Optic Nerve and Visual Pathway

Optic Disc (Optic Nerve Head)

• Manual technique count ~1.2 million RGCs

• Automated models 700,000 to 1.4 million RGC

• Other studies range from 1 million to 2.22 million

• Approximately loss of 5000 axons per year of life

• Small-diameter macular fibers to larger extramacular fibers

Retinal ganglion cell axons move towards the vitreous, then turn 90 degrees forming the retinal fiber layer, and head towards the optic disc

Arcuate bundles

Some axons do not course directly towards the disc. This pattern of fibers creates the arcuate bundles (superior, inferior, nasal radial, papillomacular).

This prevents axons from crossing the highly sensitive fovea to avoid scattering light and degrading visual acuity

The superior and inferior arcuate are separated by the horizontal raphe. This anatomical feature is vital in identifying optic nerve pathology

Optic Disc Location and Dimensions

Located nasal to the macula and slightly superior to the fovea

~15 degrees from the fovea

It is a physiological blind spot

Slightly vertically elongated

The size of the optic disc ranges

Different studies range (H x V) • 1.5 H x1.7 V mm

Lacks all retinal elements except nerve fiber layer and internal limiting membrane

Müller cells processes are replaced with those of astrocytes

Optic Disc Color

No RPE gives it a lighter-colored look compared to the rest of the retina

Pink, salmon color of the optic disc is a combination of the scleral lamina cribrosa and capillary network

The lamina is more visible in some individuals

Flat with distinct margins

Cup-to-disc ratio

The nerve fibers around the edge make up the neuroretinal rim.

The space in the center, not containing nerve fibers, is called the cup.

The cup-to-disc ratio ranges from 0 to 1.0

Split into horizontal and vertical C/D.

Slightly larger horizontally

ISNT rule

Enlarging C/D ratio can be indicative of pathology (especially vertically)

Normal C/D ratio is slightly different between different ethnicities

Optic Nerve

RNFL turn another 90 degrees at optic Disc and exit as the “optic nerve”

90% will terminate in the LGN, 10% control pupils or circadian rhythm.

5-6 cm long

4 segments based on location

• Intraocular – 0.7 to 1 mm (shortest)

• Intraorbital – 30 mm (longest)

• Intracanalicular – 6 to 10 mm

• Intracranial – 10 to 16 mm (8-19 in some sources)

Intraocular (Intrascleral) Optic Nerve

Prelaminar and laminar regions

Axons from more peripheral retinal ganglion cells (RGCs) are peripheral within the optic nerve head

Glial tissue provides structural support for the nerve fibers

Astrocytes sheaths bundle the nerve fibers into fascicles, each containing 1000 fibers. Separated by pia-derived septa.

The marginal (border) tissue of Elschnig is a ring of collagenous tissue of scleral derivation that lies outer to the glial sheath

• Tight junctions in the border tissue prevent any leakage from adjacent choriocapillary vessels.

The intermediary tissue of Kuhnt is a ring of glial tissue separating the optic nerve fibers from the retinal layers

The border tissue of Jacoby is the continuation of the intermediary tissue, and it separates the choroid from the optic nerve.

Lamina Cribrosa

Posterior Scleral Foramen

Interwoven collagen fibers form canals where optic nerve fibers pass

Intraorbital (post laminar) Optic Nerve

Exceeds the distance from the globe to the apex of the orbit

Shaped like a slight sine-wave shaped curve allowing for complete eye movements without stretching the nerve.

The superior and medial rectus muscles are adherent to the sheath of the optic nerve

Source of pain on eye movements in conditions like optic neuritis

Surrounded by three meningeal sheaths – all three fuse and become continuous with the sclera

• Dura – outside; dense connective tissue full of elastic fibers

• Arachnoid – middle; thin collagenous layer

  • Subarachnoid space is continuous with intracranial subarachnoid space and contains cerebrospinal fluid (CSF) – source of papilledema

• Pia – innermost layer made of loose vascular connective tissue. Allows for blood vessels and connective tissue septa into the nerve. Only one to continue along the intracranial optic nerve

Unmyelinated retinal fibers pass through scleral perforations of the lamina cribrosa

• Fibers from nasal macula continue to be located centrally

They become myelinated by oligodendrocytes as they pass through the pores

• Myelin insulates axons and increase their efficiency and speed of conduction

The additional sheath of connective tissue from the pia mater and each fascicle nearly doubles the diameter of the optic nerve.

