Neuroscience and memory Dr' Simon J' Davies

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Neuroscience and memory
Dr. Simon J. Davies
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Interesting facts!

• 100 150 billion neurons in your brain!
• Each neuron connects to approx 10,000 other
  neurons.
• If each neuron connected to all other neurons,
  your brain would be the size of Greater London.
• Neurons make up only 10 of your brain and 2 of
  your body weight.
• It is a myth that you only use 10 of your brain.
  
• Neurons can be regenerated despite the myth
  that they only die.

Source Ward, 2006.
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Horizontal section
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The Basal Ganglia
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Limbic System
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The Ventricles
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The brain (local structures)
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Cells of the nervous system

• There are two primary cell types in the CNS and
  PNS neurons and neuroglia.
• Neurons have the ability to send signals by
  having permeable membranes.
• Glial cells vary and carry out a number of basic
  functions supporting neurons.

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Neuroglia

• Microglia phagocytes.
• Astrocytes structural support and nutrition.
• Oligodendroglia support and myelination of
  axons.
• Schwann cells support and myelination.

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Neuroglia
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Myelination of neurons
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Case study - Multiple Sclerosis

• Multiple Sclerosis (MS) is thought to be an
  autoimmune disease at the level of the brain and
  spinal cord (CNS).
• It cause demyelination of neurons.
• Myelin helps nerve signals to be conducted from
  and to the CNS.
• MS results in multiple scarring (sclerosis)
  affecting movement and perception.

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Neuroma - glioma
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Neurons

• Receive, process and send signals.
• Varieties (functions) interneurons, sensory
  neurons, pre- post-synaptic, association etc.
• Varieties (shape) bipolar, unipolar, Purkinje,
  etc.
• Have permeable membranes whose properties permit
  the generation of electrical signals.

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Types of neuron
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Neuron basic transmitting nerve cell
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Sensory receptors

• Differ according to the sensory stimuli they
  transduce.
• Differ in transduction processes thermal
  nociceptor, mechanoreceptor, photoreceptor etc.
• Can be modified bipolar neurons but generally
  dont have an axonal process.
• Differ in the rates at which they adapt to a
  stimulus (fast or slow).

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Photoreceptor
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Communication in the brain
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Neural signals - potentials

• A potential (i.e. voltage) difference exists
  between inside and outside all nerve cells. This
  can be changed in three ways
• Receptor potential change in membrane potential
  of a sensory receptor (slow and graded).
• Synaptic potential change in membrane potential
  of a postsynaptic neuron (slow and graded).
• Action potential change in membrane potential
  of an axon (fast and all-or-nothing).

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The action potential
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Voltage-gated ion channels
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Active ion transport
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Ion gradients

• Ion movement is affected by electric and chemical
  concentration differences.
• Chemical gradients the relative difference in
  the concentration of a particular chemical.
• Electrical gradients the relative difference
  between voltage of the outside compared to the
  inside of the cell.

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Synapses

• Two types of synapse electrical (quick)
  chemical (slow).
• Chemical synapses have two forms of receptor that
  respond differently to transmitter.
• 1. Ionotropic ligand-gated (binds with a
  transmitter opens channel)
• 2. Metabotropic indirect, g-protein-gated
  receptor.

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Electrical synapse
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Chemical synapse
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Neural processing

• Two types of postsynaptic potential
• Excitatory PSP (towards action potential)
• Inhibitory PSP (away from action potential)
• PSPs summed in two ways
• Spatial summation (PSP from different input
  summed)
• Temporal summation (PSP from same input summed)

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The effect of different synapses
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EPSP and IPSP
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Temporal summation
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Spatial summation
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Neural stimulus coding

• There are four qualities a neuron can code for
• Quality what type of receptor responds.
• Duration timing of stimulus.
• Location label-line code ensures body areas are
  topographically mapped.
• Intensity - number of action potentials or size
  of receptor potential.

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Neural information processing

• Cells only respond is stimulated within a certain
  area their receptive field.
• Receptive fields differ widely in the nervous
  system but many have a centre-surround
  organisation.
• Cells can send a diffuse signal (divergence) or a
  sharp signal (convergence).
• Cells can excite or inhibit another cell.

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Memory stages

• Encoding learning information. Likelihood of
  entering LTM dependent upon attention, motivation
  and association.
• Consolidation how new memories are changed from
  fragile to stable.
• Retrieval involves bringing together different
  kinds of information. A constructive process.

