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PSYC 2240 (40)


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York University
PSYC 2240
Rebecca Jubis

Chapter 23 Wiring the BrainIntroductionAll retinal ganglion cells extend axons into the optic nerve but only ganglion cell axons from the nasal retinas cross at the optic chiasm Axons from the two eyes are mixed in the optic tract but in the lateral geniculate nucleus LGN they are sorted out againoBy ganglion cell typeoBy eye of origin ipsilateral or contralateraloBy retinotopic position LGN neurons project axons into the optic radiations that travel via the internal capsule to the primary visual cortex here they terminateoOnly in cortical area 17oOnly in specific cortical layers mainly layer IV oAgain according to cell type and retinotopic position The neurons in layer IV make very specific connections with cells in other cortical layers that are appropriate for binocular vision and are specialized to enable the detection of contrast borders Most of the wiring in the brain is specified by genetic programs that allow axons to detect the correct pathways and the correct targetsoHowever a small but important component of the final wiring depends on sensory information about the world around us during early childhood The Genesis of NeuronsFirst step in wiring the nervous system together is the generation of neurons In the primary visual cortex striate cortex in the adult there are six cortical layers and the neurons in each of these layers have characteristic appearances and connections that distinguish striate cortex from other areas Neuronal structure develops in three major stagesoCell proliferationoCell migrationoCell differentiation Cell ProliferationRecall the brain develops from the walls of the five fluidfilled vesicles oThese fluidfilled spaces remain in the adult and constitute the ventricular system Very early in development walls of the vesicles consist of only two layersoVentricular zone and the marginal zone The ventricular zone lines the inside of each vesicle and the marginal zone faces the overlying piaWithin these layers of the telencephalic vesicle a cellular ballet is performed that gives rise to all the neurons and glia of the visual cortex Choreography of cell proliferationoFirst position A cell in the ventricular zone extends a process that reaches upward toward the piaoSecond position The nucleus of the cell migrates upward from the ventricular surface toward the pial surface the cells DNA is copied oThird position the nucleus containing two complete copies of the genetic instructions settles back to the ventricular surfaceoFourth position The cell retracts its arm from the pial surfaceoFifth position The cell divides into two Fate of the newly formed daughter cell depends on a number of factorsA ventricular zone precursor cell that is cleaved vertically during division has a different fate than one that is cleaved horizontallyAfter vertical cleavage both daughter cells remain in teh ventricular zone to divide again and again oThis mode of cell division takes place in early development to expand the population of neuronal precursors
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