Visual pathway Anatomy and Physiology.pptx

Anatomy and Physiology

1. VISUAL PATHWAY ANATOMY & PHYSIOLOGY Presented By :- Dr. Yunita Yadav Sec.DNB Resident Dept.Of Ophthalmology DDUH

2. Visual pathway consists of a series of cells & synapses that carry visual information from environment to brain for processing.

3. RETINA

4. ARRANGEMENT OF NERVE FIBRES IN RETINA

5. OPTIC NERVE 2 nd optic nerve Composed of 1-2 million nerve fibers ; approximately 1.5 mm in diameter, enlarges to 3.5 mm posterior to lamina cribrosa due to myelin sheath; located 3–4 mm from fovea Comparable to sensory tract(white mater) causes absolute scotoma 15° temporal to fixation and slightly below horizontal meridian; approximately 45-50 mm in length ,acquires myelin posterior to lamina cribosa Surrounded by 3 layers of meninges:  dura mater (outer layer; merges with sclera), arachnoid layer, pia mater (inner layer, fused to surface of nerve); space between arachnoid and pia contains cerebrospinal fluid (CSF) ON runs through annulus of Zinn (ring of tendinous origins of the rectus muscles) and enters the optic canal Optic canal:  9 mm long and 5–7 mm wide; thinnest medially, adjacent to ethmoid and sphenoid sinuses; dura of ON fuses with periosteum of canal

6. Parts of optic nerve

7. INTRAOCULAR PART Passes through sclera,choroid and appears in eye as optic disc 1.5mm in diameter Expands to 3mm behind sclera,and acquires meningeal sheath Divides into 4 portion (ant. to post.)

8. INTRAORBITAL PART Extends from back of eyeball to the optic foramina Sinuous course Covered by dura,arachnoid and pia Pial sheath contains capillaries and sends septa to divide nerve into fasciculi The SAS containing CSF ends blindly at sclera but continues intracranially Central retinal artery accompaining vein crosses SAS inferomedially about 10mm from eyeball

9. INTRACANALICULAR PART Closely related to ophthalmic artery OA crosses the nerve inferiorly from medial to lateral side in dural sheath Leaves the sheath at the orbital end of the canal Sphenoid and post ethmoidal sinuses lie medial to it and are separated by a thin bony lamina Applied – may account for retrobulbar neuritis following sinusitis

10. INTRACRANIAL PART Lies above cavernous sinus and converges with its fellow to form optic chiasm Ensheath in pia mater Receives arachnoid and dural sheaths at the point of its entry into optic canal Internal carotid artery runs at first below then lateral to it Medial root of olfactory tract and the anterior cerebral artery lie above it

11. Both ciliary and retinal circulation; centripetal branches from central retinal artery

12. BLOOD SUPPLY INTRA ORBITAL PORTION:  Ophthalmic artery with meningeal anastomoses INTRACANALICULAR PORTION:  pial branches from ophthalmic artery; possibly internal carotid artery (ICA) INTRACRANIAL PORTION:  small vessels from ICA, anterior cerebral and anterior communicating arteries

13. OPTIC CHIASMA Flattened structure 12mm horizontally and 8mm anteropsteriorly Ensheathed by pia and surrounded by CSF Lies over diaphragma sellae 10 mm above pituitary gland Relations of chiasma Anterior : anterior cerebral arteries and communicating arteries Posterior : tuber cinereum,infundibulum,pituitary body,mamillary body,posterior perforated substance Superior : third ventricle Inferior : hypophysis Lateral : extracavernous part of internal carotid artery and anterior perforated substance 55% of ON fibers cross in chiasm:  nasal retinal fibers cross in chiasm to contralateral optic tract (decussating nasal fibers); temporal fibers remain uncrossed; macular fibers run posteriorly (posterior compression leads to bitemporal defect) ‘Knee of von Willebrand’ :  inferonasal retinal fibers cross in chiasm and course anteriorly approximately 4 mm into contralateral ON before running posteriorly; produces junctional scotoma Carotid arteries course on either side of chiasm Blood supply:  ICA; occasionally by anterior cerebral and anterior communicating arteries

14. BLOOD SUPPLY OF OPTIC CHIASMA Arterial supply Branches of anterior cerebral and internal carotid artery via pial plexus Venous drainage - Superior aspect by superior chiasmal vein which ends in anterior cerebral vein - Inferior aspect by pre infundibular vein which drains into basal vein