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Memory and neurons

• It is important to note, that it is not the
  neuron that has memory.
• Memory is a pattern of activation of a set of
  neurons.
• This is achieved by temporal co-incidence of
  firing linked to related neurons.
• Synchronous firing of neural populations is also
  hypothesised for the recognition and coding of
  several features of an object.

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Learning Memory - Conditioning

• Classical or associative conditioning (Pavlov)
  relationship between conditional stimulus and
  conditional response
• Instrumental conditioning (Skinner) learning
  based on reward or punishment the result is a
  declarative memory.

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Hebbian learning.

• Donald Hebb (1949) explains classical conditional
  by
• Pre- and postsynaptic neurons in a relationship
• Subsequent stimulation increase likelihood of
  postsynaptic cell responding (synaptic
  efficiency)
• Thus, learning (and memory) in classical
  conditioning are explained by this Hebbian
  learning via a Hebbian synapse.

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But how???

• An unconditional neuron (bell) fires at the same
  time as an influential neuron (food) linked to a
  post-synaptic neuron (salivation).
• There is massive interconnectivity in the brain
  so weak links to most centres.
• With repetition, the link becomes strengthened
  (synaptic efficiency) , so that eventually
  unconditioned neuron becomes able to stimulate
  salivation.

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Long term potentiation (LTP)

• LTP refers to a Hebbian effect in stimulated
  neurons.
• Essentially, repeated stimulation eventually
  results in a continued increase in synaptic
  response.
• Importantly, LTP is shown to last thus,
  supporting the notion that Hebbian synapses can
  lead to LTM.
• For LTP to be successfully induced a certain
  threshold must be reached thus the need for the
  normal stimulus in the presence of the
  unconditional stimulus.

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Amnesia LTP deficit?

• Amnesia is impaired declarative/episodic memory
  (anterograde or retrograde).
• One mechanistic explanation of amnesia is a
  problem with consolidation.
• However, amnesiacs can learn some things.
• LTP now thought to involve two types short term
  and long term consolidation.

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The Garcia Effect

• Does all memory require repeated stimulation?
• No. The Garcia Effect shows learning through a
  single exposure.
• Rats, when given food that makes them sick will
  not touch the food again.
• Humans, also appear to have preparedness with
  respect to specific events (e.g. spiders, snakes,
  heights, etc.).

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H.M. again???

• Bilateral excision of the medial temporal lobe,
  including the hippocampus for epilepsy.
• Epilepsy cured.
• No new declarative memories.
• Still able to create new procedural memories.
• However, can not remember learning new motor
  skills!

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Hippocampus and LTM

• Hippocampus not the site of LTM.
• Creates new memory, then transfers to cerebral
  cortical storage system.
• However, damage to the association cortices, does
  influence semantic and episodic memories.
• These will be dependent on which association
  cortex is influenced.

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Bird hippocampal lesions
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Example of association dissociation

• Patient A associative agnosia given a set of
  drawings, can draw accurately but cannot name.
• Patient B apperceptive agnosia cannot draw but
  can name.
• Thus episodic and semantic memory not localised,
  but supported by multiple representations in
  different areas of the brain.

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Spatial memory

• The hippocampus is also used to develop rapid
  spatial memories.
• London cabbies have a larger posterior
  hippocampus.
• Scrub Jays (birds) have spatial memories for up
  to 33,000 items of food in 7,000 locations.
• Scrub Jays are born with a potential for a larger
  hippocampus.

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Alzheimers disease and memory

• Alzheimers disease affects attention and memory.
• Most recent symptoms are declarative memory loss
  and lack of attention.
• Associated with neuron loss within the
  hippocampus and medial lobe.
• Thus, cellular loss directly related to memory
  structures of the brain.

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Recognition Memory

• Some memory involves the depression of synaptic
  responsiveness.
• One example is recognition memory and the neurons
  in the perirhinal cortex.
• Recordings from the perirhinal cortex show
  increased activity with novelty, but decreased
  activity with familiarity.
• Unlike LTP, we now have long-term depression
  (LTD).

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Perirhinal cortex
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Working memory

• Unlike LTM (supported by LTP), WM is affected by
  short-term increase in AP firing.
• Prefrontal cortex involved in WM.
• In monkey expts, prefrontal neurons increase
  firing rate, not at encoding, but during periods
  of delay.
• Can you think why the AP firing rate increases
  during the maintenance period?

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Forgetting

• 84 of psychologists agree with the statement
  everything we learn is permanently stored.
• Lurias patient S could remember everything,
  but not understand simple passages.
• Forgetting, reduction in LTP, is important to
  selectively maintain material that is relevant to
  us.