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Chapter 10

NROC64 - Chapter #10 Notes.docx

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Matthias Neimier

NROC64 – Chapter #10: The Central Visual System:  Retina extracts information about different facets of the visual image  More than 100 million photoreceptors in the retina but only 1 million axons leaving the eye carrying info to the rest of the brain  Pathway leaving conscious visual perception includes the LGN of the thalamus and the primary visual cortex (Area 17), V1 or striate cortex The Retinofugal Projection:  Retinofugal projection - neural pathway that leaves the eye beginning with the optic nerve o Centrifugal projection = away from the center o Corticogugal projection goes away from the cortex  Retinofugal = away from the retina The Optic Nerve, Optic Chiasm and Optic Tract:  Ganglion axons pass through three structures before reaching the brain stem  Components are: optic nerve  optic chiasm  optic tract  Optic nerves – exit the right and left eyes at the optic disks and pass through holes in the floor of the skull  Optic chiasm – axons originating in the nasal retinas cross from one side to the other o Decussation - crossing of a fiber bundle from one side of the brain to the other o b/c only axons from the nasal retinas cross = partial decussation at the optic chiasm  Optic tracts – axons form these tracts which run just under the pia along the lateral surfaces of the diencephalon Right and Left Visual Hemifields:  Left visual hemifield – objects appearing to the left of the midline (vice versa) o Objects in the binocular region of the left visual hemfield are imaged on the nasal retina of the left eye and on the temporal retina of the right eye o This is because the temporal regions are encoded by the opposite side of the brain o Nasal are encoded by the same side of the brain (ipsilateral) o Optic nerve fibers cross in the optic chiasm  Binocular visual field – central portion viewed by both retinas Targets of the Optic Tract:  Small # of optic tract axons form synaptic connections with cells in the hypothalamus  10% continue past thalamus to innervate the midbrain  Most innervate the LGN of the dorsal thalamus o Neurons in LGN give rise to axons that project to the primary visual cortex o Projection from LGN  cortex = optic radiation  Lesions in the retinofugal projection cause blindness  Cut in the left optic nerve = blind in left eye  Cut in the left optic tract = blindness in right visual field  Midline cut in the optic chiasm = only affects fibers that cross the midline Nonthalamic Targets of the Optic Tract:  Direct projections to the hypothalamus play an important role in synchronizing a variety of biological rhythms including sleep, wakefulness with the daily dark-light cycle  Direct projection to the midbrain (pretectum) control the size of the pupil and certain types of eye movement  10% (150 000 neurons) of the ganglion cells in the retina project to a part of the midbrain tectum called the superior colliculus (little hill)  Tectum of the midbrain major target of retinofugal projection in all nonmammalian vertebrates o In these the superior colliculus = optic tectum o Projection from retina  superior colliculus is called the retinotectal projection in mammals  Superior colliculus a patch of neurons activated by a point of light commands head, eye movements to bring the image to the fovea Box 10.1 – David and Goliath:  Body size is regulated by secretion of growth hormone form the anterior lobe of the pituitary gland  If anterior lobe swollen (hypertrophied) it produces excess amounts of hormone resulting in body growth of unusually large proportions  This is called pituitary hypertrophy – also disrupts normal vision  Loss of peripheral vision b/c enlarged pituitary, results in a loss of peripheral vision = tunnel vision/bitemporal vision The Lateral Geniculate Nucleus:  Right and left lateral geniculate nuclei in the dorsal thalamus = major targets of the 2 optic tracts  Each LGN appears to be viewed in 6 layers of cells (most ventral layer = 1)  Layers arranged like a pancake bent around the optic tract  LGN = gateway to visual cortex (visual perception) The Segregation of Input by Eye and by Ganglion Cell Type:  LGN neurons receive synaptic input from the retinal ganglion cells  Segregation of LGN neurons in layers suggest that diff types of retinal info are being kept separate o Axons from M-type, P-type, nonM-nonP ganglion cells in the 2 retinas synapse of diff LGN layers o LGN: right eye (ipsilateral) axons synapse on LGN layers 2, 3, 5 o Left eye (contralateral axons synapse on layer 1, 4, 6 o Layers 1 and 2 contain larger neurons and the 4 more dorsal layer 3-6 contain smaller cells o Ventral layers = magnocellular LGN layers and dorsal layers = parvocellular LGN layers o P-type ganglion cells in retina project to parvocellular LGN o M-type ganglion cells  magnoceullular LGN o Koniocellular layers- tiny neurons b/w layers that receive inpit from the nonM-nonP types of retinal ganglion cells and project to visual cortex  Info derived from 3 categories of retinal ganglion cells and from 2 eyes remains