15. OPTIC TRACTS Cylindrical bundle of nerve fibres Running outwards and backwards from optic chiasma Ends posteriorly in lateral geniculate body Special fibers run to the hypothalamus, contributing to neuroendocrine systems that control diurnal rhythms A major projection leaves the optic tract just before the lateral geniculate body (LGB) to form the brachium of the superior colliculus (also called  optic tectum ) Superior colliculus:  involved in foveation reflexes (receives input from pupillary fibers); injury disrupts eye movements but does not cause visual field (VF) defect Pupillary fibers pass through brachium of superior colliculus to pretectal area, which innervates both Edinger-Westphal subnuclei of CN 3 Optic tract provides retinal input to visual nucleus (pulvinar) in the thalamus Damage to optic tract results in contralateral relative afferent pupillary defect (RAPD) because 55% of fibers cross (greater quantity of nasal fibers [nasal to foveal]), including the large monocular crescent (which corresponds with the extreme nasal retina) Blood supply:  anterior choroidal artery; branches from posterior communicating artery;venous drainage by anterior cerebral vein and basal vein

16. LATERAL GENICULATE BODY Lower fibers lie laterally in optic tract and LGB (90° rotation of fibers) Crossed fibers  (contralateral eye): project to layers 1, 4, and 6 Uncrossed fibers  (ipsilateral eye): project to layers 2, 3, and 5 Layers of LGB can also be categorized by neuronal size: MAGNOCELLULAR NEURONS  (M cells): layers 1 and 2; subserve motion detection, stereoacuity, and contrast sensitivity; project to layer 4C alpha of visual cortex PARVOCELLULAR NEURONS  (P cells): layers 3 to 6; subserve fine spatial resolution and color vision; project to layer 4C beta of visual cortex KONIOCELLULAR NEURONS  (K cells): sit in interlaminar zones and superficial layers; receive input from both retinas and the superior colliculus; may modulate information among other pathways BLOOD SUPPLY:  anterior communicating artery and choroidal arteries Venous drainage by basal veins

17. DORSAL LATERAL GENICULATE NUCLEUS OF THALAMUS 1.Relay function Paired visual transmission from both eyes Dorsal geniculate nucleus: Layer 2,3,5 from lateral half of ipsilateral retina Layer 1,4,6 from medial half of contralateral retina Gating function ( selective information) Inhibitory corticofugal from primary visual cortex Inhibitory reticular areas of mesencephalon Parvocellular pathway Magnocellular pathway Smaller receptive fields Larger receptive fields Slower conduction of impulse Faster conduction of impulse Layer 3,4,5,6 Layer 1,2 More sensitive to color vision More sensitive to black and white vision Less sensitive to low contrast signals more sensitive to low contrast signals and rapidly changing visual stimuli Receive input from P retinal ganglion cells Receive input from M retinal ganglion cells

18. Maunsell JH, Nealey TA, DePriest DD. Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey. J Neurosci . 1990 Oct;10(10):3323-34. doi : 10.1523/JNEUROSCI.10-10-03323.1990. PMID: 2213142; PMCID: PMC6570195.

19. OPTIC RADIATIONS myelinated nerve fibers ; connect LGB to occipital cortex Geniculo -calcarine pathway Superior retinal fibers  (inferior VF): travel in white matter underneath parietal cortex to occipital lobe Inferior retinal fibers  (superior VF): travel around ventricular system into temporal lobe (Meyer’s loop); Meyer’s loop is about 5 cm from tip of temporal lobe; temporal lobe injury causes incongruous homonymous superior quadrantanopia, or a ‘pie-in-the-sky’ VF defect Macular fibers :  travel more centrally than do inferior retinal fibers Blood supply:  middle cerebral arteries

20. VISUAL CORTEX Primary visual cortex (striate cortex, V1, Brodmann’s area 17) medial face of occipital lobe, divided horizontally by calcarine fissure Visual cortex contains a topographic map of the contralateral hemifield; central portion of VF is highly magnified Macular region is posterior, extending slightly onto lateral aspect of occipital lobe Peripheral VF is located anteriorly along calcarine fissure Temporal crescent in each VF (from 55° to 100°) is seen only by nasal retina of ipsilateral eye; located most anteriorly; only site posterior to chiasm that if injured would cause a monocular VF defect; temporal crescent may also be the only portion of VF spared after occipital lobe damage Blood supply:  middle and posterior cerebral arteries;venous drainage by internal occipital vein Visual association areas Areas 18 and 19

