Lecture 16 - Visual Pathway

Ocular Anatomy

What is the visual pathway?

A network of cells and synapses that sends visual information from the environment to the brain.

Why is the visual pathway clinically important?

Damage anywhere along it causes vision changes and visual field defects, which helps determine cause and treatment.

What are the major components of the visual pathway?

Retina → Optic nerve → Optic chiasm → Optic tract → LGN → Optic radiations → Visual cortex

What are the first three cells in the visual pathway?

Photoreceptors → Bipolar cells → Ganglion cells

Where are the first two synapses in the visual pathway?

Outer plexiform layer and inner plexiform layer.

How long is the optic nerve?

5–6 cm

How many fibers does the optic nerve contain?

1–2 million ganglion cell axons

What percentage of optic nerve fibers terminate in the LGN?

90%

Where do the remaining 10% of optic nerve fibers project?

To non-LGN areas like superior colliculus, pretectal nucleus, and hypothalamus.

What tissue surrounds the optic nerve?

Three meningeal sheaths: dura mater, arachnoid, pia mater.

Which meningeal sheath around the optic nerve is made of tough, dense, elastic connective tissue?

The dura mater.

Which optic nerve meningeal sheath is a thin collagenous membrane?

The arachnoid mater.

What is found inside the optic nerve’s subarachnoid space?

CSF continuous with intracranial CSF.

Which meningeal sheath of the optic nerve is loose and highly vascular?

The pia mater.

What happens to the optic nerve meninges at the sclera?

All meninges fuse and become continuous with the sclera and periorbita.

What causes papilledema?

Increased intracranial pressure causing CSF buildup around the optic nerve.

What happens to the axons during papilledema?

Compression of optic nerve axons and vascular blockage.

What are the four optic nerve segments?

Intraocular, intraorbital, intracanalicular, intracranial

What are the two sub-regions of the intraocular optic nerve?

Pre-laminar and laminar.

Are ganglion cell axons myelinated in the intraocular segment?

No, they are unmyelinated until after the lamina cribrosa.

What structure allows ganglion cell axons to exit the sclera?

The lamina cribrosa.

What is the peripapillary scleral flange?

Opening in the posterior sclera where the optic nerve exits.

Which cells bundle nerve fibers into ~1000 fascicles?

Astrocytes.

What is the border tissue of Elschnig?

The peripapillary border tissue of the scleral flange; surrounds the optic nerve and separates it from the sclera.

What is the border tissue of Jacoby?

The peripapillary border tissue of the choroid; separates the choroid from the optic nerve.

What is the intermediary tissue of Kuhnt?

Tissue separating outer retina from ganglion cell/optic nerve fibers.

What is the length of the intraocular segment of the optic nerve?

0.7–1.0 mm

What is the length of the intraorbital optic nerve?

30 mm

Why is the intraorbital optic nerve longer than the globe-to-apex distance?

To allow eye movement.

What shape does the intraorbital optic nerve form?

An S-shaped curve.

What is the length of the intracanalicular segment?

6–10 mm

What is the length of the intracranial segment?

10–16 mm

Where does myelination of the optic nerve begin?

Just after the lamina cribrosa.

Which artery enters the optic nerve sheath 10–12 mm behind the globe?

Central retinal artery

What are common causes of optic neuritis?

Demyelinating disease, infections, autoimmune disease.

What muscles surround the intraorbital optic nerve?

The rectus muscles.

Why does optic neuritis often cause pain with eye movement?

Because optic nerve sheaths adhere to superior and medial rectus muscle sheaths.

What enters the meningeal sheath in the intracanalicular segment?

The ophthalmic artery.

Which meningeal sheath continues beyond the optic canal?

Only the pia mater.

What structures lie near the intracranial optic nerve?

• Cavernous sinus (inferior)

• Internal carotid artery (inferolateral)

• Sphenoid sinus (medial)

Where could an aneurysm compress the intracranial optic nerve?

At the internal carotid artery.

Which blood vessels supply the entire length of the optic nerve?

Pial vessels

What supplies the optic disc?

Circle of Zinn–Haller from short posterior ciliary arteries.

What supplies the intraorbital and intracanalicular nerve?

The ophthalmic artery.

What supplies the intracranial optic nerve?

Superior hypophyseal, ophthalmic, and anterior cerebral arteries.

Where is the optic chiasm located?

In the subarachnoid space, surrounded by meningeal sheaths and CSF.

What are the dimensions of the optic chiasm?

