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Kenneth Campbell

Chapter 23- Wiring the Brain Introduction • All 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 again: o By ganglion cell type o By eye of origin (ipsilateral or contralateral) o By retinotopic position • LGN neurons project axons into the optic radiations that travel via the internal capsule to the primary visual cortex, here, they terminate o Only in cortical area 17 o Only in specific cortical layers (mainly layer IV) o Again 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 targets o However, a small but important component of the final wiring depends on sensory information about the world around us during early childhood The Genesis of Neurons • First 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 stages: o Cell proliferation o Cell migration o Cell differentiation Cell Proliferation • Recall, the brain develops from the walls of the five fluid-filled vesicles o These fluid-filled spaces remain in the adult and constitute the ventricular system • Very early in development, walls of the vesicles consist of only two layers: o Ventricular zone and the marginal zone • The ventricular zone lines the inside of each vesicle, and the marginal zone faces the overlying pia • Within 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 proliferation o First position: A cell in the ventricular zone extends a process that reaches upward toward the pia o Second position: The nucleus of the cell migrates upward from the ventricular surface toward the pial surface; the cell’s DNA is copied o Third position: the nucleus, containing two complete copies of the genetic instructions, settles back to the ventricular surface o Fourth position: The cell retracts its arm from the pial surface o Fifth position: The cell divides into two. • Fate of the newly formed daughter cell depends on a number of factors • A ventricular zone precursor cell that is cleaved vertically during division has a different fate than one that is cleaved horizontally • After vertical cleavage, both daughter cells remain in teh ventricular zone to divide again and again o This mode of cell division takes place in early development to expand the population of neuronal precursors • Later in development, horizontal cleavage is the rule • In this case, the daughter cell lying farthest away from the ventricular surface migrates away to take up its position in the cortex, where it will never divide again • The other daughter remains in the ventricular zone to undergo more divisions • Ventricular zone precursor cells repeat this pattern until all the neurons and glia of the cortex have been generated • Once a cell commits to a neuronal fate, it will never divide again • How does cleavage plane during cell division determine the cell’s fate? o All of our cells contain the same complement of DNA inherited from our parents, so every daughter cell has the same genes o The factor that makes one cell different from another is the specific genes that generate mRNA and ultimately protein o Cell fate is regulated by differences in gene expression during development o Gene expression is regulated by cellular proteins called transcription factors • Multiple cell types, including neurons and glia, can arise from the same precursor cell • Neural stem cells- ability of the cell to give rise to many different types of tissue • Ultimate fate of the migrating daughter cell is determined by a combination of factors, including the age of the precursor cell, its position within the ventricular zone, and its environment at the time of division • Cortical pyramidal neurons and astrocytes derive from the dorsal ventricular zone, whereas inhibitory interneurons and oligodendroglia derive from the ventral telencephalon • Subplate- consists of the cells that are the first to migrate away from the dorsal ventricular zone o The subplate eventually disappears as development proceeds • Next cells to divide become layer VI neurons, followed by the neurons of layers V, IV, III, and II Cell Migration • Many daughter cells migrate by slithering along thin fibers that radiate from the ventricular zone toward the pia o These fibers are derived from specialized radial glial cells, providing the scaffold on which the cortex is built • Immature neurons called neuroblasts, follow this radial path from the ventricular zone toward the surface of the brain • Some neurons actually derive from radial glia o In this case, migration occurs by the radial movement of the soma within the fiber that connects the ventricular zone and pia • When cortical assembly is complete, the radial glia withdraw their radial processes • Not all migrating cells follow the path provided by the radial glial cells o 1/3 of the neurobalsts wander horizontally on their way to the cortex • Neuroblasts destined to become subplate cells are among the first to migrate away from the ventricular zone • Neuroblasts destined to become the adult cortex migrate next o They cross the subplate and form another cell layer called the cortical plate • First cells to arrive in the cortical plate are those that will become layer VI neurons • Next come the layer V cells, followed by layer IV cells, and so on o Notice that each new wave of neuroblasts migrates right past those in the existing cortical plate  In this way, the cortex is said to be assembled inside-out • This orderly process can disrupted by a number of gene mutations Cell Differentiation • The process in which a cell takes on the appearance and characteristics of a neuron is called cell differentiation • Differentiation is the consequence of a specific spatiotemporal pattern of gene expression • As seen, neuroblast differentiation begins as soon as the precursor cells divide with the uneven distribution of cell constituents • Neuronal differentiation occurs when the neuroblast arrives in the cortical plate o Thus layer V and VI neurons have differentiated into recognizable pyramidal cells even before layer II cells hav migrated into the cortical plate o Neuronal differentiation occurs first, followed by astrocyte differentiation that peaks at about the time of birth o Oligodendrocytes are the last cells to differentiate • Differentiation of the neuroblast into a neuron begins with the appearance of neurites sprouting off the cell body o At first these neurites are indistinguishable, but later they are visible to be the axons and dendrites • Differentiation will occur even if the neuroblast is removed from the brain and placed in tissue culture o For ex. Cells destined to become neocortical pyramidal cells will often assume the same characteristic dendritic architecture in tissue culture  Therefore differentiation is programmed well before the neuroblast arrives at its final resting place • However, stereotypical architecture of cortical dendrites and axons also depends on intercellular signals • Pyramidal neurons are characterized by a large apical dendrite that extends radially toward the pia, and an axon that projects in the opposite direction Differentiation of Cortical Areas • Most cortical neurons are born in the ventricular zone and then migrate along radial glia to take up their final position in one of the cortical layers • It is reasonable to conclude that cortical areas in the adult brain simply reflect an organization that is already present in the ventricular zone of the fetal telencephalon • The thalamus is important for specifying the pattern of cortical areas • How did the appropriate thalamic axons come to lie in wait under the parietal cortex in the first place? o Subplate neu
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