• ~1.5 mm at the level of the retina to 3 mm after it exits the globe

Astrocytes provide structure, store glycogen, and a regular extracellular concentration of ions

Travels within the muscle cone formed by the superior, lateral, inferior, and medial rectus muscles

Tumors within this cone can become a source of compressive optic neuropathy

Enlargement of the muscles secondary to systemic conditions like Grave’s disease, can also compress the nerve.

Intracanalicular

Optic Canal is a 5-12 mm passage located superonasal to the superior orbital fissure.

Passage for axons of the sympathetic pathway as well as the ophthalmic artery (inferolateral to the optic nerve, covered in dura)

Intracranial

Highly variable length 8-19 mm with average of 12 mm

Superior to optic nerve:

• Anterior perforated substance

• Root of olfactory tract

• Anterior cerebral artery

Medial to optic nerve – sphenoid sinus

Inferior to optic nerve – internal carotid artery (inferior at first)

Lateral to optic nerve – internal carotid artery (inferior then lateral)

Ophthalmic artery enters the Dural sheath of the optic nerve as it passes through the optic canal

Cytology- Oligodendrocytes

myelin sheath. allow for transduction of signals

Cytology- Meninges

protective layers that surround the nerve

(inner) pia, subarachnoid, dura (outer)

Cytology- Astrocytes

Named for their stellate appearance

Glial tissue in the central nervous system

Regulates ionic and energy homeostasis in white matter like optic nerve

• Highly efficient at transporting potassium

• Accumulate glycogen serving as an energy source in the absence of glucose (ischemia)

Concentrated at the nodes of Ranvier and in contact with nearby capillaries

• Transports substances between circulation and axons

• Signal blood vessels to dilate and constrict to regulate metabolism of nerve

Pathology: Most common intrinsic tumors is astrocytoma (optic nerve glioma)

• Usually found in childhood

Cytology- Microglia

Macrophage

Likely from peripheral bone marrow origin and not neuroectoderm like astrocytes and oligodendrocytes

Phagocytizes extracellular material

Associated with axon bundles

Involved in stimulation of immune system

Blood Supply – Optic Nerve Head

The central retinal artery enters the optic nerve approximately 12 mm behind the globe

• Its branches supplies the inner retina

• As it branches at the optic disc, it also provides partial perfusion of the superficial optic disc via small capillaries

Anastomoses of the posterior ciliary artery branches form the circle of Zinn-Haller which contributes significant perfusion to the optic nerve head.

Optic nerve vessels are non-fenestrated endothelial cells with tight junctions surrounded by pericytes and share the same blood-nerve barrier as the blood-brain barrier.

Optic nerve head vessels can also autoregulate to maintain blood flow despite intraocular pressure changes

Blood Supply – Intraorbital Optic Nerve and Optic Canal

Intraorbital perfused primarily through the pial circulation

• Branches off of the ophthalmic artery either directly or indirectly via recurrent branches of the short posterior ciliary arteries.

Intracanalicular perfused by three branches of the ophthalmic artery (medial collateral, lateral collateral, and ventral branch) which perfuse the pial surface and then penetrate the nerve

Removing optic nerve sheath meningiomas is very delicate because any removal of the pial supply will lead to a possible infraction and blindness

Axonal Action Potential

RGC axons transmit information via action potentials

All-or-nothing spikes of electrical activity

• This is in contrast with the graded potentials of the retina

The actual amount of voltage-change, i.e., depolarization, is the same while the number of impulses per second and the distribution of impulses within the axons is the way visual information is carried down the optic nerve

Resting potential

• Potential across the membrane is driven by the concentration of potassium

• Concentration are achieved via the Na+ -K+ -ATPase.