21. BLOOD SUPPLY OF VISUAL CORTEX

22. Professor Sohan Singh Hayreh Sohan Singh Hayreh  was an  ophthalmologist , clinical scientist, and professor emeritus of ophthalmology at the  University of Iowa . As one of the pioneers in the field of  fluorescein angiography , he was generally acknowledged to be a leading authority in vascular diseases of the eye and the  optic nerve .For over 60 years, Hayreh was actively involved in basic, experimental, and clinical research in ophthalmology, publishing over 400 original peer-reviewed articles in various international ophthalmic journals, six classical monographs and books in his field of research, and more than 50 chapters in ophthalmic books. He made many seminal observations dealing with the ocular circulation in health and disease, the optic disc and the optic nerve, retinal and choroidal vascular disorders, glaucomatous optic neuropathy, fundus changes in malignant arterial hypertension, ocular neovascularization, rheumatologic disorders of the eye, and nocturnal arterial hypotension. [4]  He was an elected fellow of the  National Academy of Medical Sciences . Much of Professor Hayreh’s research legacy has become so engrained in the modern practice of ophthalmology that many are not aware of the full extent of his work. For example, he was a pioneer in the use of fluorescein angiography to study ocular circulation in health and disease. He was the first to distinguish between ischaemic and non- ischaemic central retinal vein occlusions, and to classify ischaemic optic neuropathies as anterior or posterior. He identified nocturnal hypotension as a risk factor for both non- arteritic anterior ischaemic optic neuropathy and glaucoma. He made significant contributions to the vasogenic theory of glaucoma, and his work on the pathogenesis of papilloedema led to the reintroduction of optic nerve sheath fenestration as a treatment for this condition. Prof. Sohan Singh Hayreh ( 1927–2022)

23. Other terminations of visual fibres Suprachiasmatic nucleus of hypothalamus - Cicadian rhythm Pre tectal nuclei in midbrain - reflex movement of eye - Pupillary light reflex Superior colliculus - Rapid directional movement of the two eyes Ventral geniculate nucleus of thalamus - Body’s behavioural function

24. https://webvision.org.es/part-x-brain-visual-areas/9-1-primary-visual-cortex-by-matthew-schmolesky / Hubel DH. Exploration of the primary visual cortex, 1955–78. Nature. 1982;299:515–524. 

25. LAYERS OF VISUAL CORTEX Simple cells Respond to bars of light,lines,or edges but only when they have a specific orientation 2. Complex cells Preferred orientation of a linear stimulus Less dependent on the location of a stimulus 3. Hypercomplex cells Stimulated only by lines or borders of specific length by specific angulated shapes or by images that have other characteristics

26. SIMPLE CELLS Hubel, D. H., & Wiesel, T. N. (1965). Receptive fields and functional architecture in two nonstriate visual areas (18 and 19) of the cat. Journal of Neurophysiology, 28(2), 230-289.

27.  The retina and the LGN "see" the position of an object, the simple cells see its axis of orientation, the complex cells see the movement of this axis, and the hypercomplex cells see the object's edges and angles

28. OCLULAR DOMINANCE COLUMNS Ocular dominance (OD) columns are alternating, striped regions within the visual cortex's layer 4, specifically in the primary visual cortex (V1), where neurons preferentially process input from either the left or right eye Adams DL, Horton JC. Ocular dominance columns: enigmas and challenges. Neuroscientist. 2009 Feb;15(1):62-77. doi : 10.1177/1073858408327806. PMID: 19218231; PMCID: PMC6913877.

29. CORTICAL MECHANISMS OF COLOUR VISION three types of cone photoreceptor in the retina, which have different but overlapping wavelength tuning curves  'cardinal' mechanisms, which are usually termed black–white, red–green and blue–yellow  primary visual cortex (area V1), a large proportion of neurons respond selectively to colour information Most of these neurons also respond to variations in the brightness of visual stimuli and have preferences to a particular color https://webvision.org.es/part-viii-neural-basis-of-color-vision/7-1-color-vision-by-peter-gouras/

30. COLOR BLOBS Blobs are sections of visual cortex where groups of neurons that are sensitive to colour assemble in cylindrical shapes Present in layer 2 & 3 contain high concentration of the mitochondrial enzyme cytochrome oxidase blobs are arranged in lines and centred on an ocular dominance band in layer IV C Boyd JD, Casagrande VA. Relationships between cytochrome oxidase (CO) blobs in primary visual cortex (V1) and the distribution of neurons projecting to the middle temporal area (MT). J Comp Neurol. 1999;409:573–591.

