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Chapter 10

NROC64 Chapter 10 Notes.docx

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Matthias Niemeier

NROC64 Chapter 10 Notes Retinofugal projection- neural pathway that starts at optic nerve. (going away from center) Ganglion cell axons fleeing the retina pass through 3 structures before they synapse on the brain stem: the optic nerve, the optic chiasm and the optic tract. Optic nerves- exit the left and right eye at fatty disks, travel through fatty tissue behind eyes and pass through holes in the skill. They form an x to form the optic chiasm which is at the base of the brain, anterior to the pituitary. At optic chiasm, axons originating in the nasal retinas cross from one side to the other. Decussation- A crossing of fibre from one side of the brain to the other This is only a partial decussation- only axons originating in the nasal retinas cross. Then the axons form optic tracts and run under the pia along lateral surface of the diencephalon Full visual field- what can be seen with both right and left eye- divided into left and right visual hemifield. The central portion of both hemifields is viewed by both retinas- this is called the binocular visual field. Objects in the binocular region of the left visual hemifield will be imaged on the nasal retina of left eye and on temporal of right eye. All the info about left field is directed to right hemispheres because of the decussation. Some optic tract axons peel off to form synaptic connections with cells in the hypothalamus and another 10% continue past the thalamus to innervate the midbrain Most of them innervate the lateral geniculate nucleus (LGN) of the dorsal thalamus. Neurons from LGN project to primary visual cortex, this is called the optic radiation. Lesions anywhere in the retinofugal projection from eye to LGN cause blindness in humans. Transection of left optic nerve- blind in left eye Transection of left optic tract- blindness in the right visual field If the midline of the optic chiasm is transected, there would be blindness in the regions of the visual field as viewed by the nasal retinas- the peripheral visual fields on both sides. Other than LGN- the hypothalamus is also innervated. Part of hypothalamus plays a role in synchronizing biological rhythyms, sleep and wakefulness with darklight cycles. - the pretectum, an area of the midbrain controls the size of the pupil and certain types of eye movement. - About 10% of projections go to a part of the midbrain tectum called the superior colliculus (this is the major target of the retinofugal projection in all nonmammalian vertebrates – fish, birds, reptiles). In these groups, the superior colliculus is called the optic tectum.- sometimes the projection form the retina to the superior colliculus is called the retinotectal projection (for mammals too). IN the superior colliculus, a patch of neurons activated by a point of light via indirect connections with motor neurons in the brain stem commands head and eye movements to bring the image of this point in space onto the fovea. – Superior colliculus branch- involved in orienting eyes in response to NEW stimuli in the visual periphery LGN - located in dorsal thalamus, have 6 different layers of cell, layer 1 being the most ventral. The layers are bent around the optic tract. - LGN = gateway to visual cortex - LGN receive input from retinal ganglion and project on to axons of primary visual cortex via optic radiation. The segregation of neurons into layers suggests that different types of retinal info are kept separate. - the right LGN receives info about the left visual field (viewed by the temporal right retina and the nasal left) - at LGN both eyes are kept separate. Right LGN- right eye (ipsilateral) axons synapse on cells in layers 2,3, and 5. The left eye, (contralateral) synapses on LGN layers 1,4,6 . - 2 ventral layers, 1 and 2 – contain larger neurons. Ventral layers- Magnocellular LGN layers - 4 dorsal layers 3-6 – contain smaller cells.- Parvocellular LGN layers. - P type ganglion cells in the retina project exclusively to the parvocellular LGN and M type cells project entirely to the magnocellular. - There are also numerous tiny neurons that lie ventral to each layer- cells in these koniocellular layers receive input from the nonM-nonP types ganglion cells and project to visual cortex. These cells aren’t numbered - The different types of ganglion cells (P,M, NonM-nonP) respond diff. to light and colout. The diff. ingo derived remains segregated. - Retina gives rise to streams of info processed in parallel. - the visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them. Ex. Magnocellular LGN neurons- have large-center surround receptive fields, respond to stimulation of their receptive field centeres with transient burst of AP and are insensitive to difference in wavelength- this is just like Mtype Ganglion cells. - Parvocellular LGN cells – like P type retinal cells- have small center surround receptive fields, respond to stimulation of receptive field centers with a sustained increase in frequency of AP and many of them exhibit color opponency. - Receptive fields of cells in the koniocellular layers are center surround and have either light/dark or color opponency. - Within all layers of LGN- neurons are activated by only one eye, they are monocular and ON – center and OFF-center cells are intermixed Nonretinal Inputs to the LGN - retina is not the main source of synaptic input to the LGN- the major input (80%) of excitatory synapse comes from primary visual cortex. - LGN also receives synaptic inputs from neurons in the brain stem – activity related to alertness and attentiveness. The LGN is more than a simple relay from retina to cortex, it is the first site in the ascending visual pathway where what we see is influenced by how we feel (you see a flash of light when startled in a dark room) - LGN’s major synaptic target is the primary visual cortex – Brodmann’s area 17- located in occipital lobe (medial surface of the hemisphere, surrounded by the calcarine fissure). Area 17 is