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Behavioural Neuroscience (NS III) .pdf

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Psych 1XX3 –Behavioural Neuroscience (NSIII) Notes – Feb 2, 2010 Introduction to Cognitive Behavioural Neuroscience: Role of Cognitive Neuroscientists:  Understand abstract mental processes in a neural framework.  Traditional paradigms used to study cognitive functions such as learning, memory language, and problem solving are complemented by techniques such as neuroimaging to trace the routes of neural processing. Role of Behavioural Neuroscientists:  Understand the neural processes underlying behaviours such as reward, sexual motivation, and feeding mechanisms.  Typically, these complex behaviours are simplified into component behaviours that are modeled in simple animal systems to use the full range of techniques available in neuroscience such as electrophysiology, pharmacology and behavioural genetics. Learning, Memory and Behaviour:  Focus of lecture will be on addressing the question of how you learn about the world, remember information, and apply knowledge moving from lower to higher levels of processing. Neural Plasticity:  Head injuries: Depending on the severity and the region of the insult, deficits can affect changes in planning, motor control, language production or even induce coma or brain death.  However in some cases, rehabilitation can lead to miraculous recovery allowing an injured person to regain lost abilities. Everyday Neural Plasticity:  The brain is changing in every interaction with the environment. This 'everyday neural plasticity' allows your brain to adapt incoming stimuli and rewire itself to optimize interactions with the outside world.  Beginning in the 1950s, researchers were well aware that environmental influences can lead to enduring changes in complex behaviour that can be observed.  Studies (Bingham and colleagues – 1952): demonstrated that exposure to complex environments made animal subjects into better problem solvers.  However it was not until about a decade later that researchers realized the importance of environmental experience on enduring changes in the physical structure and functional organization of the brain.  In 1964, Bennett and colleagues compared the brains of rats raised in enriched or impoverished environments. The enriched environment was like a little piece of rat heaven, where the rats lived in social groups in a complex environment filled with toys, ladders, tunnels and running wheels to explore.  In the impoverished environment, rats lived alone in small cages with access to food and water only.  Researchers found that brains from the two groups were wired very differently. The brains of rats exposed to the enriched environment had a much richer network of neurons with more dendrites and synaptic connections compared to the brains of rats raised in the impoverished environment.  Another dramatic example of the role of environmental input on enduring changes in neural structure comes from studies on maternal care in rat pups.  Meany and colleagues found that rats raised by mothers that engaged extensively in maternal care behaviours (like licking and grooming) later grow to become less fearful and less responsive to stress than do those raised by mothers that do not engage as frequently in these maternal care behaviours.  These stress and fear behavioural traits observed in adulthood were matched by measureable stress and fear changes in the brain including increased expression of glucocorticoid receptors in the adult rat's hippocampus [Weaver et al., 2004).  Many neuroscientists believe that similar effects may also be observed in humans.  These experimental findings demonstrate that the brain is adaptive and can detect contingencies between particular stimuli. This process of contingency learning allows you to learn, remember and navigate through the world. Coincidence Detection:  The first practical theory based on neuroscience principles came in 1949 from a Canadian neuroscientist named Donald Hebb in his seminal work "The Organization of Behaviour".  Hebb's theory described how connections between individual neurons can be changed, and how combinations of connected neurons can be grouped together as processing units. Hebb called these flexible units “cell-assemblies" which could adapt to the constant adjustments necessary to direct the brain's response to stimuli.  TF: All your complex thoughts can be built from the sequential activation of neurons.  Hebb's Law is often paraphrased as “neurons that fire together wire together." Long-Term Potentiation:  A promising candidate mechanism for Hebbian learning is Long-term potentiation (LTP)  LTP is the strengthening of the connection between two neurons and this effect can last for an extended period of time; from minutes to a lifetime. It is also sometimes referred to as an increase in synaptic efficacy, meaning a presynaptic neuron becomes more efficient at generating a larger response in the postsynaptic neuron.  In the lab, this LTP can be measured by the change in amplitude of the EPSP.  