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Neuroanatomy of the Visual Pathways

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NEUROANATOMY OF THE VISUAL PATHWAYS Magrane Basic Science Course June 2018 Central Nervous System All 5 divisions involved in ocular function and/or adnexa Telencephalon Cerebral hemispheres Diencephalon Thalamus and hypothalamus Mesencephalon Midbrain Metencephalon Cerebellum and pons Myelencephalon Medulla oblongata Brain Divisions (midsagittal plane) Telencephalon (cerebrum) Cerebellum (dorsal metencephalon) Diencephalon Mesencephalon Myelencephalon (midbrain) Pons (medulla oblongata) (ventral metencephalon) Brain Divisions on MRI (midsagittal) Telencephalon (cerebrum) Cerebellum (dorsal metencephalon) Foramen magnum – Diencephalon junction of brain and spinal cord Mesencephalon Myelencephalon (midbrain) Pons (medulla oblongata) (ventral metencephalon) Telencephalon Site of awareness, initiation of voluntary movements and perception stimuli Functions in perception and integration of vision as well as voluntary control of eye movements Cerebral Lobes Sensory Processing Audition Sensory Processing Behavior Somatosensory Memory Parietal lobe Temporal lobe Learning Behavior Occipital lobe Intellect Frontal lobe Sensory Processing Vision Motor function Piriform lobe Sensory Processing Olfaction Occipital Lobe Caudal part of cerebral hemisphere Not well delineated Contains visual cortex Visual perception Occipital Lobe Occipital lobe occupies the caudal 1/3 of cerebral hemispheres Right and left occipital lobes meet between the cerebral hemispheres across the longitudinal fissure There is no definite line or sulcus to demarcate the borders of the occipital lobe Dog: includes parts of the marginal, ectomarginal, caudal suprasylvian, caudal composite, splenial and occipital gyri. Dog: endomarginal and ectomarginal gyri and sulci are part of the parietal lobe (rostral 2/3) and the occipital lobe (caudal 1/3). 15. ectomarginal 16. marginal 17. endomarginal 18. caudal suprasylvian 19. occipital Occipital Lobe ectomarginal, marginal, endomarginal, caudal suprasylvian Occipital Lobe ectomarginal, marginal, endomarginal, caudal suprasylvian, caudal composite Occipital Lobe Splenial, occipital Occipital Lobe Splenial, occipital Occipital Lobe Brodmann’s area 17 = striate = visual I Lies in the posterior pole of the occipital lobe In the cat, it occupies the posteromedial portion of the cortex, extending from the crown of the lateral gyrus on the dorsal surface to the superior bank of the splenial sulcus on the medial surface In the dog, it is located at the junction of the marginal and endomarginal gyri Brodmann’s area 18 = parastriate = visual II Brodmann’s area 19 = peristriate = visual III Area 18 & 19 – may be referred to as extrastriate cortex or as visual association area Occipital Lobe extent of visual resolution in a species reflected in the surface area of the striate cortex the magnification factor (a term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis) the number of visually responsive areas hedgehog: 3; mouse: 4; cat: > 12; nonhuman primates: > 30 Occipital Lobe point of central vision is located in the striate cortex varies in position between species and perhaps between breeds important when recording visual evoked potentials stereotaxic coordinates of the area centralis in the feline cortex are P3-L5 (3 mm posterior to the interaural plane and 5 mm lateral to the midline). Anatomically, it is located on the crown of the lateral gyrus, near the junction of the lateral and posterior lateral gyri. Another source listed this area in the cat as the junction of marginal and endomarginal gyri. on average, the projection of the canine area centralis is 13.4 mm anterior to the interaural plane and 8.4 mm lateral to the midline. Beagle: 11.3 mm rostral to the interaural plane and 8.3 mm lateral to the midline; Greyhound: 15.6 mm rostral to the interaural plane and 8.5 mm lateral to the midline. Visual areas 17, 18, and 19 of the cat brain Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri Occipital Lobe cells of the primary visual cortex, visual I, are arranged in 6 layers layer 4 is heavily myelinated; in this layer the incoming LGN axons synapse with cortical neurons magnocellular (stereopsis, movement, directionality and contrast sensitivity) projections synapse in layer 4Ca parvocellular (spatial resolution and color sensitivity) projections synapse in layer 4Cb most of the neurons are GABAnergic, inhibitory neurons that do not project outside of visual I and are devoted to processing of the signal in the striate cortex a minority of the cells are excitatory, spiny (stellate or pyramidal) neurons that project outside of area 17 Occipital Lobe basic cortical unit that processes an incoming signal is termed a column, which descends through all six layers of the cortex vertical penetration through six cortical layers of the column will result in passage through cells with approximately identical receptive fields therefore, adjacent retinal receptive fields project onto adjacent columns in visual I no difference in size between columns serving central and peripheral retina rather, more columns are used to process visual input from the central retina Occipital Lobe area visual I receives input from the DLGN input consists of the entire contralateral visual hemifield as projected on both retinas this visual hemifield is mapped on the surface of the cortex in a retinotopic manner (adjacent loci of the contralateral visual hemifield are projected onto adjacent loci of the cortex in a simple, point to point manner) each visual hemifield projects onto the cortical surface vertical meridian is a vertical line of demarcation that passes through the area centralis (or fovea) and divides the retina into a nasal hemifield (projected to the contralateral cortex) and a temporal hemifield (projected to the ipsilateral cortex) Visual hemifield on the cat cortex Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri Occipital Lobe Large cortical areas are devoted to processing signals originating from the area centralis magnification factor: term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis In the retina, each degree of the visual field is projected onto a similar-sized retinal area, regardless of whether it is peripheral or central increased resolution and processing achieved by the central retina is obtained by increasing the density of ganglion cell or photoreceptor population In the cortex, the density of neurons serving the peripheral or central fields is identical increased cortical visual discrimination from the area centralis is a result of the increased cortical area devoted to representing the area centralis larger area results in magnification of its representation Occipital Lobe Hedgehog: the surface area of the striate cortex is 20 mm2 and ½ of this area is devoted to representing the central 35 degrees of the contralateral visual hemifield Cat: the surface area is 380 mm2 and ½ of this area is devoted to the central 20 degrees of the visual field. In other words, ½ of visual I is devoted to the central 20 degrees of the visual field and ½ is devoted to the rest of the visual field Occipital Lobe Afferent connections to visual areas come from the LGN via optic radiation of internal capsule (main white matter connection between hemisphere and rest of brain) there are also reciprocal connections with other lobes and with parastriate/peristriate areas Efferent connections from visual areas include the association areas (long and short association fibers connect visual cortex with other lobes of the same hemisphere such as motor cortex at frontal/parietal lobe) the opposite hemisphere via corpus callosum the brain stem (to LGN and rostral colliculus, pontine nuclei and reticular formation) Telencephalon Structure of the telencephalon involved with vision is the occipital cortex Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri Diencephalon thalamus, hypothalamus thalamus receives, processes and relays to the cerebral cortex information from other regions of the brain and most sensory information. Only the olfactory information does not pass through the thalamic relay Structures of the diencephalon involved with vision are the optic chiasm, optic tract, lateral geniculate body (LGB)/lateral geniculate nucleus (LGN), internal capsule and hypothalamus Diencephalon Diencephalon Landmarks - Ventral View Optic chiasm Mamillary bodies Pituitary (hypophysis) Optic chiasm Usually detached Hypothalamus Infundibulum Opening communicates with third ventricle Hypothalamus surrounds infundibulum Diencephalon – Visual Pathway Optic nerve (CN II) Optic chiasm Optic nerve Majority of optic n. fibers cross midline Optic chiasm Optic tract Optic tract Projection from chiasm toward brain Lateral to remainder of diencephalon Travels in caudodorsal direction to lateral geniculate nucleus Lateral Lateral geniculate nucleus geniculate sends input to occipital nucleus cortex in telencephalon Optic tract Optic chiasm Diencephalon Structures of the diencephalon involved with vision are the optic chiasm, optic tract, lateral geniculate nucleus (LGN), internal capsule and hypothalamus Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology
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