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Physiology of memory & learning.

1. PHYSIOLOGY OF MEMORY AND LEARNING 2. ➢ Memory is the ability to capture externally or internally presented information, store it and reconstruct it later. ➢ We are…
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  • 2. ➢ Memory is the ability to capture externally or internally presented information, store it and reconstruct it later. ➢ We are consistently presented with a flow of new information, which needs to be processed and sometimes acted upon. ➢ For us to adapt and survive, our brain, through the evolutionary process, developed a well-calibrated mechanism to capture our experiences, which then shape our actions. ➢ This mechanism enables the species to adapt more quickly to a changing environment and to respond to a stimulus by comparing it with past experiences. Definition of memory
  • 3. Three temporal stages of memory i.Immediate memory – seconds ii.Recent memory – minutes to days iii.Remote memory – years Memory systems ▪ Short term memory Working memory ▪ Long term memory Declarative memory (explicit) Non declarative memory (implicit) Types of memory
  • 4. ➢ Explicit & many forms of implicit memory involve: short-term memory, which lasts secs to hrs, during which processing in hippocampus & elsewhere lays down long-term changes in synaptic strength and long-term memory, which stores memories for yrs and sometimes for life. ➢ During short-term memory, the memory traces are subject to disruption by trauma and various drugs, whereas long-term memory traces are remarkably resistant to disruption. ➢ Working memory is a form of short-term memory that keeps information available, usually for very short periods, while the individual plans action based on it. Types of memory
  • 5. Explict memory ( non declarative memory ) ➢ Factual knowledge of people, places, things and meaning of facts. ➢ Conscious process and recall requires conscious search of memory. ➢ Expressed mainly in verbal form 1.Episodic - events and personal experience 2.Semantic - memory for facts Implict memory ( declarative memory ) ➢ Involved in training reflexive motor or perceptual skills. ➢ Builds up slowly through repetition over many trails. ➢ Recalled unconsciously. ➢ Expressed mainly in form of performance.
  • 6. Long term memory
  • 7. Classification of memory Two forms of long term memory Explicit (declarative) Implicit (Nondeclarative) Facts (Semantic) Events (Episodic) •Medial Temporal Lobe •Hippocampus Priming Procedural (skills & habits) Associative Learning: Classical & operant conditioning Nonassociative learning: Habituation & sensitization Emotional responses Skeletal musculature Neocortex Striatum Amygdala Cerebellum Reflex pathways
  • 8. Stages of memory process ➢ How does information get into memory ? Reception and registration ➢ How is information maintained in memory ? Storage and retention ➢ How is information pulled back out of memory ? Recall and retrieval
  • 9. Explicit memory
  • 10. ➢ Episodic memory refers to the explicit and declarative memory system used to recall personal experiences framed in our own context, such as a short story or what you had for dinner last night. ➢ This memory system depends on the medial temporal lobes (including the hippocampus and the entorhinal and perirhinal cortexes). ➢ Other structures include the basal forebrain with the medial septum and diagonal band of Broca’s area, the retrosplenial cortex, the presubiculum, the fornix, mammillary bodies, the mammillothalamic tract and the anterior nucleus of the thalamus. Episodic memory
  • 11. Papez Circuit Mammillary bodies Other hypothalamic nuclei Septal nuclei Substantia innominata (Basal nucleus of Meynert) Hippocampal Formation (hippocampus and dentate gyrus) Anterior Thalamic nuclear group Cortex of Cingulate Gyrus Parahippocampal Gyrus Neocortex Fornix Mammillothalami c tract
  • 12. ➢ Memory loss attributable to dysfunction of the episodic memory system follows a predictable pattern known as Ribot’s law, which states that events just before an ictus are most vulnerable to dissolution, whereas remote memories are most resistant. ➢ Thus, the ability to learn new information is impaired (anterograde amnesia), recently learned information cannot be retrieved (retrograde amnesia) and remotely learned information is usually spared.
  • 13. ➢ Frontal lobes are involved in the registration, acquisition, or encoding of information, the retrieval of information without contextual and other cues, the recollection of the source of information and the assessment of the temporal sequence and recency of events. ➢ They permit the person to focus on the information to be remembered and to engage the medial temporal lobes. ➢ Dysfunction of the frontal lobes may cause distortions of episodic memory as well as false memories, such as information that is associated with the wrong context or with incorrect specific details.