• Small amounts of leakage occur constantly

• High concentration of Potassium [K+] inside of axon compared to extracellular

• Higher concentration of Sodium [NA+] outside of axon compared to intracellular

• Overall, negative resting potential

Depolarization

• Potential across the membrane is driven by the concentration of sodium

• Adjacent sections of the membrane of the axons gets depolarized

• Opens voltage-sensitive sodium channels

• Rush of sodium enters the axons causing it to be more positive (depolarized)

• Each segment in the axons goes through the same process and the action potential travels throughout the axon

Repolarization

• Voltage-sensitive sodium channels close and a transient opening of voltage-sensitive potassium channels open

• The potential is then returned to be weighed by the potassium equilibrium and the resting potential is restored

Hyperpolarization

Role of Oligodendrocytes and Myelin

Decreases capacitance

• Less sodium needed to enter the axon in order to depolarize the membrane

Increases resistance

• Less leakage of charge across the membrane, thus saving energy that is spent by the Na+ -K+ - ATPase.

Ion channels in adult myelinated axons are not distributed uniformly

They are grouped into patches within the small areas where the axons are unmyelinated called nodes of Ranvier

Saltatory conduction

• much faster

• jumping from one node to the other

Such clustering is induced by oligodendrocytes

Axonal Transport

Orthograde – away from cell body and towards the brain.

Retrograde – towards the cell body and away from the brain.

Different rates of transfer

• Fast and slow orthograde transport

• Retrograde transport occurs at about half the velocity of fast orthograde

Axonal Repair

Adult Retinal ganglion cells (RGC) loss is irreversible, if true cell death occurred

• Elsewhere in the CNS, embryonic neurons can regenerate and lose that ability with development.

Rate and timing of RGC death after optic nerve injury depends on the species, age, RGC cell body size, location of optic nerve transection in relation to the vascular supply

The distance from the site of the injury to the RGC is controversial

• Some suggest that the shorter the distance, the more rapid degeneration. Others suggest that the two are not correlated

Axonal injury, binding of tumor necrosis factor to its receptors, and glutamate excitotoxicity all may result in apoptosis

Why does the apoptosis occur?

Lack of neurotrophic factors from the target tissue

• Typically, these factors promote axon regeneration

Phagocytosis and immune activation

• If macrophages and microglia are activated, they are poorly able to phagocytose degraded myelin, then the inhibitory signals found in myelin may prevent axonal regeneration

Gliosis

• Astrocytes hypertrophy

• Actively inhibits axons from regenerating

Clinical Cases- Ischemic Optic Neuropathy

Occlusion of the posterior ciliary arteries - not enough oxygen getting in

Non-arteritic anterior ischemic optic neuropathy (NAION) - associated with vascular disease and sleep apnea

Arteritic anterior ischemic optic neuropathy (AAION) - ocular emergency/ from GCA

Altitudinal defects; not distinct or flat edges → nerve swelling

Clinical Cases- Optic Neuritis

Young to middle aged adults

More common in females

Often a presentation of multiple sclerosis

Inflammation results in demyelination of the optic nerve. Multiple rounds of such damage causes axonal loss and optic atrophy

Clinical Cases- Glaucoma

Group of distinct optic neuropathies distinguished by a distinct characteristic pattern of progressive excavation of the nerve head without significant pallor

Affects RGC and their axons

Increased cup-to-disc ratio

Clinical Cases- Compressive Optic Neuropathy

Tumors - Typically presenting unilaterally

Grave’s disease

Aneurysms

Clinical Cases- Papilledema

Papilledema is defined as optic disk edema secondary to elevated intracranial pressure.

• Bilateral!

Should not be used to denote to disc edema. (if monocular, just optic edema)

It is unclear whether the visual loss associated with chronic papilledema results from disturbances of axonal transport, or from ischemia due to congestion of the optic nerve head.

Overview

Optic Disc (Optic Nerve Head)

Optic Nerve (CN II)

Optic Chiasm

Optic Tract (one large axon from disc → tract)

Lateral Geniculate Nucleus (LGN) - 1st synapse

Optic Radiations

Striate Complex (visual cortex)

Optic Chiasm

Roughly rectangular

15 mm width, 8 mm anterior to posterior, 4 mm high

Located within the circle of Willis

Above the chiasm is the floor of the third ventricle, 1 cm below the chiasm is the pituitary gland

The chiasm is located above the sella turcica (the fossa in which the pituitary sits) ~10 mm above

• Locations directly above (75%)

• Prefixed – if the optic nerves are short and gland lies below the posterior part of the chiasm (10%)

• Postfixed – if the optic nerves are long and the gland is situated toward the anterior of the chiasm (15%)

