## The basic visual pathway
<ul>
 <li>The first-order neurone is the photoreceptor (rod or cone), synapses with</li>
 <li>The second-order neurone, a bipolar cell, which synapses with</li>
 <li>The ganglion cell</li>
 <li>
 Ganglion cell axons cross the retina as the nerve fibre layer>
 </li>
 <ul>
 <li>
 The orientation of these fibres causes the patterns of visual field loss if they become
 damaged
 </li>
 <li>
 Two populations: parvocellular >for fine VA and colour;{' '}
 magnocellular >for motion detection and coarser vision
 </li>
 </ul>
 <li>(NFL becomes) the optic nerve</li>
 <li>At the optic chiasm, the nasal fibres cross and the temporal fibres remain uncrossed</li>
 <li>The optic tracts progress to the&nbsp;</li>
 <li>Lateral geniculate body on the posterior surface of the thalamus</li>
 <ul>
 <li>Crossed fibres to laminae 1, 4 and 6</li>
 <li>Uncrossed to laminae 2, 3 and 5</li>
 </ul>
 <li>
 In the LGN the ganglion cell fibres synapse with cells that connect via the optic radiations
 to&nbsp;
 </li>
 <li>Occipital cortex, area 17&nbsp;</li>
</ul>

## Embryology
<ul>
 <li>
 The edges of the optic stalk seal near the forebrain from approx 5-6 weeks and proceed
 distally.&nbsp;
 </li>
 <ul>
 <li>Note: the optic cup seals first</li>
 </ul>
 <li>
 Retinal ganglion cell axons grow towards the stalk and through it to the brain, surrounding the
 hyaloid vessels
 </li>
 <li>
 Bergmeister&rsquo;s papilla: a remnant of glial cells and posterior hyaloid vessels that may
 persist at the optic nerve head
 </li>
 <li>
 The anterior remnant of the hyaloid vessels forms Mittendorf&rsquo;s dot on the posterior lens
 capsule
 </li>
</ul>
<HotTopic>
 <ul>
 <li>The three clinically recognisable remnants of fetal hyaloid vasculature:</li>
 <ul>
 <li>Bergmeister&rsquo;s papilla</li>
 <li>Mittendorf&rsquo;s dot</li>
 <li>Vascular loops (95% of which are arterial)</li>
 </ul>
 </ul>
</HotTopic>
<ul>
 <li>Condensed mesenchyme forms the outer dural layer of the optic nerve</li>
</ul>
<ClinicalCorrelate>
 <ul>
 <li>Coloboma and optic pit</li>
 <ul>
 <li>
 Failure of closure of the choroidal fissure leads to deformity of the optic nerve head at
 the inferonasal border and scleral bulging termed scleractasia
 </li>
 <li>
 Optic pit is a recess with herniation of the neighbouring retina into the meninges
 associated with exudation beneath the macula and serous retinal detachment (with a
 unilateral centrocecal scotoma)
 </li>
 <li>
 Optic disc colobomas> cause enlarged blind spot, arcuate scotomas,
 altitudinal field defects. Causes include
 </li>
 <ul>
 <li>Idiopathic</li>
 <li>CHARGE syndrome</li>
 <li>Sjogren Larson syndrome</li>
 <li>Mechel syndrome</li>
 <li>Aicardi syndrome</li>
 <li>Warburg&rsquo;s syndrome</li>
 <li>Trisomy 22</li>
 <li>Trisomy 18</li>
 </ul>
 </ul>
 </ul>
</ClinicalCorrelate>
<ul>
 <li>Optic nerve fibres are maximal in utero and decrease before birth</li>
</ul>

## Optic nerve
<ul>
 <li>1.2 million ganglion cell axons (myelinated once exited lamina cribosa)</li>
 <li>
 90% are small diameter (1 micron) representing midget ganglion cells associated with cones
 </li>
 <li>Remainder are larger diameter (2-10 microns) representing axons associated with rods</li>
 <li>Average length of 40mm</li>
 <li>Contains</li>
 <ul>
 <li>No neuronal cell bodies</li>
 <li>No interneurones</li>
 <li>Microglial cells: resident macrophages</li>
 <li>Astrocytes: regulate potassium</li>
 <li>Oligodendrocytes: for myelination</li>
 </ul>
</ul>