15 mm wide × 8 mm deep × 4 mm high

What sits directly above the optic chiasm?

The floor of the third ventricle and the hypothalamus.

Which fibers cross at the optic chiasm?

Nasal retinal fibers

What are the knees of Wilbrand?

Small loops of nasal fibers:

• Posterior knee: superior nasal fibers loop backward

• Anterior knee: inferior nasal fibers loop forward

What lies directly below the optic chiasm?

The pituitary gland (~1 cm below).

What are the three chiasm positions relative to the pituitary?

Prefixed (10%), normal (75%), postfixed (15%)

Why do pituitary tumors affect vision?

They compress the optic chiasm.

What arteries supply the optic chiasm?

Internal carotid arteries and Circle of Willis.

What does the optic tract connect?

The optic chiasm → LGN

What are the dimensions of the optic tract?

3.5 mm high × 5.1 mm long

The optic tract runs parallel to which artery?

Posterior cerebral artery.

What two deep brain structures sit near the optic tract?

It lies below the globus pallidus and above the hippocampus.

What percentage of optic tract fibers go to the LGN?

90%

Where do the other 10% of optic tract fibers go?

• Pretectal nucleus (pupillary light reflex)
• Hypothalamus (circadian rhythm)
• Superior colliculus (saccades, fixation)

What artery supplies the optic tract?

Anterior choroidal artery (branch of middle cerebral artery)

Where is the LGN located?

On the dorsolateral (posterior) thalamus.

What are the LGN layers?

• Magnocellular (1–2)

• Parvocellular (3–6)

• Koniocellular (K1–K6) in between M and P layers

Which LGN layers receive crossed fibers?

1, 4, 6

Which LGN layers receive uncrossed fibers?

2, 3, 5

Where are the optic radiations located anatomically?

In the white matter of the cerebral cortex, lateral to the temporal horns of the lateral ventricles.

What are the LGN efferents?

Optic radiations → visual cortex

What arteries supply the LGN?

Anterior choroidal + lateral/posterior choroidal (PCA)

What are optic radiations also called?

Geniculocalcarine tract

What are the two major divisions of optic radiations?

• Superior radiations (parietal lobe)
• Inferior radiations (temporal lobe)

What is Meyer’s loop?

Anteriorly looping inferior radiations in the temporal lobe.

What supplies the anterior radiations?

Anterior choroidal + MCA

What supplies the middle radiations?

Deep optic branch of MCA

What supplies the posterior radiations?

Calcarine branch of PCA + MCA branches

How thick is the primary visual cortex (V1)?

2 mm thick

What are other names for V1?

Striate cortex, Brodmann area 17

Where is V1 located?

Medial occipital lobe, around calcarine fissure.

What divides V1 into upper and lower portions?

The calcarine fissure

Where does the calcarine fissure extend?

From the parieto-occipital sulcus to the posterior pole of the occipital lobe.

What region lies superior to the calcarine fissure?

The cuneus gyrus.

What region lies inferior to the calcarine fissure?

The lingual gyrus.

Which gyrus represents the superior visual field?

Lingual gyrus (inferior cortex)

Which gyrus represents the inferior visual field?

Cuneus gyrus (superior cortex)

Which fibers occupy the most posterior cortex?

Macular fibers

Why does the macula take up so much cortical space?

It processes fine detailed vision.

Which layer of V1 receives most LGN input?

Layer IV

Which LGN layers project to V1 Layer IV?

Both magnocellular and parvocellular inputs.

What do magnocellular pathways process?

Motion and low spatial frequency

What do parvocellular pathways process?

Color and high spatial frequency

What are ocular dominance columns?

Vertical columns responding to one eye preferentially.

What are orientation columns?

Columns responding to specific stimulus orientations.

What structures receive output from V1?

• Superior colliculus
• Frontal eye fields
• Hypothalamus
• Extrastriate cortex (V2, V3, V4, V5)

What midbrain structure receives V1 output for reflexive eye movements?

The superior colliculus.

What cortical area controls voluntary saccades?

The frontal eye fields.

What deep brain structure receives visual information for circadian rhythm regulation?

The hypothalamus.

What is the main blood supply to V1?

Calcarine branch of the PCA

Why can strokes sometimes spare central vision?

The macular fibers in the occipital lobe receive dual blood supply from the PCA and the MCA.

What are the two divisions of the visual field?

Central and peripheral.

How does the visual field map onto the retina?

It is flipped vertically and horizontally.

What visual field does superior retina represent?

Inferior visual field