Temporal fibers remain ipsilateral

Nasal fibers cross at the chiasm and course towards the contralateral brain

Site of decussation

Optic Tract

Cylindric, slightly flattened

3.5 mm high x 5.1 mm long

Hippocampus is below the optic tract

Runs from the posterolateral corner of the chiasm to the LGN

Fibers of the pupillomotor reflex leave the tract before reaching the LGN and pass by way of the superior brachium to the pretectal nucleus in the midbrain

Some project to the hypothalamus involved in the circadian rhythm

Some terminate in the superior colliculus - orientation/ eye saccades

Blood Supply – Intracranial optic nerve, chiasm and optic tract

The intracranial optic nerve – branches from the ophthalmic, anterior cerebral, anterior communicating and internal carotid

Chiasm – circle of Willis forming two capillary beds:

• Superior: anterior cerebral and anterior communicating arteries

• Inferior: internal carotid, posterior cerebral, posterior communicating arteries

Optic tract is perfused by branches of the posterior communicating and anterior choroidal arteries (branches of the internal carotid)

Lateral Geniculate Nucleus

It is the location where retinal axons terminate

Information from all the sensory systems except the olfactory pass through the thalamus before being transferred to the cerebral cortex

Visual information is processed in the LGN then related to higher cortical centers

Located on the dorsolateral aspect of the thalamus

Inferior horn of the lateral ventricle is posterolateral to the LGN

Not simply a relay station, but a center of complex processing

Receives input from cortical and subcortical centers as well as reciprocal innervation from the visual cortex.

Regulates flow of visual information

Axons leaving the LGN are called optic radiations

Lateral Geniculate Nucleus - 6 major layers

Each layer is composed of the same single cell type

Three types according to size:

Magnocellular (M cells) – large (2 layers located inferiorly)

Parvocellular (P cells) – medium (4 layers located superiorly)

Koniocellular (K cells) – small (below each of the six layers)

Lateral Geniculate Nucleus - Visual Field Processing

Most of the processing is dedicated to the central 2-17 degrees of the visual field

Mapped in specific layers

• Superior visual field (inferior fibers) → lateral zone

• Inferior visual field (superior fibers) → medial zone

Lateral Geniculate Nucleus - grouped in two cells classes

1. Relay cells – cells that send an axon to the visual cortex

   • Use glutamate

   • M, P, and K are all relay cells

2. Interneurons – cells with axons that remain within the LGN

   • Use gamma-amino butyric acid (GABA)

Ratio of 4:1 (relay to interneurons)

Lateral Geniculate Nucleus - inputs

Retina

Primary visual cortex

Some extrastriate areas

Pretectum

Superior colliculus

Parabigeminal nucleus (PBG) - satellite region of superior colliculus

Visual sector of the thalamic reticular nucleus (TRN) - send back info to LGN

*know red (input for visual stimuli), bolded (heaviest input), neurotransmitters used

Lateral Geniculate Nucleus - outputs

Bulk primary visual cortex

Visual sector of the thalamic reticular nucleus (TRN)

Some terminate in extrastriate visual areas, the most well documented of these is the middle temporal area (MT).

• Appears to originate from K LGN cells

• ~1 percent of all of the LGN cells

• Responsible for residual vision referred to as “blindsight” in those who have lost their primary visual cortex.

Feedback pathway

Triad of retinal terminal synapses, relay cell dendrite, and a dendrite of an inhibitory interneuron

Retinal axons are the “drivers of the input” for 3 reasons:

• They terminate closer than other excitatory inputs to the axon hillock of relay cells.

• They have much larger terminals.

• They signal through fast ionotropic glutamate channels located at these synaptic sites on LGN relay cell dendrites.

V1 cells get feedback from higher visual cortical areas, and these are very fast compared to slower messages arriving from the LGN to the cortex, thus intersecting continually, and adjusting messages from the retina continuously. (signal going out of LGN is slower)

Lateral Geniculate Nucleus - M, P, K cells

M Cells - hilus (bottom 2 layers)

• Highest temporal frequency (temporal resolution)

• Detect motion

• Transient response: only at the entrance and withdrawal of a stimulus in their receptive fields

P Cells - crest (top 4 layers)

• Higher sensitivity to spatial frequency (spatial resolution)

• Responsible for red – green color detection

• Sustained response: continue to respond to a visual stimulus as long as it remains present