## Intraocular portion
<ul>
 <li>Optic disc, 3mm nasal to macula, 1.5mm in diameter</li>
 <li>
 Axons from the central retina/cone-system (macula) pass centrally and peripheral axons lie
 peripherally
 </li>
 <li>Arcuate bands from temporal retina/periphery bend around to disc</li>
 <li>Unmyelinated as they bend posteriorly, supported by astrocytes</li>
</ul>
<HotTopic>
 
 The optic nerve becomes myelinated as passes through the lamina cribrosa and doubles in diameter
 
</HotTopic>
<ul>
 <li>
 Distally: macular fibres lie laterally along with temporal fibres and nasal fibres lie medially
 </li>
 <li>Proximally: macular fibres move to occupy central part of nerve</li>
 <li>Blood supply:&nbsp;</li>
 <ul>
 <li>
 Circle of Zinn derived from short posterior ciliary arteries: supply the prelaminar and lamina
 cribrosa regions
 </li>
 <li>
 Note: the surface nerve fibre layer of the ON head is supplied by branches of the central
 retinal artery
 </li>
 </ul>
</ul>

## Orbital portion
<ul>
 <li>25mm long: sinuous course to allow some movement of eyeball</li>
 <li>
 Myelinated from lamina cribosa (formed by the sclera) by oligodendrocytes therefore larger
 diameter
 </li>
 <li>Surrounded by meningeal coverings including dura</li>
 <li>Surrounded by orbital fat</li>
 <li>Ciliary ganglion lies laterally between nerve and lateral rectus</li>
 <li>Ophthalmic artery and nasociliary nerve cross over the nerve above it</li>
 <li>
 Central retinal artery and vein pierce dural coverings 12mm behind eyeball on inferomedial
 aspect
 </li>
 <li>At apex the nerve lies within tendinous coverings of the recti</li>
 <li>Fibres arranged into bundles separated by septa created by pia mater</li>
 <li>
 Blood supply: pial plexus of vessels (derived from branches of ophthalmic). Extraneural part of
 central retinal artery provides some branches (&lsquo;central collateral arteries&rsquo;)
 </li>
</ul>

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

## Intracranial portion
<ul>
 <li>
 Nerve exits canal and passes backwards, medially and upwards to form optic chiasm on floor of
 the third ventricle
 </li>
 <li>Passes within subarachnoid space of middle cranial fossa</li>
 <li>Internal carotid lies laterally</li>
 <li>
 Blood supply: pial plexus derived from superior hypophyseal artery from internal carotid and
 ophthalmic artery
 </li>
</ul>

## Optic chiasm
<ul>
 <li>At junction between anterior wall and floor of third ventricle</li>
 <li>Below lamina terminalis (wall of the forebrain)</li>
 <li>Superior to diaphragma sella (and pituitary)</li>
 <li>
 Posterior to anterior cerebral arteries and anterior communicating artery (and sphenoid sinus)
 </li>
 <li>Medial to internal carotid and cavernous sinus</li>
 <li>12 x 8mm size</li>
</ul>
<HotTopic>
 
 Fibres from the nasal hemi-retinae decussate, take a short loop into contralateral optic nerve
 and then pass into optic tracts (ie. fibres nasal to the fovea)
 
</HotTopic>
<ul>
 <li>
 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)
 </li>
</ul>
<ul>
 <li>Normally: 53% of retinal fibres decussate</li>
</ul>
<ClinicalCorrelate>
 Albinism increases the degree of decussation
</ClinicalCorrelate>
<ul>
 <li>Inferior nasal fibres pass in anterior and inferior chiasm</li>
 <li>Superior nasal fibres pass in posterior and superior chiasm</li>
</ul>
<ClinicalCorrelate>
 <>
 
 Lesions compressing the chiasm result in bitemporal hemianopia but the area of highest density
 can indicate the position of the lesion
 
 <ul>
 <li>
 Inferiorly dense bitemporal hemianopia suggests superior compression eg from a
 craniopharyngioma
 </li>
 <li>
 Superiorly dense bitemporal hemianopia suggests inferior compression eg from a pituitary
 adenoma
 </li>
 </ul>
 </>
</ClinicalCorrelate>
<ul>
 <li>Macular fibres travel in the centre of the optic nerve</li>
 <ul>
 <li>Macular fibres decussate in the posterior superior portion</li>
 <li>
 Temporal macular fibres pass uncrossed and nasal fibres decussate but in albinism some
 temporal fibres also cross
 </li>
 </ul>
 <li>Blood supply: pial plexus from</li>
 <ul>
 <li>Internal carotid</li>
 <li>Superior hypophyseal</li>
 <li>Anterior cerebral</li>
 <li>Anterior and posterior communicating arteries</li>
 </ul>
</ul>

