Visual Pathway

The basic visual pathway

• The first-order neurone is the photoreceptor (rod or cone), synapses with
• The second-order neurone, a bipolar cell, which synapses with
• The ganglion cell

• Ganglion cell axons cross the retina as the nerve fibre layer>

• The orientation of these fibres causes the patterns of visual field loss if they become damaged

• Two populations: parvocellular >for fine VA and colour; magnocellular >for motion detection and coarser vision

• (NFL becomes) the optic nerve
• At the optic chiasm, the nasal fibres cross and the temporal fibres remain uncrossed
• The optic tracts progress to the 
• Lateral geniculate body on the posterior surface of the thalamus
• Crossed fibres to laminae 1, 4 and 6
• Uncrossed to laminae 2, 3 and 5

• In the LGN the ganglion cell fibres synapse with cells that connect via the optic radiations to 

• Occipital cortex, area 17 

Embryology

• The edges of the optic stalk seal near the forebrain from approx 5-6 weeks and proceed distally. 

• Note: the optic cup seals first

• Retinal ganglion cell axons grow towards the stalk and through it to the brain, surrounding the hyaloid vessels

• Bergmeister’s papilla: a remnant of glial cells and posterior hyaloid vessels that may persist at the optic nerve head

• The anterior remnant of the hyaloid vessels forms Mittendorf’s dot on the posterior lens capsule

• Condensed mesenchyme forms the outer dural layer of the optic nerve

• Optic nerve fibres are maximal in utero and decrease before birth

Optic nerve

• 1.2 million ganglion cell axons (myelinated once exited lamina cribosa)

• 90% are small diameter (1 micron) representing midget ganglion cells associated with cones

• Remainder are larger diameter (2-10 microns) representing axons associated with rods
• Average length of 40mm
• Contains
• No neuronal cell bodies
• No interneurones
• Microglial cells: resident macrophages
• Astrocytes: regulate potassium
• Oligodendrocytes: for myelination

Intraocular portion

• Optic disc, 3mm nasal to macula, 1.5mm in diameter

• Axons from the central retina/cone-system (macula) pass centrally and peripheral axons lie peripherally

• Arcuate bands from temporal retina/periphery bend around to disc
• Unmyelinated as they bend posteriorly, supported by astrocytes

• Distally: macular fibres lie laterally along with temporal fibres and nasal fibres lie medially

• Proximally: macular fibres move to occupy central part of nerve
• Blood supply: 

• Circle of Zinn derived from short posterior ciliary arteries: supply the prelaminar and lamina cribrosa regions

• Note: the surface nerve fibre layer of the ON head is supplied by branches of the central retinal artery

Orbital portion

• 25mm long: sinuous course to allow some movement of eyeball

• Myelinated from lamina cribosa (formed by the sclera) by oligodendrocytes therefore larger diameter

• Surrounded by meningeal coverings including dura
• Surrounded by orbital fat
• Ciliary ganglion lies laterally between nerve and lateral rectus
• Ophthalmic artery and nasociliary nerve cross over the nerve above it

• Central retinal artery and vein pierce dural coverings 12mm behind eyeball on inferomedial aspect

• At apex the nerve lies within tendinous coverings of the recti
• Fibres arranged into bundles separated by septa created by pia mater

• Blood supply: pial plexus of vessels (derived from branches of ophthalmic). Extraneural part of central retinal artery provides some branches (‘central collateral arteries’)

Intracanalicular portion

• Optic canal lies within the lesser wing of the sphenoid (only)
• 5mm long
• Surrounded by meningeal coverings
• Dural sheath fuses with periorbita
• Ophthalmic artery and postganglionic sympathetic fibres also lie within canal
• Blood supply: pial plexus derived from recurrent branches of ophthalmic

Intracranial portion

• Nerve exits canal and passes backwards, medially and upwards to form optic chiasm on floor of the third ventricle

• Passes within subarachnoid space of middle cranial fossa
• Internal carotid lies laterally

• Blood supply: pial plexus derived from superior hypophyseal artery from internal carotid and ophthalmic artery

Optic chiasm

• At junction between anterior wall and floor of third ventricle
• Below lamina terminalis (wall of the forebrain)
• Superior to diaphragma sella (and pituitary)

• Posterior to anterior cerebral arteries and anterior communicating artery (and sphenoid sinus)

• Medial to internal carotid and cavernous sinus
• 12 x 8mm size

• The knee of von Willebrand: infero-nasal fibres cross the chiasm and pass anteriorly into the contralateral optic nerve before running posteriorly. Hence a unilateral lesion of the chiasm/optic nerve junction causes an ipsilateral optic nerve defect and a contralateral supero-temporal field defect (so-called junctional scotoma)

• Normally: 53% of retinal fibres decussate

• Inferior nasal fibres pass in anterior and inferior chiasm
• Superior nasal fibres pass in posterior and superior chiasm

• Macular fibres travel in the centre of the optic nerve
• Macular fibres decussate in the posterior superior portion

• Temporal macular fibres pass uncrossed and nasal fibres decussate but in albinism some temporal fibres also cross

• Blood supply: pial plexus from
• Internal carotid
• Superior hypophyseal
• Anterior cerebral
• Anterior and posterior communicating arteries

Optic tracts

• Curve around cerebral peduncles adherent to the midbrain

• Fibres from corresponding retinal areas become associated: parvocellular fibres run centrally and magnocellular fibres run peripherally

• Large lateral root terminates in the LGN
• Responsible for conscious visual sensation
• Lateral aspect attached to third ventricle
• Smaller medial root (about 10% of fibres) terminates in subcortical targets:

