CHAPTER 13- LEARNING AND MEMORY (pg 431-440) The Nature of Learning Learning: refers to the process by which experiences change our nervous system and hence our behavior. The changes are called memories Experiences change the way we perceive, perform, think and plan by physically changing the structure and the nervous system. Learning can take @ least 4 basic forms: Perceptual learning: the ability to learn and recognize stimuli that have been perceived before. Primary function is the ability to identify and recognize objects and situations. Stimulus-response learning: the ability to learn to perform a particular behavior when a particular stimulus is presented. It involves the establishment of connections between circuits involved in perception and those involved in movement. It includes 2 categories of learning: classical conditioning and instrumental conditioning. Classical conditioning involves association between 2 stimuli Hebb Rule: if a synapse repeatedly becomes active at about the same time that the postsynaptic neuron fires, changes will take place in the structure or chemistry of the synapse that will strengthen it. Eq: (on pg 432) If the tone is presented first, then weak synapse T becomes active. If the puff is presented immediately afterward, then strong synapse P becomes active and makes the motor neuron fire. The act of firing then strengthens any synapse with the motor neuron that has just been active. After several increments of strengthening, synapse T becomes strong enough to cause the motor neuron to fire by itself. Instrumental/Operant Conditioning: Involves behaviors that have been learned Involves an association between a response and a stimulus. It permits an organism to adjust its behavior according to the consequences of that behavior. ?favorable consequences? are referred to as reinforcing stimuli ?unfavorable consequences? are referred to as punishing stimuli Reinforcement causes changes in an animal?s nervous system that increase the likelihood that a particular stimulus will elicit a particular response. Reinforcement strengthens a connection between neural circuits involved in perception and those involved in movement. Motor Learning: the establishment of changes within the motor system Can?t occur without sensory guidance from the environment The more novel the behavior, the more neural circuits in the motor system s of the brain must be modified. If we tech an animal to make a new response whenever we present a new stimulus, it must learn to recognize the stimulus (perceptual learning) and make the response (motor learning), and a connection must be established between these two new memories (stimulus-response learning). Synaptic Plasticity: Long-term Potentiation and Long-Term Depression Induction of Long-Term Potentiation ? Long-Term Potentiation(LTP): A long-term increase in the excitability of a neuron to a particular synaptic neuron input caused by repeated high-frequency activity of that input. Hippocampal Formation: Specialized region of the limbic cortex located in the temporal lobe. It includes the hippocampus itself and the dentate gyrus The major neocortical inputs and outputs of the hippocampal formation are channeled through the entorhinal cortex. Neurons in the entorhinal cortex relay incoming info to the granule cells of the dentate gyrus through a bundle of axons known as the perforant path. These neurons then send axons into field CA3 of the hippocampus itself. * entorhinal cortex (perforant path) dentate gyrus field CA3* *Hippocampus a.k.a Ammon?s horn* Pyramidal cell: large neurons found in the cerebral cortex an Ammon?s horn of the hippocampal formation. The terminals of the fibers from the dentate gyrus form synapses with dendritic spines of the pyramidal cells. Spines are site of structural and biochemical changes that are responsible for lon-term potentiation. Field CA1: receives inputs from field CA3 and projects out to the hippocampal formation via the subiculum. Procedure for producing LTP: Population EPSP: extracellular measurementof the excitatory post-synaptic potentials(EPSP) produced by the synapses of the perforant path axons with the dentate granule cells. Stimulating electrode is placed among axons in perforant path and recording electrode is placed in the dentate gyrus, near the granule cells that receive input from these axons. A single pulse electrical stimulation is delivered to the perforant path and the resulting EPSP is recorded in the dentate gyrus. Long-term potentiation can be induced by stimulating the axons in the perforant path. LTP can be produced in isolated slices of the hippocampal formation LTP in hippocampal slices can follow the Hebb ruleassociative long-term potentiation: when weak and strong synapses on a single neuron are stimulated at approximately the same time, the weak synapse becomes strengthened. Role of NMDA Receptors Non associative long- term potentiation?only a series of pulses delivered at a high rate all in one burst will produce LTP Rapid stimulation depolarizes postsynaptic membrane much more than slow stimulation does. Synaptic strengthening occurs when molecules of neurotransmitter bind with postsynaptic receptors located in a dendritic spine that is already polarized. LTP requires two events: Activation of synapses Depolarization of the postsynaptic neuron NMDA receptor: Found in the hippocampal formation (field CA1) Controls a calcium ion channel normally blocked by Mg2+(prevents Ca2+ ions from entering the cell) If the postsynaptic membrane is depolarized, Mg2+ is ejected from the ion channel, the channel is free to admit Ca2+ Ca2+ ions enter the cell through channels controlled by NMDA receptors only when glutamate is present and when the postsynaptic membrane is depolarized. Ion channel controlled by NMDA receptor is a NT-and-voltage-dependent ion channel! AP5: A drug that blocks NMDA receptors Prevents CA2+ ions from entering the dendritic spines and thus blocks the establishment of LTP. If the activity of strong synapses located elsewhere on the postsynaptic cell have caused the cell to fire, then a dendritic spike will depolarized the postsynaptic membrane enough to eject the magnesium ions from the calcium channels of the NMDA receptors in the dendritic spines. Mechanism of Synaptic Plasticity ? AMPA receptors: Strengthens individual synapses by being added to the postsynaptic membrane of the dendritic spine. Are non-NMDA glutamate receptors Control sodium channels Produce EPSPs in the membrane of the dendritic spine when activated by glutamate. With more AMPA receptors, the release of glutamate by the terminal button causes a larger excitatory postsynaptic potential.
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