## Optic tracts
<ul>
 <li>Curve around cerebral peduncles adherent to the midbrain</li>
 <li>
 Fibres from corresponding retinal areas become associated: parvocellular fibres run centrally
 and magnocellular fibres run peripherally
 </li>
 <li>Large lateral root terminates in the LGN</li>
 <ul>
 <li>Responsible for conscious visual sensation</li>
 <li>Lateral aspect attached to third ventricle</li>
 </ul>
 <li>Smaller medial root (about 10% of fibres) terminates in subcortical targets:</li>
 <ul>
 <li>
 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
 </li>
 <li>
 Pretectal nucleus>: pupillary light reflexes
 </li>
 <li>Suprachiasmatic nucleus: circadian rhythm</li>
 <li>Pulvinar nucleus: visual attention and motor integration</li>
 <li>Parvocellular reticular formation: arousal</li>
 </ul>
 <li>
 Blood supply: posterior communicating artery and anterior choroidal artery forming pial arteries
 </li>
 <li>Note: the posterior cerebral artery travels beneath and parallel with the optic tract</li>
</ul>

## Pupillary light reflexes
<ul>
 <li>
 As above, a small bundle of fibres breaks off before reaching the LGN and synapses at the
 pretectal nucleus
 </li>
 <li>Fibres from there pass to the bilateral Edinger-Westphal nuclei</li>
 <li>Parasympathetic fibres travel in the oculomotor nerve to the ciliary ganglion</li>
 <li>Postganglionic fibres then travel in the short ciliary nerves to the constrictor pupillae</li>
</ul>

## Lateral geniculate nuclei
<ul>
 <li>Paired thalamic nuclei where retinal ganglion cells synapse</li>
 <li>Round swelling on pulvinar of thalamus</li>
 <li>Connected to superior colliculus via the superior brachium</li>
 <li>Structure: body, head, spur, hilum&nbsp;</li>
 <li>
 Six lamellae of cells, labelled 1-6 (&ldquo;stack of hats&rdquo;) separated by white matter of
 the optic tract fibres
 </li>
 <ul>
 <li>Crossed fibres from the contralateral eye terminate in layers 1, 4, 6</li>
 <li>Uncrossed fibres from the ipsilateral eye terminate in layer 2, 3, 5</li>
 <li>Layers 1 and 2: data from peripheral retina transmitted via the magnocellular pathway</li>
 <li>Layers 3-6: data from fovea transmitted by the parvocellular pathway</li>
 </ul>
 <li>Each retinal ganglion cell may terminate with six geniculate cells within one lamina</li>
 <li>Retinotopic mapping:</li>
 <ul>
 <li>Upper quadrant fibres (inferior visual field) terminate in medial LGN</li>
 <li>Lower quadrant fibres (superior visual field) terminate in lateral LGN</li>
 <li>Note: fibres are rotated 90 degrees</li>
 </ul>
 <li>Macular fibres occupy large central wedge of LGN</li>
 <li>Optic radiations emerge from dome-shaped posterior aspect</li>
 <li>
 Blood supply: anterior choroidal branch of ICA (may come off MCA) (+ thalamogeniculate branches
 of the PCA and lateral choroidal artery)
 </li>
</ul>
<ClinicalCorrelate>
 
 Lesions of the visual pathway posterior to the LGN cause hemianopias without optic atrophy
 whereas lesions anterior to the LGN cause damage to the ganglion cell axons and hence optic
 atrophy.
 