  • 14. ➢These differences between deficits in episodic memory that occur because of damage to the medial temporal lobes (and the Papez circuit) and those that occur because of damage to the frontal lobes can be conceptualized in a clinically useful analogy. ➢The frontal lobes are analogous to the “file clerk” of the episodic memory system, the medial temporal lobes to the “recent memory file cabinet,” and other cortical regions to the “remote memory file cabinet.” Thus, if the frontal lobes are impaired, it is difficult — but not impossible — to get information in and out of storage. ➢However, the information may be distorted owing to “improper filing” that leads to an inaccurate source, context, or sequence. ➢
  • 15. ➢If, however, the medial temporal lobes are rendered completely dysfunctional, it will be impossible for recent information to be retained. ➢ Older information that has been consolidated over a period of months or years is thought to be stored in other cortical regions and will therefore be available even when medial temporal lobes and the Papez circuit are damaged. ➢ For example, although patients with depression and those with Alzheimer’s disease may exhibit episodic memory dysfunction, the former have a dysfunctional “file clerk” and the latter have a dysfunctional “recent memory file cabinet.
  • 16. ➢ Semantic memory refers to our general store of conceptual and factual knowledge, such as the color of a lion or the first president of the United States. This is a declarative and explicit memory system. ➢ There is evidence, for example, that visual images are stored in nearby visual-association areas. However, a more restrictive view of semantic memory, one that is justified in light of the naming and categorization tasks by which it is usually measured, localizes semantic memory to the inferolateral temporal lobes. Semantic memory
  • 17. ➢ Disorders of semantic memory should be suspected when patients have difficulty naming items whose names they previously knew. ➢ Patients with mild dysfunction of semantic memory may show only reduced generation of words for semantic categories (e.g., the number of names of animals that can be generated in one minute). ➢ Patients with a more severe impairment of semantic memory typically show a two-way naming deficit (i.e., they are unable to name an item when it is described and are also unable to describe an item when they are given its name). ➢ These more severely affected patients also show impoverished general knowledge.
  • 18. Anatomical basis
  • 19. Memory processing ➢ Information is first acquired through unimodal and polymodal association areas – prefrontal, limbic and parieto-occipito-temporal cortex – which synthesize visual and somatic information.
  • 20. • Look at some face •Processed in visual ass. area in inferotemporal cortex • Parahippocampal cortex, perirhinal cortex , entorhinal cortex. •Hippocampus • Via entorhinal cortex •Neocortex storage system Association areas are the ‘ultimate repositories’
  • 21. ➢ Therefore entorhinal cortex have dual functions – both input and output. ➢ Damage causes severe memory loss and all sensory modalities involved. ➢ Earliest pathological change in AD – entorhinal cortex involvement and so explict memory lost early. ➢ Hippocampus Right side – spatial memories stored (lesions cause defect in spatial orientation) Left side – memories for words, objects and people (lesions cause defect in verbal memory)
  • 22. ➢ Hippocampus is only a temporary way station for LTM. ➢ Unimodal and polymodal association areas of cortex are concerned with LTM storage. ➢ Amygdyla – stores component of memory concerned with emotion. It doesn't store factual information. (damage has no effect on explict memory) ➢ In hippocampus , it takes days-wks to facilitate storage of information about the face initially processed by ass. areas. ➢ There is relatively slow addition of information to neocortex, which permits new data to get stored without disrupting information.
  • 23. ➢ A long-term increase in the excitability of a neuron to a particular synaptic input caused by repeated high-frequency activity of that input. ➢ It occurs in many parts of the nervous system but has been studied in greatest detail in the hippocampus. Long term potentiation
  • 24. ▶ 3 major pathways Perforant pathway Mossy fiber pathway Schaffer collateral pathway • Long term potentiation - physiological mechanism.
  • 25. Hippocampal pathways
  • 26. Associativity: LTP will not occur unless the presynaptic fiber and the postsynaptic cell are coincidentally active. Cooperativity: More than one presynaptic fiber needs to be active. Input specificity: LTP that is produced at a given synapse is specific for a given site. ➢ In other words, fibers that produce LTP at a certain synaptic location will not produce LTP at another. Properties of LTP at the CA3-CA1 Synapse
  • 27. “When an axon of cell A… excites cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells so that A's efficiency as one of the cells firing B is increased.” “Cells that fire together, wire together” Hebb's rule (1949- Donald Hebb)
  • 28. ▶ There are 2 forms in the hippocampus: mossy fiber LTP, which is presynaptic and independent of NMDA receptors; and Schaffer collateral LTP, which is postsynaptic and NMDA receptor-dependent.