LTP was first observed in 1973 by a PhD candidate studying the functional circuitry of the hippocampus, which plays an important role in memory.  Terje Lomo made the startling observation that following activation by brief, repeated bursts of high frequency stimulation, a single test pulse could make it easier for adjacent cells to fire action potentials - an effect which could last for several hours over the duration of an experiment.  This long term potentiation of signalling provided a cellular mechanism for the synaptic change described in Hebbian Learning.  There are several properties of LTP that make it such a promising candidate for the neural basis of learning and memory.  LTP occurs very rapidly and is long-lasting, giving it a dynamic flexibility to form new memories. Like memories, LTP is input specific, facilitating only the synapses activated during the original stimulation.  The strong activity in Pathway 1 initiates LTP at the synapse, without initiating LTP at the inactive synapse of Pathway 2.  Finally, LTP is associative, meaning that it can strengthen inputs from multiple pathways if they are active simultaneously, as might naturally occur when two related events are presented.  In this example, the weak stimulation of Pathway 2 alone does not trigger LTP, However, when the weak input from Pathway 2 occurs together with the strong input from pathway 1, both sets of synapses are strengthened. Mechanism of Classical Hippocampal LTP:  Lomo's original observations were made using an in vivo hippocampal preparation, which limited the techniques that could be used in an experiment.  However; with the development of an in vitro tissue preparation, a fresh hippocampal tissue slice could be kept alive in a dish, allowing many new experimental tools to be used. The Mechanism of LTP:  When the neurotransmitter glutamate is released from the presynaptic neuron it binds to the AMPA receptor, which is in fact both a receptor and an ion channel.  This binding causes the channel to open, allowing the flow of positively-charged ions in.  This depolarizes the post-synaptic cell moving it away from its' -70 mv resting potential, and closer to the -50mv threshold for an action potential to occur.  TF: binding of glutamate at the AMPA receptor alone can be sufficient to cause a short-lived EPSP. Diagram shown on next page.  Classical LTP begins with the presynaptic release of the neurotransmitter glutamate, which can bind to the AMPA receptor and another receptor called the NMDA receptor.  Glutamate binding to the AMPA receptor is associated with the normal synaptic transmission that you are already familiar with  Glutamate binding to both its NMDA and AMPA receptor types is associated with the induction or development of LTP (Image below)  Glutamate binding to both AMPA and NMDA receptors turns out to be important for another key player necessary for LTP induction.  The concentration of positively charged calcium ions inside the postsynaptic cell must exceed a critical threshold.  This process of calcium ion flow can only occur when glutamate binds to the NMDA receptor. However, at the resting state, the NMDA receptor-channels are blocked by magnesium, preventing calcium from entering the cell. (Image below)  So what's a postsynaptic cell to do? Fortunately, successive EPSPs via the binding of glutamate to the AMPA receptors, leads to sufficient depolarization that unblocks the Mg. This allows calcium to enter the post-synaptic cell and induce LTP.  This takes a lot of coordination. Calcium entry requires both presynaptic and postsynaptic activity to occur.  The depolarisation of the postsynaptic neuron must be perfectly timed with the firing of the presynaptic neuron to allow the calcium channel to fully open. The requirement of this coincident activity is what makes LTP so compellingly related to learning. (Diagram shown below)  Why does calcium entry lead to a strengthening of the synaptic connection?  Calcium entry into the postsynaptic neuron has a number of complex effects, but one important result of this activity is to promote the expression of more AMPA receptors in a specific region of the post-synaptic neuron.  A specific synapse on the postsynaptic neuron becomes more sensitive to glutamate release from a specific presynaptic cell, strengthening the connection.  Diagram shown below. LTP and LTD:  The increased expression of AMPA receptors in the post-synaptic cell can last for hours, weeks and even months in lab preparations.  Just as there are IPSPs to counter EPSPs, another mechanism, called long-term depression (LTD), exists to decrease the sensitivity of synaptic connections,  Together, research on LTP and LTD help to explain why neurons that fire together wire together A Functional Role for LTP:  How is LTP directly involved in behaviours associated with learning and memory? Simple Circuits for Memory:  Eric Kandel has been responsible for much of the pioneering work in linking behaviours associated with learning and memory with synaptic changes in the nervous system. 
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