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Lecture 7

Lecture 7 Behavioural Neuroscience.docx

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McMaster University
Joe Kim

Lecture 7: Behavioural Neuroscience The Structure of the Brain  Cognitive neuroscientists try to understand abstract mental processes in a neural framework  And so, 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  Behavioural neuroscientists seek to 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 o Ex. Research of feeding can be divided into hunger and satiety mechanisms Neural Plasticity  Although recovery from brain injury is a particularly dramatic demonstration of its flexibility, your 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  In 1950, researchers were well area that environmental influences can lead to enduring changes in complex behaviour that can be observed  Ex. Classic studies by Bingham and colleagues 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 important 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 o The enriched environments 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 o In the impoverished environment, rats lived alone in small cages with access to food and water only o Researchers found that brains from the two groups were wired very differently o The brains of rats exposed to the enriched environment had a much richer network of neurons with more dendrites and synaptic connections compared with brains of rats raised in the impoverished environment  Another example of role of environmental input on enduring changes in neural structures comes from studies on maternal care in rat pups o Meany and colleagues have found that rats raised by mothers that engaged extensively in maternal care behaviours (licking and grooming) later grow to become less fearful and less responsive to stress than do those raised by mothers that do not engages as frequently in these maternal care behaviours o These stress and fear behavioural traits observed in adulthood were matched by measurable stress and fear changes in brain including increased expression of glucocorticoids receptors in adult rat’s hippocampus  Although research on role of environmental enrichment has been in animal models, many neuroscientists believe that similar effects may also be observed in humans o Important implications for education, parental care and treating neurological disorders of aging  Coincidence Detection o The first practical theory based on neuroscience for understanding the connection between the mind and the brain came from a Canadian neuroscientists named Donald Hebb o 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 o Hebb called these flexible units “cell-assemblies”, which could adapt to the constant adjustments necessary to direct the brain’s response to stimuli o And so, complex thoughts can be built from sequential activation of neurons o Hebb himself summarized, “When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of those cells firing, is increased” o Hebb’s Law is often paraphrased as “neurons that fire together wire together”  Long-Term Potentiation o A promising candidate mechanism for Hebbian learning is Long-term potentiation or LTP o 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 o It is also sometimes referred to as an increase in synaptic efficiency, meaning a presynaptic neuron becomes more efficient at generating a large response in the postsynaptic neuron o In the lab, this LTP can be measured by the change in amplitude of the EPSP o LTP first observed by student studying the functional circuitry of hippocampus, which plays an important role in memory o Lomo observed 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 o This LTP of signalling provided a cellular mechanism for the synaptic change described in Hebbian learning o Several properties of LTP that make it a promising candidate for neural basis of learning and memory  LTP occurs 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  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  The weak stimulation of Pathway 2 alone does not trigger LTP  However, when weak input from Pathway 2 occurs together with strong input from Pathway 1, both sets of synapses are strengthened  Mechanism of Classical Hippocampal LTP o Lomo’s original observations were made using an in vivo hippocampal preparation, which limited the techniques that could be used in an experiment o 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  Mechanism of LTP o When the neurotransmitter glutamate is released from the presynaptic neuron, it binds to AMPA receptor, which is both a receptor and an ion channel o This binding causes the channel to open, allowing the flow of positively-charged ions o This depolarizes the post-synaptic cell, moving it away from its -70 mv resting potential and closer to the -50 mv threshold for an action potential to occur o And so, binding of glutamate at the AMPA receptor alone can be sufficient to cause a short-lived EPSP o Classical LTP begins with the presynaptic release of glutamate, which can bind to AMPA receptor and another receptor called NMDA receptor o Glutamate binding to AMPA receptor is associated with normal synaptic transmission o Glutamate binding to both NMDA and AMPA receptor types is associated with induction or development of LTP  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 NMDA receptor  However, at the resting state, the NMDA receptor-channels are blocked by magnesium, preventing calcium from entering cell o Fortunately, successive EPSPs via the binding of glutamate to AMPA receptor leads to sufficient depolarization that unblocks the Mg o This allows calcium to enter postsynaptic cell and induce LTP o LTP requires two events  Postsynaptic activity to remove magnesium block  Presynaptic activity to release glutamate o Calcium entry requires both presynaptic and postsynaptic activity to occur o Depolarization of postsynaptic neuron must be perfectly timed with firing of presynaptic neuron to allow calcium channel to fully open o Calcium entry into 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 postsynaptic neuron o And so, a specific synapse on the postsynaptic neuron becomes more sensitive to glutamate release from a specific presynaptic cell, strengthening the connection  LTP and LTD o Increased expression of AMPA receptors in postsynaptic cell can last for a long time o Just as there are IPSPs to counter EPSPs, another mechanism, called long-term depression (LTD) exists to decrease the sensitivity of synaptic connections Simple Circuits for Memory  Eric Kandel has been responsible for linking behaviours associated with learning and memory with synaptic changes in the nervous system  His research addressed the question, “what happens in a neural circuit when learning occurs?”  Much of his research conducted on a slug called Aplysia o Nervous system of Aplysia has only about 20,000 neurons o This limited nervous system and its repertoire of simple behaviours makes the sea slug an ideal model to study neural basis of learning and memory o In his research, Kandel focused primarily on one particular behaviour, gill withdrawal reflex o At rest, Aplysia extends its gill outward to assist in collection of oxygen o In response to danger, it rapidly retracts these organs for safety o This simple reflex is mediated by a relatively small circuit of neurons that can be used to study fundamental learning processes such as habituation and classical conditioning  Habituation o Simple form of learning which involves decreased response to a repeated or constant stimulus o In observing Aplysia, Kandel found that the strength of the gill withdrawal reflex became progressively smaller after repeated stimulation o It turns out that this behaviour is mediated by a relatively simple neural circuit o At the surface of the skin, a set of sensory neurons receive signals from the outside world o The sensory neurons synapse with motor neurons that return to the gill and control its withdrawal behaviours o After mapping the neural circuit responsible for gill withdrawal reflex, the next step was to understand which part of the circuit was modified during habituation o Kandel observed that across trials, presynaptic sensory neuron continued to fire just as many action potentials and receptors in the postsynaptic neuron remained just as sensitive o However, presynaptic neuron seemed to be releasing less neurotransmitter o In fact, Kandel found that fewer vesicles were fusing with membrane, this releasing less neurotransmitters o With repeated training, this change in efficacy of synapse would last for weeks, suggesting a mechanism, for long term memory  Classical Conditioning o Involves the learning of a contingency between a CS and a US o Prior to conditioning, US elicits a UR o After conditioning trials in which the CS and US are paired, the CS alone can now elicit a response, called the CR o Kandel demonstrated that classical conditioning could readily be demonstrated in Aplysia o In these experiments, a separate area of the body called the mantle was stimulated before stimulating the gill withdrawal circuit itself o The mantle stimulation served as a kind of CS which reliably predicted stimulation of the gill withdrawal reflex, which served as the US o Eventually this mantle stimulation alone was sufficient to elicit the gill withdrawal reflex o The sensory neuron of the Mantle synapses with the postsynaptic motor neuron of the reflex circuit o The stimulation from the mantle causes a slight depolariz
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