anatomy of the visual pathways and visual cortex

Anatomy and Anatomy and Physiology of the Physiology of the Visual pathwaysVisual pathwaysRaed Behbehani , MD, ABORaed Behbehani , MD, ABO

OutlineOutline

The optic nerveThe optic nerve

1.2 Milllion RGC axons.1.2 Milllion RGC axons. Different types of RGC (parvocellular, Different types of RGC (parvocellular,

magnocellular).magnocellular). Parvocellular : high spatial resolution, Parvocellular : high spatial resolution,

color vision, fine stereopsis.color vision, fine stereopsis. Magnocellular : low spatial resolution, Magnocellular : low spatial resolution,

motion, gorss stereopsis.motion, gorss stereopsis. ParvocellulaeParvocellulae layer 4c-alpha layer 4c-alpha MagnocellularMagnocellular layer 4c beta. layer 4c beta.

RGC axonsRGC axons

http://www.city.ac.uk

Optic NerveOptic Nerve

Lamina cribrosa provides structural Lamina cribrosa provides structural support.support.

Lamia cribrosa: Type I,III,V,VI with Lamia cribrosa: Type I,III,V,VI with intersposed elastic fibers.intersposed elastic fibers.

Ophthalmic arteryOphthalmic artery short PCA short PCA (segmental)(segmental)

Watershed area (PCA anastomoses Watershed area (PCA anastomoses are scant).are scant).

Circle of Zinn-Haller.Circle of Zinn-Haller.

ONH Blood SupplyONH Blood Supply

Optic Nerve Cross Optic Nerve Cross SectionSection

http://neuromedia.neurobio.ucla.edu

Intracanalicular Optic Intracanalicular Optic NerveNerve

Within the two bases of the LWS.Within the two bases of the LWS. Medial wall of canal forms lateral wall Medial wall of canal forms lateral wall

of sphenoid sinus (can be absent !).of sphenoid sinus (can be absent !). Within canal : meninges, ophthalmic Within canal : meninges, ophthalmic

artery and sympathetic plexus.artery and sympathetic plexus. 10 mm length.10 mm length. Tight space !Tight space ! Internal carotid artery.Internal carotid artery.

Intra-orbital Optic NerveIntra-orbital Optic Nerve

Myelination (oligodendrocytes).Myelination (oligodendrocytes). 20-30 mm Long.20-30 mm Long. Axons: mylein and glial cell Axons: mylein and glial cell

(metabolic support at the nodes of (metabolic support at the nodes of Ranvier).Ranvier).

Optic nerve Optic nerve (Intracranial)(Intracranial)

Leaves the cranial end of the optic canal Leaves the cranial end of the optic canal (medially, backwards, upwards).(medially, backwards, upwards).

4-15 m (depending on the position of 4-15 m (depending on the position of chiasm).chiasm).

Upward 45 degree-angle.Upward 45 degree-angle. Anterior cerebral and anterior Anterior cerebral and anterior

comunicating artery lie superior.comunicating artery lie superior.

Physiology of the Optic Physiology of the Optic NerveNerve

Axoplasmic transport : clearance of Axoplasmic transport : clearance of expired organelles, structural expired organelles, structural maintainance, and energy requirements.maintainance, and energy requirements.

Interruption of axoplasmic transport : Interruption of axoplasmic transport : ischemia, compression, inflammation.ischemia, compression, inflammation.

Orthograde axonal transport : Orthograde axonal transport : away away from the cell bodyfrom the cell body LGN. LGN.

Retrograde axonal transportRetrograde axonal transport : toward : toward cell body.cell body.

Physiology of the Optic Physiology of the Optic NerveNerve

Orthograde Orthograde fast and slow speeds. fast and slow speeds. Proteins, transmitters transported in Proteins, transmitters transported in

smooth surface vesicles at 400 smooth surface vesicles at 400 mm/day (5 hours to LGB).mm/day (5 hours to LGB).

Elements of cytoskeleton Elements of cytoskeleton (microtubules, neurofilaments) (microtubules, neurofilaments) dependant on actin, kinesin and dependant on actin, kinesin and dynein.dynein.

Mitochondria moving in both Mitochondria moving in both orthograde and retrograde fashion.orthograde and retrograde fashion.

ChiasmChiasm

ChiasmChiasm

Optic chisamOptic chisam

Floor of the third ventricle.Floor of the third ventricle. 5-10 mm above the diphragma sella and the 5-10 mm above the diphragma sella and the

hypophysis cerebri.hypophysis cerebri. 12mm wide, 8mm A-P , 4 mm thick.12mm wide, 8mm A-P , 4 mm thick. Important relations: 3Important relations: 3rdrd ventricle, hypothalmus, ventricle, hypothalmus,

pituitary stalk, sella, dorsum sellam anterior pituitary stalk, sella, dorsum sellam anterior and posterior clinoid processes, cavernous and posterior clinoid processes, cavernous sinus.sinus.

