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E.De Rosa

Themes Guiding Current Science of Learning & Memory 1. Learning and memory are closely intertwined stages of informational processing. Learning is a process that generates memories; memories reflect successful learning. Neurons: The Cellular Units of Information Processing - Early researchers observed brain tissue to be a network of jumble interconnected threads, stains b/c they are closely intertwined and stains colored all cells similarly - Reticular theory of brain circuitry: the brains wiring involved a fixed network of fine wires like a spiders web - Cajal discovered that brain was not a reticular network, but composed of separate nerve cells connected to one another via specialized elements - there was no evidence of stain continuing to next cell as reticular theory suggested, thus he concluded cells communicated w/o joining their membranes and proposed the neuron doctrine - Neuron Doctrine: hypothesis that brain is composed of discrete nerve cells that are essential units of info processing - proposed that info flows one way (dendrite -> cell body -> axon) - more connections between nerve cells could be basis of intellectual power Type Defn: Principal Neuron - highly organized set of dendrites and an axon connecting w/ local cells or send signals to another brain region Interneuron - receive and send signals within local region of NS Motor Neuron - many branching dendrites, each neuron has single axon extending over long distance sending signals to muscles Sensory Neuron - specialized dendrite endings to receive signals from sensory organs (E.g., eyes/skin/nose etc..) - synaptic potentials are small and variable in size, decrease as they travel along the dendrite toward cell body, the cell body adds together all of the synaptic potentials from the dendrites - if signals reach a specific threshold at the origin of axon, the cell fires and generates a larger signal (action potential) travelling along axon in longer distances - resting potential is the natural electrical potential, a difference in charge between inside and outside of their cell membranes - Na ions are much more concentrated outside, while K is more inside + - when at rest, a neuron allows K ions to flow out leaving a ve charge inside the cell (~70mV) - when synaptic transmission changes, Na flows from outside to inside/high to low concentration, raising the synaptic potential to be less ve or even +ve, a change of +15-20mV is enough to fire a neuron and generate action potential - K flows out of cell to restore balance of concentrations after depolarization, bringing it back to -70mV - an action potential is all-or-none, the full action potentials appears or it doesnt, maintaining its size as it travels through the axon regardless of distance - Electroencephalogram (EEG) can be used to record synaptic/action potentials, if a EEG produces large waves the degree of coordinated neural activity is high, if it is small and irregular then synapses and neurons are active at different times Chemical Transmission & Communication of Neurons at Synapses 2+ - When an action potential reaches the presynaptic axon terminal, Ca flows in causing a rise in Ca , vesicles fuse w/ the membrane allowing release of neurotransmitters - process opens up Na channels in postsynaptic membrane, causing an EPSP - Excitatory Postsynaptic Potential (EPSP): bringing a neuron closer to its threshold for generating an action potential - Cl is a variant of synaptic transmission, causing an IPSP and allows for regulation of duration and type of postsynaptic potentials - Inhibitory Postsynaptic Potential (IPSP): inhibits generation of action potential by bringing postsynaptic membrane more ve - EPSP/IPSPs are added together to determine whether the threshold for an action potential will be reached, the pattern and rate of the AP Interconnected Neurons in Brain Circuits Serve Specific Functions - Hebb suggested cell assemblies form when cells were active together, which would enhance structural connections between the cells. Memories - Memories are encoded within the activities of whole cell group rather than individual cells, consistent upon Lashleys findings that memories are not stored at discrete sites - Long Term Potentiation (LTP): cellular mechanism for strengthening connections between neurons - most commonly studied in hippocampus (important for memory) of rats through hippocampal slice preparation - Discovered by Bliss after seeing in the synapse two signs of increased efficiency through repetitive high-frequency electrical stimulation of a pathway: 1. single stimulation caused rapid increase in excitatory synaptic potential 2. single stimulation recruited more cells that reached action potential - Hippocampal LTP: - when postsynaptic site at rest, AMPA receptors allow glutamate to bind while NMDA receptor blocked by Mg 2+ - when the presynaptic neuron is activated, glutamate released and binds to AMPA causing EPSP; init2+l activation causes an AP in the postsynaptic neuron removing the Mg , unblocking NMDA receptors - binding of glutamate to NMDA receptors allow Ca to enter the postsynaptic cell which activate previously inactive AMPA receptors 2+ - Ca helps synthesize neurotrophins, enlarging synapses and permanently sensitize them- Long Term Depression (LTD) of synaptic efficacy depotentiates, and occurs when either presynaptic or post-synaptic activity occurs alone - Cellular mechanisms of LTP are also the same mechanisms that support memory - Evidence 1: training rats to retrieve food pellets through small hole in box - rats brain was removed and measured the area of brain involving motor cortex controlling paw movements, through EPSP - result: EPSP was larger on the side of brain controlling trained paw, also it was difficult to induce LTP by stimulation on the trained side of brain - suggests synaptic potentiation resulting from motor learning uses same synapses and mechanisms as synaptic enhancement produced by LTP - Evidence 2: Morriss Water Maze - AP5 used to block NMDA receptors which are essential for induction of LTP, prevented new spatial learning - result: drug-treated rats swam normally but did not reach platform as fast as normal rats, drug-treated showed no bias of swimming The Brain Operates Hierarchically - Hindbrain - contains many cranial nerves and nuclei (group of neurons) coordinating impulses to and from spinal cord and cranial nerves - on surface of hindbrain is the cerebellum:
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