Development of the Nervous System Chapter 5 Nervous system development begins to take form at about an embryonic age of 2 weeks in humans CNS development occurs around a fluid filled cavity called the neutral tube which eventually becomes the ventricles and the central canal Cells lining the ventricles divide Some of the daughter cells are primitive neurons and glia which begin to migrate towards their eventual destinations in the CNS The process that makes one neuron different from another is called differentiation Differentiation begins as the neuron is migrating but is also dependent on the local environment of its target destination Proliferation cells lining the ventricles divide Some of the daughter cells are primitive neurons and glia Migration the proliferating cells begin to migrate toward their eventual destinations in the CNS The migration is guided by immunoglobins and chemokines as well as radial glia Differentiation The process that makes one neuron different from another is called differentiation Differentiation begins as the neuron is migrating but is also dependent on the local environment of its target destination Axons develop first and the cell shape and dendrites develop once neuron reaches target site Myelination production of myelin sheaths around axons First occurs in spinal cord then hindbrain midbrain and forebrain It occurs gradually through decades Synaptogenesis formation of new synapses The process of synapse formation and removal occurs throughout life Axons grow before dendrites while the neuron is migrating Axons are guided to their targets by following gradients of guidance molecules found on the surface of cells and in the extracellular matrix Once the neuron reaches its destination then dendritic growth begins and increases as incoming axons arrive Myelination of axons begins during fetal development and continues for many years after birth There are more neurons and axons generated during fetal development than are ultimately found in the adult brain Several factors determine which neurons and connections survive Neurotrophins Chemicals that promote neuron survival and growth Various types nerve growth factor NGF brain derived neurotrophic factor BNDF neurotrophins 3 4 5 and 6 NT3 NT4 5 NT6 Early in development they promote survival and growth of selected neurons Experience can lead to increases in secretion of neurotrophins which promote axonal sprouting Promote regrowth of injured axons Apoptosis apo tosis Programmed cell death Once an axon forms a synapse neurotrophins secreted from the target cell strengthen the connection and prevent the neuron from committing suicide Axons that do not receive sufficient amounts of neurotrophins e g NGF degenerate and the cell body dies If the neuron does not make sufficient connections by a certain age then it kills itself Postnatal development CNS development continues after birth With some notable exceptions e g olfactory neurons generally neurons cannot be replaces There are however some examples of new neurons being formed in certain parts of the adult brain e g hippocampus and cortex The formation of dendrites dendritic spines axon branches and synapses continues to occur after birth Experience can have an impact on these processes Recovery of Function after Nervous System Damage Behavioral Compensation subject learns how to use remaining portions of the nervous system to compensate for the damage Involves the development of new strategies to achieve tasks Neural Response to Injury Axons in peripheral nerves can regenerate is they are damaged More likely after nerve is crushed than if it is cut Axons in CNS cannot regenerate over appreciable distances It is thought that scar tissue and growth inhibiting proteins suppress regeneration in the CNS Axons can respond to neurotrophins released by cells in the vicinity of their terminals by sprouting collateral axon terminals that can fill synapses vacated by degenerating axons Reorganizational Events Phantom limb refers to the continuation of sensation of an amputated body art and reflects this process The cortex reorganizes itself after the amputation of a body part by becoming responsive to other parts of the body Original axons degenerate leaving vacant synapses into which other axons sprout Heightened sensitivity to a neurotransmitter after the destruction of an incoming axon or of Denervation Disuse Supersensitivity inactivity Potential Mechanisms o Up regulation of receptor proteins o Up regulation of a following cascade component e g ion channels second messengers Stroke Types of strokes include Ischemia the most common type of stroke resulting from a blood clot or obstruction of an artery Neurons lose their oxygen and glucose supply Hemorrhage a less frequent type of stroke resulting from a ruptured artery Neurons are flooded with excess calcium oxygen and other products Ischemia and hemorrhage also cause Edema the accumulation of fluid in the brain resulting in increased pressure on the brain and increasing the probability of further strokes Disruption of the sodium potassium pump leading to the accumulation of sodium ions inside neurons Edema and excess sodium Triggers the release of the excitatory neurotransmitter glutamate Overstimulation of neurons leads to sodium and other ions entering the neuron in excessive amounts Excess positive ions in the neuron block metabolism in the mitochondria and kill the neuron Stroke Treatment A drug called tissue plasminogen activator tPA breaks up blood clots and reduces the effects of an ischemic strokes Research has begun to attempt to save cells in the penumbra or region that surrounds the immediate damage by blocking glutamate synapses but the results have not been promising Cannabinoids have also been shown to potentially minimize cell loss through anti oxidant and anti inflammatory actions One of the most effective laboratory methods used to minimize damage caused by strokes is to cool the brain Additional Information Biological Psychology textbook Stop and Check questions 1 Which develops first a neuron s axon or its dendrites a The axon forms first 2 In which brain areas do new neurons form in adults a Olfactory receptors neurons in the hippocampus and neurons in the song producing 3 What process assures that the spinal cord has the right number of axons to innervate al the areas of some birds muscle cells a The nervous system builds far more neurons than it needs and discards through apoptosis
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