segregated  Retina give rise to streams of info that are processed in parallel Receptive Fields:  Inserting a microelectrode into LGN can study potential discharges of geniculate neurons in response to visual stimuli  Visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them  Magnocellular LGN neurons: relatively large centre-surround receptor fields, respond to stimulation with a transient burst of APsand are insensitive to diff in wavelength (like M-type ganglion cells)  Parvocellular LGN cells are like P-type retinal ganglion cells – small centre-surround receptive fields, respond to stimulation of receptive field centers with a sustained increase in the freq of Aps, exhibit colour opponency  Receptive field of cells in koniocellular layers – center-surround and have either light/dark opponency  Within all layers of LGN the neurons are activated by only one eye Non-Retinal Inputs of the LGN:  Retina is not the main source of synaptic input to the LGN  Major input (about 80% excitatory synapses) comes from the primary visual cortex  LGN also receives synaptic inputs from neurons in the brain stem (activity: related to alertness and attentiveness)  LGN is the 1 site in the ascending visual pathway where what we see is influenced by how we feel Anatomy of the Striate Cortex:  LGN has a single major synaptic target: primary visual cortex (aka area 17, V1, striate cortex) located in the occipital lobe, surround the calcarine fissure  Close correlation b/w structure and function Retinotopy:  Projection from retina  LGN and V1  Retinotropy – organization whereby neighbouring cells in the retina feed info to neighboring places in their target structures – LGN and striate cortex o 2-D structure of retina is mapped onto 2-D surface of the subsequent structures o 3 important points:  Mapping of the visual field onto a tetinotopically organized structure is often distorted (central few degrees of the visual field are overrepresented or magnified in the retinotopic map)  Discrete point of light can activate many cells in the retina due to the overlap of receptive fields  Don’t be misled by the word “map” – perception is based on the brain’s interpretation if distributed patterns of activity Lamination of the Striate Cortex:  Neocortex and striate cortex have neuronal cell bodes arranged into about half a dozen layers  Just under the pia mater is largely devoid of neurons and consists entirely of axons and dendrites  Full thickness of the striate cortex from white matter to pia = 2 mm  At least 9 distinct layers in reality of striate cortex, however 3 layers are combined The Cells of Different Layers:  Spiny stellate cells – small neurons with spine-covered dendrites that radiate out from the cell body, primary in 2 tiers of layer IVC  Pyramidal cells – outside layer of IVC, covered with spines, a single thick apical dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that extend horizontally  Only pyramidal cells send axons out of the striate cortex to form connection with other parts of the brain  Axons of stellate cells make local connections only within the cortex  Inhibitory neurons which lack spines are in the cortical layers as well, these neurons only form local connections Inputs and Outputs of the Striate Cortex:  In LGN every layer receives retinal afferents and send efferent to the visual cortex  Visual cortex only a subset of layers receives input from the LGN or sends output to a diff cortical/subcortical area  Axons form the LGN terminate in several diff cortical layers with the largest # going to IVC layer  Output of LGN divided into streams of info  These streams anatomically segregated in layer IVC  Magnocellular LGN  IVC alpha, Parvocellular LGN  IVC beta Ocular Dominance Columns:  Wanted to know how the left and right eye LGN inputs segregated when they reach layer IVC of the striate cortex  Ocular dominance columns – distribution of axon terminals relaying info was not continuous in layer IVC but rather was split up into a series of equally spaced patches each about 0.5 mm wide Innervation of Other Cortical Layers form Layer IVC:  Radial connections – maintain the retinotopic organization established in layer IV  Therefore a cell in layer IV receives the same input as the cell above it in layer IV  Axons of some layer III pyramidal cells extend collateral branches that make horizontal connections within layer III  Radial and horizontal connections play diff roles in analysis of the visual world  Layer IVC stellate cells project axons radially up mainly to layers IVB and III where for the first time info from right and left eye mix  All layer IVC neurons receive only monocular input most neurons in layers II and III receive binocular input coming from both eyes  IVC alpha (magnocellular)  IVB, Layer IVC beta (parvocellular)  layer III  III and IVB an axon may form synapses with the dendrites of pyramidal cells of all layers Striate Cortex Outputs:  Pyramidal cells send axons out of striate cortex into the white matter  Layer II, III and IVB  axons to other cortical areas  Layer V  axons to superior colliculus and pons  Layer VI 
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