K cells

• Fall in between M and P cells in spatial and temporal resolution

• Responsible for blue – yellow color detection

• Largely unknown function

Modulators

Sleep/awake states

• Burst firing during sleep, tonic firing while awake

Attention to attended vs unattended stimulus (harder to study and prove)

Motor planning

• Likely involved in saccadic suppression

Although LGN cells are activated by single eye, there is evidence that contralateral eye may have influence

• Binocular rivalry: presenting each eye with one image, the brain will alternate the two

LGN– Blood Supply

The anterior choroidal artery (peripheral)

The posterior choroidal branches of the posterior cerebral artery (dorsal/ hilum wedge for the macula)

Optic Radiations

Optic radiations spread out fanwise sweeping laterally and inferiorly around the anterior tip of the temporal horn of the lateral ventricle

May loop in the temporal lobe or pass within the parietal lobe on their way to the occipital lobe.

Fibers leaving the lateral aspect of the LGN representing inferior retina follow an indirect route to the occipital lobe. They pass into the temporal lobe and loop around the tip of the temporal horn of the lateral ventricle forming Meyer Loops (inferior radiations)

Fibers from the medial aspect of the LGN representing superior retina lie superiorly as the pass through the parietal lobe.

The fibers from the macula are generally situated between the superior and inferior fibers

Optic Radiations - Blood Supply

The optic radiations can be divided into three sections:

1. Anterior radiations

   • Pass laterally over the inferior horn of the ventricle

   • Supplied by the anterior choroidal artery and the middle cerebral artery.

2. Middle Radiations

   • Passing lateral to the ventricle

   • Supplied by the deep optic branch of the middle cerebral artery

3. Posterior radiations

   • Supplied by branches of the posterior cerebral artery, including the calcarine branch

   • Branches from the middle cerebral artery also contribute

Striate Complex (Primary Visual Cortex)

2 main types of cells; both glutamate containing

1. Pyramidal 80%

   • Available in all layers

2. Stellate 20%

   • Mainly in layer IV

Primary Visual Cortex

Primary visual cortex (Brodmann area 17, V1)

Medial surface of the occipital lobe

Just a small portion (1 cm) extends around the posterior pole onto the lateral surface

Called striate cortex: white myelinated fiber layer called the white stria of Gennari

Calcarine fissure extends from the parieto-occipital sulcus to the posterior pole dividing the visual cortex into an upper portion (cuneus) gyrus and lower (lingual) gyrus

• The fovea is represented in the occipital pole and the far periphery is represented in the anterior margin of the calcarine fissure

• Upper visual field mapped on the lower lingual gyrus

• Lower visual field mapped on upper cuneus gyrus

Primary Visual Cortex- Organization

2 mm thick and 1400-3400 mm²

Organized into horizontal layers and vertical columns

Organization:

• Input: Fibers from thalamus ends up in middle layers IVC of V1.

• Output: superficial layers above IVC send information to other cortical areas.

• Output: Layers below IVC send signals to subcortical targets.

Ocular dominance columns: Left and right eye segregation of axons from the M and P LGN layers endings in layer IVC of V1

K LGN synapse on layers IVA, III, and I

Vertical organization:

• Alternating parallel ocular dominance columns according to the eye of origin

Primary Visual Cortex- 6 Layers

Layer 1 most superficial, few scattered neurons

Layer 2 neurons that send axons only to the deeper cortical layers

Layer 3 neurons the communicate with both near and far cortical locations

Layer 4 stria of Gennari

• subdivided into strata (receiving from either M or P cells)

• Sends axons to more superficial visual cortex as well as other visual cortical areas

Layer 5 sends axons to superior colliculus and brainstem

Layer 6 send projections back to the LGN

Extrastraite Visual cortex

Information transmitted to extrastraite cortex (higher visual association areas) provide further interpretation

These areas surround V1 located on the lateral aspects of the occipital cortex

V2-V5; previously Brodmann 18-19

Visual and visual association areas in one hemisphere are connected to the corresponding areas in the other hemisphere through the posterior portion of the corpus callosum

Information about stimulus orientation, movement direction, binocularity are all constructed in V1