</ClinicalCorrelate>

## Optic radiations
<ul>
 <li>Nerve fibres whose cell bodies lie in the LGN (second order neurones of visual pathway)</li>
 <li>Terminate in the striate visual cortex (area 17)</li>
 <li>
 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.&nbsp;
 </li>
 <ul>
 <li>Those from the upper quadrants pass through the parietal lobe radiations.</li>
 <li>The loop of Meyer passes around the temporal horn of the lateral ventricle</li>
 <li>
 The fibres that flare widest are associated with the peripheral retina; those of the macula
 pass more directly
 </li>
 </ul>
 <li>
 Blood supply: anterior choroidal artery, MCA (supplies the posteriorly directed fibres via the
 lateral striate artery), PCA (produces posterior choroidal artery)
 </li>
</ul>

## Primary visual cortex (Brodmann&rsquo;s area 17)
<ul>
 <li>
 Within the calcarine sulcus/fissure extending into cortex above (cuneus gyrus) and below
 (lingual gyrus)
 </li>
 <li>Receives myelinated fibres from the optic radiations (aka, geniculocalcarine tracts)</li>
 <li>Prominent band of geniculocalcarine fibres termed the stria of Gennari</li>
 <li>Fibres for superior quadrants (inferior field) pass to upper edge of sulcus</li>
 <li>
 Fibres for the macula pass to the posterior part (tip of the occipital lobe) and occupy 1/3rd of
 space
 </li>
 <ul>
 <li>Posterior sulcus lies at the watershed region between the MCA and PCA</li>
 </ul>
</ul>
<HotTopic>
 This accounts for macular sparing seen in some cortical lesions
</HotTopic>
<ul>
 <li>
 Fibres for inferior quadrants (superior field) pass to lower edge of sulcus (below calcarine
 sulcus)
 </li>
 <li>
 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)
 </li>
</ul>
<ul>
 <li>Cortex is arranged into six basic histological layers (similar to all cerebral cortex)</li>
 <ul>
 <li>Layer II: cells synapsing with the contralateral visual cortex via the corpus callosum</li>
 <li>Layers II and III: project to the secondary visual cortex</li>
 <li>Unique white stripe in layer IV: stripe of Gennari formed by myelinated fibres</li>
 <li>
 Layer IV is thickest: receives afferents from LGN (myelinated fibres of the geniculocalcarine
 tract) and contains mostly stellate cells.
 </li>
 <ul>
 <li>Fibres from the macula</li>
 <li>Expanded into distinct sublayers</li>
 </ul>
 <li>
 Layer V: connect to superior colliculus which connects to the MLF enabling ocular movements to
 be coordinated with light
 </li>
 <li>
 Layer VI: sends efferents to the LGN (&ldquo;feed-forward&rdquo;) and contains mostly
 pyramidal cells (cortical geniculate cells)
 </li>
 <ul>
 <li>These synapse with ganglion cells and interneurones in the LGN.</li>
 </ul>
 </ul>
 <li>
 Blood supply: PCA (+/- anastomoses with MCA which account for macular sparing in cortical
 infarcts)
 </li>
 <ul>
 <li>
 Ischaemia of a calcarine artery produces a congruous contralateral homonymous hemianopia with
 macular sparing
 </li>
 </ul>
</ul>
<HotTopic>
 
 The more posterior the lesion, the more congruous the defect due to convergence of fibres
 towards the occipital cortex
 
</HotTopic>

## Ocular dominance columns
<ul>
 <li>
 Stripes of neurones within the visual cortex that respond preferentially to input from one eye
 over the other
 </li>
 <li>Dominance columns from each eye lie next to each other</li>
 <li>The temporal crescent of the visual field is monocular (no dominance column)</li>
 <li>
 Monocular deprivation in early life leads to uneven distribution of dominance columns but the
 distribution is equal if development is normal
 </li>
</ul>

## Magnocellular vs parvocellular fibres
<ul>
 <li>
 Parvocellular: slow fibres carrying foveal and parafoveal signals including for spatial
 discrimination and colour to layers 3-6 of the LGN
 </li>
 <ul>
 <li>Slow conduction, slower response and smaller receptive fields</li>
 </ul>
 <li>
 Magnocellular: fast fibres transmit light detection (from peripheral retina) to layers 1 and 2
 of the LGN
 </li>
 <ul>
 <li>Responsive to motion, flicker and light&nbsp;</li>
 <li>Large dendritic trees contributing to large receptive fields</li>
 <li>Non-linear spatial summation</li>
 </ul>
</ul>

The basic visual pathway The first-order neurone is the photoreceptor (rod or cone), synapses with The second-order neurone, a bipolar cell, which…

Vision

Visual Pathway

Intro

FRCOphth Part 1 Study Notes

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