  • 29. ▶ LTP can be demonstrated in CA1 cells by stimulating the afferent pathways to the cells and recording EPSPs. ▶ After stimulation, there was increased EPSP activity in CA1 cells, which lasted several wks. ▶ This may be regarded as a form of neuronal memory within a single nerve cell. LTP in Hippocampus
  • 30. ▶ From granule cells of the dentate gyrus, NT- glutamate. ▶ Binds to both NMDA & non-NMDA receptors on the target pyramidal cells. ▶ Activates Ca/calmodulin dependent adenylyl cyclase cAMP activates PKA enhances NT release. Mossy fiber pathway (LTP)
  • 31. ▶ Schaffer pathway- CA3 to CA1 ▶ Perforant pathway- entorhinal cortex to dentate gyrus ▶ In both, NT- glutamate. ▶ NMDA type receptors. ▶ Subsequently- Ca influx- activation of PKC & PKA. LTP in other pathways
  • 32. ▶ Retrograde messenger ▶ Induction of LTP- requires events in postsynaptic cell ▶ Expression of LTP- presynaptic (NT release) ▶ Presynaptic cell must receive information that LTP is induced ▶ Nitric oxide- candidate LTP
  • 33. Early LTP ▶ lasts 1-3 hours. ▶ Represents only functional changes ▶ No change in the number of active zones, synapses. Late LTP ➢ requires new protein synthesis. ➢ Involves- - activation & growth of additional presynaptic NT vesicles - insertion of new clusters of postsynaptic receptors. ➢ Involves cAMP-PKA-MAPK-CREB pathway. Early & Late LTP
  • 34. ▶ Mutant mice. ▶ NMDA & AMPA receptors gene knock out. ▶ Loss of LTP. ▶ Impaired memory formation. Gene disruption & LTP
  • 35. ➢ Procedural memory refers to the ability to learn behavioral and cognitive skills and algorithms that are used at an automatic, unconscious level. ➢ It is nondeclarative but during acquisition may be either explicit (such as learning to drive a car with a standard transmission) or implicit (such as learning the sequence of numbers on a touch-tone phone without conscious effort). ➢ Research with the use of functional imaging has shown that brain regions involved in procedural memory, including the supplementary motor area, basal ganglia, and cerebellum, become active as a new task is being learned. Implicit memory (procedural memory)
  • 36. Types of learning (Implicit memory) Simple (Non associative) Associative Complex Habituation Classic conditioning Observational learning Sensitisation Operant conditioning Latent learning Aversion learning
  • 37. ➢ The subject learns about the properties of a single stimulus when exposed to it once or repeatedly. ➢ Two forms- Habituation & Sensitization. ➢ Habituation is a decrease in response to a benign stimulus when that stimulus is presented repeatedly. ➢ Sensitization is an enhanced response to a wide variety of stimuli after an intense or noxious stimulus. ➢ A sensitizing stimulus can also override the effects of habituation, a process called dishabituation. Simple (Non associative)
  • 38. Defensive reflexes- can be used to study sensitization & conditioning Habituation and sensitization in Aplysia (marine snail)
  • 39. Habituation
  • 40. Physiological basis - habituation ➢ Decrease in the EPSP- due to decrease in the number of quanta released. ➢ Calcium is the mediator of quanta release at neuromuscular junction. ➢ Action potential (AP)- initial part mediated by Na channels & later by calcium .