Primary Visual Cortex- Two hierarchies

1. Object – what something is “what pathway”

   • Also called the ventral stream

   • Consists of pathways going from V1 through V4 to the temporal cortex

2. Spatial - where something is “where pathway”

   • Also called the dorsal stream

   • Pathway from V2 through MT to the parietal cortex

Primary Visual Cortex - other outputs

Superior colliculus

• Complete retinotopic map of the contralateral field of vision

• Controls saccadic eye movements with input from the frontal eye fields

Frontal eye fields

• Frontal lobe

  • Receives fibers from the V1 controlling conjugate eye movements

  • Voluntary and reflex ocular movements

  • Pupillary responses to near objects

Primary Visual Cortex / V1 - Blood Supply

The calcarine branch of the posterior cerebral artery is the major blood supply for the striate cortex

Supplemented by

• The posterior temporal or parietooccipital branch of the posterior cerebral artery

• The occipital branch of the middle cerebral artery

Visual Field

The entire visible area is termed the visual field.

Information taken in by the retina is processed through the visual sensory pathway reaching the visual cortex and extrastraite cortex.

Damage along the pathway causes defects in the visual field.

Knowledge of the fiber patterns can help in identifying the location of the lesion causing the defect.

Visual Field - Basic

The visual field is divided into 4 different quadrants split by a horizontal and a vertical midline.

The optics of the eye cause an inversion and reversal of the field

• Superior field is imaged on inferior retina

• Inferior field is imaged on superior retina

• Nasal field is imaged on temporal retina

• Temporal field is imaged on nasal retina

The point of fixation is seen by the fovea (central point)

Temporal field

Temporal field is larger than nasal field

Temporal crescent:

• Far temporal periphery

• Only imaged by the nasal fibers of the ipsilateral eye and not on the temporal retina of the contralateral eye

• Secondary to the depth of the orbit and prominence of the nose

Physiologic Blind spot:

• Correlated to the location of the optic nerve head

• Secondary to a lack of photoreceptors at the optic disc.

Retina

Superior and inferior temporal fibers are separated by the horizontal retinal raphe and arch around the macular area to reach the optic disc. These fibers are called the superior and inferior arcuate, respectively.

The fibers from the macular area to the optic disc are called the papillomacular bundle

The nasal fibers travel directly to the optic disc and are described as radiating thus labeled as nasal radial bundle.

Retina → Optic Disc → Optic Nerve

Nasal fibers radiate directly to the nasal side of the disc.

Papillomacular bundle course to the temporal side of the disc.

Fibers from the superior arcuate arch around the papillomacular bundle to enter the superior pole of the disc. The same with the inferior to enter the inferior pole.

Macular fibers take up one third of the disc, despite the macular area being only one twentieth of the total retinal area.

The temporal fibers take up another third, and lastly the nasal fibers take up the last third.

Boundaries between each set of fibers are not always clear-cut.

Fibers from the peripheral retinal are more superficial than those coming from central locations

Macular fibers move to the center of the nerve.

The rest all line up in their respective logical location (e.g., superior temporal in the superior temporal portion of the nerve).

Optic Nerve → Optic Chiasm

Temporal fibers remain ipsilateral while nasal fibers cross (decussate).

The ratio of the crossed to uncrossed fibers in the chiasm is 53 to 47

Inferior nasal fibers cross inferiorly in the anterior chiasm and terminate part of the opposite optic nerve before turning back to the chiasm and into the contralateral optic tract. This is called the anterior knee of Wilbrand.

Superior nasal fibers cross superiorly in the posterior chiasm. These fibers loop posteriorly into the ipsilateral optic tract of the before crossing over. This is called the posterior knee of Wilbrand.

Small number of fibers exit the posterior chiasm and enter the suprachiasmatic nucleus in the hypothalamus and work on synchronizing the circadian rhythm.

Optic Chiasm → Optic Tract

As the fibers leave the chiasm in the optic tract, the crossed and uncrossed fibers intermingle

The superior fibers (the fibers from both the ipsilateral superior temporal retina and the contralateral superior nasal retina) move to the medial side of the tract.

The fibers from the inferior retina (ipsilateral inferior temporal retinal fibers and contralateral inferior nasal retinal fibers) occupy the lateral area of the tract.