• Superior colliculus (three groups of fibres): visual grasp reflex, scanning of images, auditory/visual association pathways, visual attention and coordination of head and eye movements

• Pretectal nucleus>: pupillary light reflexes

• Suprachiasmatic nucleus: circadian rhythm
• Pulvinar nucleus: visual attention and motor integration
• Parvocellular reticular formation: arousal

• Blood supply: posterior communicating artery and anterior choroidal artery forming pial arteries

• Note: the posterior cerebral artery travels beneath and parallel with the optic tract

Pupillary light reflexes

• As above, a small bundle of fibres breaks off before reaching the LGN and synapses at the pretectal nucleus

• Fibres from there pass to the bilateral Edinger-Westphal nuclei
• Parasympathetic fibres travel in the oculomotor nerve to the ciliary ganglion
• Postganglionic fibres then travel in the short ciliary nerves to the constrictor pupillae

Lateral geniculate nuclei

• Paired thalamic nuclei where retinal ganglion cells synapse
• Round swelling on pulvinar of thalamus
• Connected to superior colliculus via the superior brachium
• Structure: body, head, spur, hilum 

• Six lamellae of cells, labelled 1-6 (“stack of hats”) separated by white matter of the optic tract fibres

• Crossed fibres from the contralateral eye terminate in layers 1, 4, 6
• Uncrossed fibres from the ipsilateral eye terminate in layer 2, 3, 5
• Layers 1 and 2: data from peripheral retina transmitted via the magnocellular pathway
• Layers 3-6: data from fovea transmitted by the parvocellular pathway
• Each retinal ganglion cell may terminate with six geniculate cells within one lamina
• Retinotopic mapping:
• Upper quadrant fibres (inferior visual field) terminate in medial LGN
• Lower quadrant fibres (superior visual field) terminate in lateral LGN
• Note: fibres are rotated 90 degrees
• Macular fibres occupy large central wedge of LGN
• Optic radiations emerge from dome-shaped posterior aspect

• Blood supply: anterior choroidal branch of ICA (may come off MCA) (+ thalamogeniculate branches of the PCA and lateral choroidal artery)

Optic radiations

• Nerve fibres whose cell bodies lie in the LGN (second order neurones of visual pathway)
• Terminate in the striate visual cortex (area 17)

• Lateral fibres from the LGN represent lower quadrant retinal fibres (therefore the superior field) fan out into the temporal lobe and sweep anteriorly into the loop of Meyer. 

• Those from the upper quadrants pass through the parietal lobe radiations.
• The loop of Meyer passes around the temporal horn of the lateral ventricle

• The fibres that flare widest are associated with the peripheral retina; those of the macula pass more directly

• Blood supply: anterior choroidal artery, MCA (supplies the posteriorly directed fibres via the lateral striate artery), PCA (produces posterior choroidal artery)

Primary visual cortex (Brodmann’s area 17)

• Within the calcarine sulcus/fissure extending into cortex above (cuneus gyrus) and below (lingual gyrus)

• Receives myelinated fibres from the optic radiations (aka, geniculocalcarine tracts)
• Prominent band of geniculocalcarine fibres termed the stria of Gennari
• Fibres for superior quadrants (inferior field) pass to upper edge of sulcus

• Fibres for the macula pass to the posterior part (tip of the occipital lobe) and occupy 1/3rd of space

• Posterior sulcus lies at the watershed region between the MCA and PCA

• Fibres for inferior quadrants (superior field) pass to lower edge of sulcus (below calcarine sulcus)

• Fibres for peripheral retina sit anteriorly within cortex (a lesion at the most anterior point represents the peripheral nasal retinal fibres and so creates a temporal crescent in the contralateral eye only)

• Cortex is arranged into six basic histological layers (similar to all cerebral cortex)
• Layer II: cells synapsing with the contralateral visual cortex via the corpus callosum
• Layers II and III: project to the secondary visual cortex
• Unique white stripe in layer IV: stripe of Gennari formed by myelinated fibres

• Layer IV is thickest: receives afferents from LGN (myelinated fibres of the geniculocalcarine tract) and contains mostly stellate cells.

• Fibres from the macula
• Expanded into distinct sublayers

• Layer V: connect to superior colliculus which connects to the MLF enabling ocular movements to be coordinated with light

• Layer VI: sends efferents to the LGN (“feed-forward”) and contains mostly pyramidal cells (cortical geniculate cells)

• These synapse with ganglion cells and interneurones in the LGN.

• Blood supply: PCA (+/- anastomoses with MCA which account for macular sparing in cortical infarcts)

• Ischaemia of a calcarine artery produces a congruous contralateral homonymous hemianopia with macular sparing

Ocular dominance columns

• Stripes of neurones within the visual cortex that respond preferentially to input from one eye over the other

• Dominance columns from each eye lie next to each other
• The temporal crescent of the visual field is monocular (no dominance column)

• Monocular deprivation in early life leads to uneven distribution of dominance columns but the distribution is equal if development is normal

Magnocellular vs parvocellular fibres

• Parvocellular: slow fibres carrying foveal and parafoveal signals including for spatial discrimination and colour to layers 3-6 of the LGN

• Slow conduction, slower response and smaller receptive fields

• Magnocellular: fast fibres transmit light detection (from peripheral retina) to layers 1 and 2 of the LGN

• Responsive to motion, flicker and light 
• Large dendritic trees contributing to large receptive fields
• Non-linear spatial summation