  • 41. Attributed to structural changes. 1. Reduction in active zones, 2. Active zones- smaller & flatter. 3. Changes in inter-neurons. Long term habituation – Physiological basis
  • 42. Sensitization Intense stimuli Enhanced response Periodic touching Consistent response
  • 43. Activation of nociceptor pathway (serotonin). Activates specific receptors on sensory neurons. Coupling of receptors to produce cAMP. Activation of cAMP dependent protein kinase,Phosphorylation of K channels to reduce K currents. Increased Ca influx, increase quanta release. Sensitization – Physiological basis
  • 44. ➢ Increase in the active zones. ➢ Sprouting of axonal branches. ➢ Indicate that cAMP- mediated new protein synthesis. ➢ increased gene transcription- the products of which mediate positive feed back. Sensitization – long term changes
  • 45. ➢ The subject learns about the relationship between two stimuli or between a stimulus and a behavior. ➢ Three forms- Aversion learning, Classical & operant conditioning. ➢ Classical conditioning involves learning a relationship between two stimuli, whereas operant conditioning involves learning a relationship between the organism's behavior and the consequences of that behavior. Associative learning
  • 46. ➢ The essence of classical conditioning is the pairing of two stimuli. ➢ Conditioned stimulus (CS) & unconditioned stimulus (US) (reinforcement). ➢ Unconditioned response- innate; produced without learning. ➢ When a CS is followed by a US, the CS will begin to elicit a new or different response called the conditioned response. Classical conditioning
  • 47. ➢Under normal circumstances Conditioned stimulus (CS) No response Unconditioned stimulus (UCS) UR ➢During conditioning Conditioned stimulus (CS) Unconditioned stimulus (UCS) UR ➢After conditioning Conditioned stimulus (CS) Conditioned response
  • 48. Reward Punishment ➢ Also called trial-and-error learning. ➢ Learning in which behaviors are emitted to obtain rewards or avoid punishment. Operant conditioning
  • 49. ➢ When experiences are aversive, the type of learning that encodes memories of such events is called `aversive learning.’ ➢ 1960- Garcia J & Koelling R ➢ Rats- taste & sickness ➢ Rats- irradiated with strong X rays to damage the GI tract & it was paired with tasty solutions to drink ➢ After recovery- rats refused to drink tasty water ➢ Similar to classic conditioning with respect to pairing of US & CS Aversion learning
  • 50. ▶ Include Latent learning Observational learning Complex learning
  • 51. ➢ A form of learning that is not immediately expressed in an overt response & occurs without obvious reinforcement to be applied later. ➢ An organism learns something in its life, but the knowledge is not immediately expressed. ➢ Remains dormant, and may not be available to consciousness, until specific events/experiences might need this knowledge to be demonstrated. ➢ For instance a child may observe a parent setting the table or tightening a screw, but does not act on this learning for a year; then he finds he knows how to do these. Latent learning
  • 52. ➢ A type of learning that occurs as a function of observing, retaining and replicating novel behavior executed by others. ➢ Although observational learning can take place at any stage in life, it is thought to be of greater important during childhood, particularly as authority becomes important. Observational learning
  • 53. ➢ Practice makes perfect. ➢ Repeated experiences consolidates memory by converting short term to long term form. ➢ Best studied for sensitization in Aplysia. ➢ Short term- doesn't require new protein synthesis. ➢ Long term memory involves 3 major processes- Gene expression New protein synthesis Growth of synaptic connection. Long term storage of implicit memory
  • 54. Long term habituation & sensitization
  • 55. ➢ Studies of long-term sensitization of the gill-withdrawal reflex indicate that: ➢ Serotonin- along with PKA (protein kinase A) recruits another second messenger, Mitogen activated protein kinase (MAPK) ➢ These 2 kinase act on the nucleus of sensory neuron- activate the genetic switch. ➢ Activates transcription factor- CREB1 (cAMP response element binding protein). Also inhibit action of CREB2 – repressor of transcription. ➢ Activation CREB1 induces Activation of ubiquitin carboxyterminal hydrolase- makes PKA persistently active. ➢ Activates transcription factor C/EBP- necessary for the growth of new synaptic connections. Role of gene & protein
  • 56. Persistent synaptic enhancement with long-term sensitization.
  • 57. ▶ Working memory is typically defined as a storage system that holds a limited amount of information for a brief time, where that information is in a rapidly accessible state and can be changed from moment to moment. ▶ It is a combination of the traditional fields of attention, concentration, and short-term memory. ▶ Because it requires active and conscious participation, working memory is an explicit and declarative memory system. ▶ traditionally been divided into components that process phonologic information (e.g., keeping a phone number “in your head”) or spatial information (e.g., mentally following a route) and an executive system that allocates attentional resources. Working memory
  • 58. ➢ Numerous studies have shown that working memory uses a network of cortical and subcortical areas, depending on the particular task. However, virtually all tasks involving working memory require participation of the prefrontal cortex. ➢ Typically, the network of cortical and subcortical areas includes posterior brain regions (e.g., visual-association areas) that are linked with prefrontal regions to form a circuit. ➢ Studies have shown that phonologic working memory tends to involve more regions on the left side of the brain, whereas spatial working memory tends to involve more regions on the right side. ➢ More difficult tasks involving working memory require bilateral brain activation, regardless of the nature of the material being manipulated.
  • 59. ▶
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