The macular fibers remain in the middle of the groups

Optic Tract → LGN

Superior fibers from the optic tract terminate in the medial aspect of the LGN

Inferior fibers terminate in the lateral aspect of the LGN

A dorsal wedge composing 2/3 to 3/4 of the LGN is dedicated to the macula

Layers 1, 4 and 6 receive input from the contralateral nasal retina

Layers 2, 3, and 5 receive ipsilateral temporal retina

Each layer of the LGN contains a retinotopic map or representation of the contralateral hemifield of vision.

A retinotopic map is a “point-to-point localization” of the retina.

Line of projection

• A line passing through all six layers perpendicular to the surface

• All the intercepted cells would be carrying information about the same point in the visual field.

• This alignment is so precise that there is a gap in each contralateral layer along the line of projection that corresponds to the location of the optic disc.

LGN → Optic Radiations

Inferior retinal fibers leave the LGN from the lateral aspect, pass into the temporal lobe and loop around the tip of the temporal horn of the lateral ventricle. This is caller Meyer’s Loop. • Superior retinal fibers leave the LGN from the medial aspect and lie superiorly as they pass through the parietal lobe. • Macular fibers are situated between superior and inferior fibers.

Optic Radiations → Striate Complex (Primary visual cortex)

Inferior radiations terminate in the region below the calcarine sulcus called the lingual gyrus • Superior radiations terminate in the region above the calcarine sulcus called the cuneus gyrus • Fibers from the macular area terminate posteriorly in the visual cortex • Fibers from the nasal periphery terminate in the most anterior part of the visual cortex near the parietal lobe. This corresponds to the temporal crescent in the visual field seen by only the contralateral eye • Mapping of the visual cortex demonstrated that the central portion of the visual field occupies the majority of the striate cortex

Visual Field Territories

Territory 1: choroid and outer retina

Territory 2: inner retina and optic nerve

Territory 3: optic chiasm

Territory 4: Post-chiasmal

Visual Field - Territory 1

Defects in the choroid and outer retina will cause a field defect that is similar in shape to the lesion and located in the corresponding location of the field (e.g., a lesion in the superior nasal retina should be found in the inferior temporal field)

Visual Field - Territory 2

Damage to the superior, inferior and papillomacular nerve fiber bundles causes a field defect corresponding to the location and configuration of the affected nerve fiber layer.

These defects respect the horizontal midline

If the nasal radial bundle is affected, a wedge shape defect emanates from the blind spot.

Terminology for Visual Field Loss

“Anopia” or “Anopsia”- defines loss of a portion of the visual field

Hemianopsia – loss of half a visual field

Quadrantanopia OR quadranopsia – loss of one-quarter of a visual field

Sectoranopia – loss of a sector of a visual field

Heteronymous – two eyes have non-overlapping field losses

Homonymous – visual field loss is on the same side of the vertical line of both eyes

Congruity:

• A judgment of how similar an incomplete homonymous hemianopic defect is

• A characteristic of post-chiasmal VF loss only

• It is secondary to the fact that nerve fibers from corresponding points lie adjacent to one another

• Cannot judge congruity if the VF defect is total/complete (all points in the whole hemifield) and absolute (no sensitivity at all)

• Congruity increases the more posterior the lesion is in territory 4

Visual Field - Territory 3

The location of the lesion in comparison to the optic chiasm determines the location of the defect

If the lesion is directly below the chiasm, the resulting defect is a bitemporal field defect.

If a lesion affects the anterior or the posterior portion of the chiasm as it connects to the optic nerve or optic tract, respectively, it may result with an anterior junctional scotoma or a posterior junctional scotoma

Visual Field - Territory 4

The postchiasmal pathway (ipsilateral temporal fibers and contralateral nasal fibers) carries information from the contralateral visual field (e.g., right pathway from left field) thus postchiasmal lesions cause a homonymous field defect.

Optic tract: relatively small in cross section so often a single lesion will damage the majority of fibers and causes a full homonymous field defect, if patrial may be incongruent. (affect one side more than other)

• Can cause optic nerve atrophy and/or pupillary defects

LGN lesions cause a defect in the contralateral field

• Can cause optic atrophy, but no pupillary defects.

Optic radiations running through the temporal lobe cause superior defects, while those running in the parietal lobe causes inferior defects.

• Cannot cause optic atrophy or pupillary defects.

Occipital lobe: congruous hemianopsia, quadranopsia, possible macular sparing

Visual Field Defects

Visual Field Defects - LGN