood flow
Why is the superior orbital fissure different in a dry skull as opposed to in life?
In life the outer part is filled with fibrous tissue and nothing can pass through
What forms the lateral wall of the cavernous sinus?
Dura
Which foramen lie within the cavernous sinus?
Foramen rotundum and lateral edge of trigeminal ganglion depression
NOT foramen ovale and spinosum
What makes up the roof of the cavernous sinus?
Sheet of dura between the wall of the piuitary fossa and tentorial edge
What lies inferiorly to the cavernous sinus?
Middle fossa floor
What is the posterior boundary of the cavernous sinus?
Anterior slope of petrosal temporal bone
What is the contents of the cavernous sinus?
Internal carotid artery Separated from sinus by vascular endothelium but in the case of ruptured aneurysm, a fissure may form CNIII CN IV CNVI V1 V2
Where does the internal carotid artery leave the cavernous sinus?
Medially to the anterior clinoid process
In close proximity to the optic canal -this is where internal carotid gives of ophthalmic artery
How do the trochlear and occulmotor nerves pierce the dura and cavernous sinus?
Occulomotor nerve sits on top of trochlear
But CN IV eventually crosses CN III to reach superior oblique
Which is the only cranial nerve to arise from the dorsal brain stem?
Trochlear
Which nerve runs with the internal carotid artery?
Abducens
Leaves posterior cranial fossa by crossing petrous temporal bone and enters the cavernous sinus on top of the ICA and runs with it until it heads dorsally
Abducens continues on to reach the fibrous ring and the orbit
Which veins enter the cavernous sinus?
Just behind the apex of the orbit-superior ophthalmic vein
Drains through the superior orbital fissure
Inferior ophthalmic vein drains mostly to pterygoid venous plexus but some to cavernous
Sphenoparietal sinus
Superficial middle cerebral vein
Transverse sinus
Pterygoid venous plexus
Where does blood from the cavernous sinus go?
Posteriorly via inferior petrosal sinus
Inferior petrosal sinus drains back down temporal bone, lying between petrous temporal laterally and occipital medially
Opens into the jugular foramen to join internal jugular vein
Where does the transverse sinus meet the cavernous sinus?
Just as it turns to form sigmoid sinus
Which vein drains internal ear?
Superior petrosal sinus
Where does the cavernous sinus communicate with the pterygoid venous plexus?
Via foramen ovale and foramen of Vesalius when present
