Visual pathways

Editor's Notes

• #12: Begins anatomically at optic disc but physiologically & functionally within the ganglion cell layer that covers retina Outgrowth of the cerebral vesicle, develops from nerve fibre layer of retina than grow into optic stalk by passing through choroidal fissure and pass posteriorly to brain. Glial system of nerve develops from neuroectodermal cell of outer wall of optic stalk. Myelination of nerve fiber begins from chiasma at 7th month grow distally to reach lamina cribriosa before birth. Not covered by Neurilemma so does not regenerate when cut. Fibres are 2-10 um in diameter & 47-50mm long. Surrounded by meninges . Both the first order(bipolar cells) & second order(ganglion cells) neurons are in the retina.
• #14: Extends from anterior surface in contact with vitreous to a plane which is in parallel with posterior scleral surface
• #15: 1)Surface nerve fiber layer covered by inner limiting membrane of Elschnig,center portion of this membrane is thickened and is called central meniscus of Kuhnt composed of astrocytes(10%)& nerve fibers of retina is in continuity with inner limiting membrane of retina 2)Prelaminar Region Neurons & astroglial tissue Border tissue of Jacoby separates it from choroid 3)Lamina Cribrosa dense band ofConnective and elastic tissue and contains fenestrations for passage of nerve fibres and blood vessels embrased by trabeculae. Maintains IOP bet intra and extra ocular spaces. Blood Supply from Circle of Zinn 4) Retrolaminar Myelinated nerve fibres DECREASE IN ASTROCYTES, Oligodendrocites are in large amount  form the myelin This doubles the diameter from 1.5mm to 3 mm Invested by thick sheath of dura, arachnoid,pia Carry blood vessel to optic nerve
• #16: Back of the eyeball upto optic foramina Covered by all 3 meninges Posteriorly near optic foramen, optic nerve is closely surrounded by annulus of zinn and origin of 4 rectus muscle Some fibers of superior & medial rectus are adherent to sheath here painful ocular movements(elevation and adduction) seen in retrobulbar neuritis.
• #17: Lie above the cavernous sinus and converge with other side optic nerve to form optic chiasma over the diaphragm sellae Covered by pia only Above lie the gyri recti of the frontal lobes of the brain. Lateral side lie the internal carotid artery, or alternatively the anterior cerebral and middle cerebral arteries. Ophthalmic artery lies to the lateral side and below the nerve. Proximity of cavernous sinus makes it possible for tumors to produce cranial nerve palsies in combination with an optic neuropathy.
• #34: Function : 1)relay station 2) to gate the transmission of signals Large magnicellular neurons (M cells) - 1 and 2 –receive input from Y ganglion cells of retina(10% seen in periphearl retina) Motion detection transmitts only black and white information Small parvicellular neurons (P cells) - 3,4,5,6 layers-receive input from X ganglion cells of retina Colour perception –texture shape fine depth vision
• #37: Fibres from inferior retina (Meyer’s loops) Pass through temporal lobe looping around the inferior horn of lateral ventricleInformation from the superior part of visual fieldLoss of vision in superior quadrant ( quadrantanopia or ‘pie in sky defect) Fibres from superior retina (Baum’s loop) Parietal lobe occipital lobe internal capsule visual cortex Information from the inferior part of visual field. Vascular lesion of internal capsulehemiplegia & homonymous hemianopia
• #38: The fibres of optic radiation spread out fanwise to form medullary optic lamina The inferior fibres of optic radiation subserve upper visual field & the superior fibres subserve lower visual field . Ends in a extensive area of thin occipital cortex in which is a distinctive white stripe , striae of Gennari visible to naked eyehence the name areastriate
• #42: Called cortical retina since true copy of retinal image is formed here. Only in visual cortex impulse originating from corresponding point of two retina meet. Right visual cortexreceive impulse from temporal half of right retina & nasal half of left retina(left half visual field) Left visual cortexreceive impulse from temperal half of left retina & nasal half of right retina & (right half visual field)
• #46: VISUOSENSORY AREA:Medial aspect of the occipital lobe around and in the calcarine sulcus, with extension into cuneus and lingual gyrus variably into lateral aspect of occipital pole limited by sulcus lunateus (Brodmann area 17 or V1) VISUOPSYCHIC AREA:peristriate area 18 &19 V1- primary visual area V2- greater part of area 18 V3- narrow stripe over anterior part of area 18 V4- within area 19 V5- posterior end of superior temporal gyrus
• #47: Upper Nasal fibres involved first by lesions coming from above eg . Craniopharyngiomas Lower nasal fibres first affected in tumours of pituitary gland  upper temporal field defects. Ipsilateral blindness is associated with contralateral field defects.
• #48: neuron of LGB form the 3rd order neurons There is point to point representation of retina in LGB SUCH THAT AREAS FROM CORRESPONDING PART OF 2 RETINA END IN NEIGHBOURING PART OF ADJACENT LAMINAE smallest lesion of retina results in degenration of 3 lamellae of LGB in which the retinal fiber end OPTIC RADIATION commence from all 6 laminae  so lesion of visual cortyex degenration of all 6 laminae Fibres from corresponding part of 2 retina end adjacent to each other So lesion in the retina will cause degeneration of 3 laminae in which the fibres end Since the optic radiation begin from all 6 laminae , lesion in cortex will cause degeneration of all 6 laminae .
• #49: Ventral system(pavocellular):Recognition/identification, Long term stored representations,inputMainly foveal or parafoveal Dorsal system(magnocellular):spatial location, Only very short-term storage, Across retina