31. CONTRAST SENSITIVITY Ability to perceive sharp and clear outlines of very small objects, identify minute differences in the shadings and pattern Campbell and Robson proposed that independent mechanisms in the nervous system exist that are highly sensitive to a limited range of spatial frequencies. There is selective orientation and transfer of interocular adaptation effect in the visual cortex at the site of neurons. This helped explain the vital role of these interactions in complex image recognition and magnification generalization M and P Pathways The ganglion cells are subdivided into P cells - These cells have a high spatial resolution These cells are small and slow conducting cells that give input to the parvocellular layer of the lateral geniculate body. M cells - These cells have higher CS and temporal resolution and lower spatial resolution. These are large and flat conducting axons that provide input to the magnocellular layers Channel Theory  visual system is composed of 4 to 6 spatial frequency channels. The CSF channels depend on a series of ganglion cells with varied receptive fields so that they have maximum sensitivity to various spatial frequencies. A partial response is obtained from the ganglion cells when the stimulus size is smaller than the central receptive field. When the stimulus size is larger than the central receptive field, the response from the ganglion cells is reduced

32. LESIONS OF VISUAL PATHWAY

33. JUNCTIONAL SCOTOMA

34. Syndrome  Description Localization Anton syndrome Patients with cortical blindness who are unaware of their visual loss Bilateral occipital lesions Palinopsia Perseveration of the visual image once the stimulus has been removed Occipital lobe lesion Prosopagnosia Inability to recognize familiar faces Bilateral occipitotemporal lesions Achromatopsia Abnormality of color perception Bilateral occipitotemporal lesions Balint syndrome   Triad of  simultanagnosia  (impaired spatial awareness of more than one object at time),  optic apraxia  (difficulty in fixating the eyes), and  ocular ataxia  (visual misreaching ) Bilateral parieto-occipital lobes or the visual association cortex Alexia without agraphia left occipital lobe and splenium of the corpus callosum (visual information from the intact right occipital lobe is unable to reach the language areas “angular”) Occipital Lobe Lesions .

35. Abnormality Description Causes Marcus-Gunn pupil (afferent pupillary defect) The affected pupil does not react to light as briskly as the unaffected pupil unilateral optic neuropathies chiasmal and optic tract lesions Horner syndrome Miosis, with an increase in anisocoria in the dark - Ptosis of the upper and lower lids Variable anhydrosis - slow pupillary redilation in the dark Acquired (see above) or Congenital (there will be a heterochromic iris because of impairment of the pigmentary changes.) Adies -Tonic pupil dilated pupil with poor or absent response to light but preserved near response Holmes Adies syndrome. Triad of dilated pupil with poor or absent response to light but preserved near response, loss of deep tendon reflexes, and abnormalities of sweating. Unknown, may be viral infection of parasympathetic and dorsal root ganglia Argyll-Robertson pupils Pupils are small and irregular with impaired light response and intact near response diabetes or syphilis Localization of Pupil Abnormalities

36. Horner Syndrome Localization Associated Symptoms Consideration Isolated, painful Carotid dissection, cluster headache Sensory level Spinal cord Arm numbness or weakness Brachial plexus Ipsilateral face and contralateral body numbness Medulla Sixth-nerve palsy Cavernous sinus

37. Motility Disturbance Localization/Etiology Weber syndrome (Third-nerve palsy and hemiparesis) Anterior midbrain Benedikt syndrome (third-nerve palsy and contralateral tremor) Red nucleus and third-nerve fascicle Isolated pupil-involving third-nerve palsy Posterior communicating artery aneurysm Pupil-sparing third-nerve palsy Microvascular ischemia of the third nerve Isolated fourth-nerve palsy Doral midbrain/anterior medullary velum, Microvascular ischemia Isolated sixth nerve palsy Pons or sixth-nerve fascicle, Demyelination/microvascular ischemia Gaze palsy and facial weakness Dorsal pons/facial colliculus Third-, fourth-, and sixth-nerve palsies Cavernous sinus Third-, fourth-, sixth-nerve palsies, and optic neuropathy Orbital apex Internuclear ophthalmoplegia Medial longitudinal fasciculus Gaze palsy Dorsal pons Parinaud syndrome (upgaze palsy, eyelid retraction) Dorsal midbrain Localization of Ocular Motility Abnormalities

38. VISUAL ADAPTATION DARK ADAPTATION Ability to adapt to decreasing illumination Scotopic vision rods> cones Slower around 20-30 min Mechanism : retinal and opsin coverted back to light sensitive pigment Factors that affect dark adaptation : intensity and duration of the pre-adapting light size and position of the retina are used in measuring dark adaptation wavelength distribution of the light used rhodopsin regeneration LIGHT ADAPTATION Ability to adapt to increasing illumination Photopic vision cones > rods Faster around 5 min. Calcium influx dependent Mechanism : photochemicals reduced to retinal and opsin and vit.A Time course of light adaptation : Anticipatory effect : cortical threshold increases Transient effect Photochemical effect : bleaching and regeneration reach a balanced state

39. VISUAL CYCLE

40. REFERENCES Wolfs anatomy of eye and orbit 8 th edition Textbook of anatomy and physiology of eye 3 rd edition by AK Khurana and Indu Khurana The Eye Basic Sciences in Practice 4 th edition by John V. Forrester https://thebrain.mcgill.ca/

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