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DevelopmentW2010.Lecture9notes

DevelopmentW2010.Lecture9notes - Development BIO 120 Winter...

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Development BIO 120 Winter 2010 Jeremy Lee Patrick Yuh Eva Murdock Lecture 9 Neural Development I. The Problem and Complexity of Neural Development a. Vertebrate central nervous system (CNS): perhaps the most complex, ordered matter known. b. Tremendous number of specific connections must be made between different neurons, and between neurons and muscles. c. Many synapses are stereotyped ; i.e. they are the same from one individual to the next. d. System must also allow for plasticity ; i.e. modification in connections and/or neuron function, both as development proceeds and as a result of experience or exposure to the environment, to allow for changes in the animal’s behavior. II. General steps in neural development (Fig. #1) a. Differentiation of ectodermal cells into neurons = birth of neurons. b. Determination of specific neuron fate : determination of what kind of neurons they become and with which kind of cells they will ultimately synapse. c. Pathway selection = axon pathfinding : outgrowth of axon and development of a growth cone to direct the axon’s growth in the proper direction and toward the appropriate target cell(s) . Paths over which an axon grows can be up to a couple of meters. d. Target selection : recognition of the set of cells to which a neuron will form a synapse. e. Address selection : refining of target selection. Neuron forms a synapse or synapses with one or a small subset of potential targets. There often is competition between neurons for a target. Some synapses form and are maintained; this may depend on the activity of neuron. Other synapses are initiated but are not maintained. f. Survival of neurons following innervation . Many neurons die (up to 1/2 that are born!) as a result of apoptosis . They may die before or after innervating a target cell. g. Example: projection of retinal axons in vertebrates (Fig #2). 1. Sensory neurons in retina send axons out; they first grow around the inner surface of the retina. 2. As they reach the beginning of the optic nerve, the axon turns, and heads down the optic nerve. 3. As axons enter the brain, they hit the optic chiasm ; some axons cross the optic chiasm; others do not cross the chiasm. 4. These retinal axons follow a very specific pathway in brain; this is called the optic tract. 5. Eventually retinal axons reach the tectum . A. In the tectum, the retinal axons form synapses with very specific sets of neurons. B. The specific tectum neuron that a retinal cell synapses onto corresponds to the position of the retinal cell body in the retina (and from where in the visual field it responds.)
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C. In other words, there is a perfect correspondence between position of the cell body in the retina and the position in the tectum that receives the input. This is called a retinotopic projection , as if the visual field and the retina were mapped right onto the brain (see below).
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