Nasal fibers cross ; temporal fibers do not Nasal fibers cross ; temporal fibers do not (53:47).(53:47).

Wilband’s knee.Wilband’s knee.

ChiasmChiasm

ChiasmChiasm

ChiasmChiasm

ChiasmChiasm

90% of chiasmal fibers have macular 90% of chiasmal fibers have macular origin (superior and posterior origin (superior and posterior portions of chiasm).portions of chiasm).

Supraclinoid portions of carotid Supraclinoid portions of carotid artery lie lateral to chiasm.artery lie lateral to chiasm.

Wilbrand’s kneeWilbrand’s knee

Optic tractsOptic tracts

Travel around the cerebral Travel around the cerebral peduncles at dorsal midbrain.peduncles at dorsal midbrain.

Divides into lateral rootDivides into lateral root LGN , and LGN , and a smaller medial roota smaller medial root pretectal pretectal area (pupillary light reflex).area (pupillary light reflex).

Lateral Geniculate Lateral Geniculate BodiesBodies

Part of the thalamus.Part of the thalamus. Hilum, medial and lateral horn.Hilum, medial and lateral horn. Six laminae (layers 1-6), crossed Six laminae (layers 1-6), crossed

fibersfibers1,4,6 , uncrossed fibers 1,4,6 , uncrossed fibers 2,3,5.2,3,5.

medial

lateral

LGNLGN

Upper quadrantUpper quadrant medial aspect of medial aspect of LGN, Lower quadrantLGN, Lower quadrant lateral lateral aspect of LGN.aspect of LGN.

Macular fibersMacular fibers central wedge of central wedge of LGN.LGN.

Layers 1,2: magnocellular. (motion)Layers 1,2: magnocellular. (motion) Layers 3-6: Parvocellular. (color)Layers 3-6: Parvocellular. (color)

Optic radiationsOptic radiations

Nerve fibers bundles with cell bodies Nerve fibers bundles with cell bodies in the LGN.in the LGN.

Loop of Meyers (around temporal Loop of Meyers (around temporal and inferior horn of LV).and inferior horn of LV).

Inferior fascicle.Inferior fascicle. Superior fascicle.Superior fascicle.

Optic radiationsOptic radiations

Inferior fascicleInferior fascicle anterior pole of anterior pole of temporal lobetemporal lobe lower calcarine lower calcarine cortex.cortex.

Superior fascicleSuperior fascicle parietal lobe parietal lobe upper calacrine cortex.upper calacrine cortex.

Visual cortexVisual cortex

Upper bank and lower bank Upper bank and lower bank (Calcarine fissure).(Calcarine fissure).

LGN input into layer 4 (Stria of LGN input into layer 4 (Stria of Gennari ).Gennari ).

Inferior visual filed (upper bank) , Inferior visual filed (upper bank) , Superior visual field (lower bank).Superior visual field (lower bank).

Visual cortexVisual cortex

--Maculr projections Maculr projections represented by 50%-60% of represented by 50%-60% of the area of the calcarine the area of the calcarine cortex.cortex.-Occipital tip is for foveal -Occipital tip is for foveal vision.vision.-Anterior striate cortex -Anterior striate cortex (8%-10%) is monocularly (8%-10%) is monocularly innervated (temporal innervated (temporal crecsent of contralateral crecsent of contralateral eye).eye).

Visual association areasVisual association areas

Visual association areasVisual association areas

Ventral stream (occipitotemporal) : Ventral stream (occipitotemporal) : “what” pathway i.e. object recognition , “what” pathway i.e. object recognition , continuation of the parvocellular continuation of the parvocellular pathway.pathway.

V4- color perception (lingual,fusiform V4- color perception (lingual,fusiform gyri)gyri)

Dorsal stream (occipitoparietal): “where” Dorsal stream (occipitoparietal): “where” pathway i.e. spatial orientation , pathway i.e. spatial orientation , continuation of magnocellular pathway.continuation of magnocellular pathway.

V5-motion perception.V5-motion perception.

Higher visual functionsHigher visual functions

Problem 1Problem 1

72 year old with progressive vision 72 year old with progressive vision loss over the last 2 years.loss over the last 2 years.

Visual acuity: 20/20 , 20/40Visual acuity: 20/20 , 20/40

Problem 1 Problem 1

Problem 2Problem 2

A 55 year old patient with sudden, A 55 year old patient with sudden, pain less vision loss in her left eye.pain less vision loss in her left eye.

Visual acuity : OD-20/200 OS-20/25Visual acuity : OD-20/200 OS-20/25 Color vision : OD- Nil OS-8/8Color vision : OD- Nil OS-8/8

Problem 2Problem 2

Problem 2Problem 2

Problem 2Problem 2

Problem 3Problem 3