also described as the striate cortex or -Lab coat, goggles, effective ventilation, gloves VI. Retinotopy- organization whereby neighbouring cells in the retina feed info to neighboring places in their target structures (LGN and striate cortex). The surface of the retina is mapped onto subsequent structures. Note: representation of visual fiels is distorted on the striate cortex. The central few degrees of the visual field are overrepresented or magnified in the reinotropic map (many more ganglion cells with receptive fields in/near fovea) Note: discrete point of light can activte many cells in the retina and often many more cells in target structure due to overlap of receptive fields. When retina stimulated bu point of light, activity in striate cortex is broad distribution with a peak at corresponding retinotropic location Neocortex (esp. Straite cortex)- separated into 6 layers which can be seen by Nissl stain (stains the soma of neurons) - Layer I- just below pia is devoid of Neurons- consists almost exclusively of axons and dendrites of other layers. - There are actually 9 layers, but we combined 3 sublayers into IV (IVA, IVB, IVC- IVCalpha and IVCbeta) - There is a division of labor in the cortex just like what we saw in the LGN Spiny stellate cells- small neurons with spine-covered dendrites that radiate out from the cell body- seen in the two tiers of layer IVC - outside of IVC we see pyramidal cells- these are also covered with spnies and have thick apical dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that extend horizaontally - Axons of stellate- make connections only w/in the cortex - Axons of pyramidal cells send connections to other parts of the brain. Also, pyramidal cells have dendrites extending into other layers - There are also inhibitory neurons, w/out spines sprinkled throughout the layers. These form only one connection. In LGN- every layer rexeives retinal afferents and sends efferents to visual cortex. In visual cortex, it is diff. Only a subset of layers receive input from LGN or sends output to different cortical or subcortical area. - axons from LGN terminate In diff. cortical areas but mostly layer IVC. - Magnocellular LGN – project to layer IVCalpha and parvocellular project to IVCbeta. - Koniocellular LGN axons- by pass layer IV and make synapses on layers II and III Ocular dominance column How are the left and right eye LGN inputs segregated when they reach IVC layer of striate cortex? - experiment done Hubel and wiesel-inject radioactive amino acid inot one eye of monkey- it was incorporated into proteins by the ganglion cells and proteins were transported down ganglion cell axons into LGN (anterograde transport). The radioactive materials spiled out of ganglion cell axon terminals and were taken up by nearby LGN neurons- only the LGN neurons that were postsynaptic to the inputs from the injected eye incorporated the labelled proteins. These cells transported the radioactive proteins to their axon terminal in layer IVC of striate cortex - wer are able to see the radioactive axon terminals through autoradiography- place emulsion over a layer of striate and develop it like photo- silver grains - the distributon of axon terminals that relayed info from the injected eye was no continuous in layer IVC- it was split up into a series of patches, equally spaced, each about 0.5mm wide – ocular dominance columns - when it was cut tangentially, parallel to layer IV, the left and right eye inputs were laid out as a series of alternating bands, like a zebra. Innervation of Other Cortical layers from layer IVC - most intracortical connections extend perpendicular to the cortical surface along radial lines that run across layers, from white matter to layer I - pattern of radial connections- maintain retinotopic organization established in layer IV. A cell in layer VI receives info from same part of retina as cell in layer IV above it. - Axons of some layer III pyramidal cells extend collateral branches to form horizontal connections within layer III. - Layer IVC stellate cells project axons radially up to mainly IVB layers and III where info for the first time from both eyes begins to mix. - All IVC neuroncs receive monocular input. Neurons in layers II and III receive binocular input from both eyes. - Layer IVC alpa- receive magnocellular LGN input and projects mainly to cells in layer IVB - layer IVCbeta – receive parvocellular LGN input and projects mainly to layer III - n layers III and IVB- 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 white matter. Pyramidal cells in different layers innervate different structures. - Layers II, III, IVB pyramidal cells send axons to other cortical areas - Layer V pyramidal cells send axons all the way down to superior colliculus and pons - Layer VI pyramidal cells give rise to the massive axonal projection back to the LGN. - Pyramidal cell axons in all layers branch and form local connections in the cortex Layers II and III play a key role is visual processing- providing most of info that leaves VI for other cortical areas. VI output comes from 2 distinct population of neurons in superficial layers. Straite cortex stained to reveal presence fo cytochrome oxidase- mitochondrial enzyme used for cell metabolism, a colonnade appears, a series of pillars at regular intervals, running the full thickness of layers II and III and also in layers V and VI. - when the cortex is sliced tangentially through layer III, these pillars appear like splots- they are called blobs. The blobs re in a row, each blob centered on an ocular dominance stripe in layer IV. Blobs receive direct LGN input from the koniocellular layers, aand parvocellular and magnocellular input from layer IVC of striate cortex. Physiology of Striate cortex - receptive fields of neurons in the IVC layer are similar to the magnocellular and parvocelular LGN neurons providing their input- they are generally small monocular center-surround receptive fields - in layer IVCalpha, neurons are insensitive to wavelength of light, in layer IVCbeta, neurons exhibit
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