Anatomy of the visual pathways and visual cortexPresentation Transcript
Anatomy and Physiology of the Visual pathways Raed Behbehani , MD, ABO
The optic nerve 1.2 Milllion RGC axons. Different types of RGC (parvocellular, magnocellular). Parvocellular : high spatial resolution, color vision, fine stereopsis. Magnocellular : low spatial resolution, motion, gorss stereopsis. Parvocellulae layer 4c-alpha Magnocellular layer 4c beta.
Optic Nerve Lamina cribrosa provides structural support. Lamia cribrosa: Type I,III,V,VI with intersposed elastic fibers. Ophthalmic artery short PCA (segmental) Watershed area (PCA anastomoses are scant). Circle of Zinn-Haller.
Intracanalicular Optic Nerve Within the two bases of the LWS. Medial wall of canal forms lateral wall of sphenoid sinus (can be absent !). Within canal : meninges, ophthalmic artery and sympathetic plexus. 10 mm length. Tight space ! Internal carotid artery.
Intra-orbital Optic Nerve Myelination (oligodendrocytes). 20-30 mm Long. Axons: mylein and glial cell (metabolic support at the nodes of Ranvier).
Optic nerve (Intracranial) Leaves the cranial end of the optic canal (medially, backwards, upwards). 4-15 m (depending on the position of chiasm). Upward 45 degree-angle. Anterior cerebral and anterior comunicating artery lie superior.
Physiology of the Optic Nerve Axoplasmic transport : clearance of expired organelles, structural maintainance, and energy requirements. Interruption of axoplasmic transport : ischemia, compression, inflammation. Orthograde axonal transport : away from the cell body LGN. Retrograde axonal transport : toward cell body.
Physiology of the Optic Nerve Orthograde fast and slow speeds. Proteins, transmitters transported in smooth surface vesicles at 400 mm/day (5 hours to LGB). Elements of cytoskeleton (microtubules, neurofilaments) dependant on actin, kinesin and dynein. Mitochondria moving in both orthograde and retrograde fashion.
Optic chisam Floor of the third ventricle. 5-10 mm above the diphragma sella and the hypophysis cerebri. 12mm wide, 8mm A-P , 4 mm thick. Important relations: 3rd ventricle, hypothalmus, pituitary stalk, sella, dorsum sellam anterior and posterior clinoid processes, cavernous sinus. Nasal fibers cross ; temporal fibers do not (53:47). Wilband’s knee.
Chiasm 90% of chiasmal fibers have macular origin (superior and posterior portions of chiasm). Supraclinoid portions of carotid artery lie lateral to chiasm.
Optic tracts Travel around the cerebral peduncles at dorsal midbrain. Divides into lateral root LGN , and a smaller medial root pretectal area (pupillary light reflex).
Lateral Geniculate Bodies Part of the thalamus. Hilum, medial and lateral horn. Six laminae (layers 1-6), crossed fibers1,4,6 , uncrossed fibers 2,3,5. medial lateral
LGN Upper quadrant medial aspect of LGN, Lower quadrant lateral aspect of LGN. Macular fibers central wedge of LGN. Layers 1,2: magnocellular. (motion) Layers 3-6: Parvocellular. (color)
Optic radiations Nerve fibers bundles with cell bodies in the LGN. Loop of Meyers (around temporal and inferior horn of LV). Inferior fascicle. Superior fascicle.
Optic radiations Inferior fascicle anterior pole of temporal lobe lower calcarine cortex. Superior fascicle parietal lobe upper calacrine cortex.
Visual cortex Upper bank and lower bank (Calcarine fissure). LGN input into layer 4 (Stria of Gennari ). Inferior visual filed (upper bank) , Superior visual field (lower bank).
Visual cortex-Maculr projections represented by50%-60% of the area of the calcarinecortex.-Occipital tip is for foveal vision.-Anterior striate cortex (8%-10%) ismonocularly innervated (temporalcrecsent of contralateral eye).
Visual association areas
Visual association areas Ventral stream (occipitotemporal) : “what” pathway i.e. object recognition , continuation of the parvocellular pathway. V4- color perception (lingual,fusiform gyri) Dorsal stream (occipitoparietal): “where” pathway i.e. spatial orientation , continuation of magnocellular pathway. V5-motion perception.
Higher visual functions
Problem 1 72 year old with progressive vision loss over the last 2 years. Visual acuity: 20/20 , 20/40
Problem 2 A 55 year old patient with sudden, pain less vision loss in her left eye. Visual acuity : OD-20/200 OS-20/25 Color vision : OD- Nil OS-8/8