Neuro 430 Spring 06 Lecture

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Neuro 430 Spring 06 Lecture 27

• Announcements

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Donald Hebb 1904-1985
Postulated that elements of learning and memory
could be explained by modifications in synaptic
strength. A simple multi-neuron circuit can
completely change its response by modifying the
synapses, not require any real physical change in
the circuit.
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Development Requires Sensory Experience

• Molecular cues guide the initial formation of
  pathways and projections from source to target.
• These circuits must have sensory experience, or
  be exercised in order to refine the connections.
• The presence of sensory experience will enhance
  the connections.
• The lack of sensory experience will cause the
  connections to degrade.

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Ocular Dominance Columns in the Visual System
Molecular cues and axonal guidance cause ganglion
cells from the retina to project through the
thalamus to the visual cortex. How then are the
patterns of the ocular dominance columns refined?
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Neural Responses to Visual Stimulation
- Most cells in V1 respond with a certain
probability to stimulation of either eye. -
Ocular dominance columns are defined by
alternating columns of cells that respond
preferentially to stimulation in one eye over the
other. - However, if one eye is sutured shut from
age 2 weeks to 18 months, cells will only fire to
stimulation of the open eye. - Cells still
respond to stimulation of both eyes after
binocular deprivation. This means the development
of connections is dependent on activity.
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Ocular Dominance is Organized After Birth
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Dendritic Arborization is Stimulation Dependent
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Asynchronous Activity is Required for Proper
Development

• Ocular dominance columns fail to form if retinal
  activity is blocked by TTX in one eye.
• When the two optic nerves are artificially
  stimulated simultaneously, ocular dominance
  columns also fail to form
• Only when the optic nerves are stimulated
  asynchronously do the columns form.
• Simultaneous activity cooperate to enhance
  connectivity, thus the two eyes must compete for
  synapses by all of the axons from the same eye
  working together.

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Artificial Induction of Ocular Dominance Columns
Frog retinal projections are normally not divided
into visual fields. The implanted eye competes
for connections just as in ocular dominance. NMDA
receptor activation controls the segregation of
the inputs to the frog optic tectum.
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Segregation of Retinal Inputs in the Thalamus is
Driven by Spontaneous Synchronized Activity
In-Utero
Initially, axon terminals are randomly
arranged. - Synchronous activity from each eye
causes axons to segregate. Some mammals like the
ferret exhibit additional types of segregation.
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Trophic Factors May Be Responsible for Synapse
Formation
Release of neurotrophic factor by the
post-synaptic neuron such as BDNF (Brain Derived
Neurotrophic Factor) or NT-4 may be taken up by
activated neurons and strengthen the
connections. Release of NTF requires a
threshhold level of stimulation.
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Activity-Dependent Refinement of Connections

• A general feature of circuits in the CNS
• Auditory inputs form tonotopic maps, auditory
  deprivation disrupts these maps
• Critical periods in development define the
  ability of animals to perform coordinated
  movement, learning paradigms, and social
  behaviors.

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Activity Dependent Spine Formation
GFP expressing mice (glowing cells),
Trachtenberg et al., 2002
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Activity Dependent Spines (cont)
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Exercise
Your long time friend invited you to a dinner
party, and has seated you next to a teacher who
has just graduated with a PhD in elementary
school education. This person has just finished
explaining to you about some new methods that are
being implemented around the country that will
change the way that children learn. Apparently,
education research suggests that memorization is
no longer necessary in this new age of
information and computers. Much more time can be
spent on developing problem solving skills since
all the of the pertinent facts can easily be
looked up. Decide the neural implications of this
idea, is this a good or bad idea? In the context
of what you learned in your Neuro 430 class,
explain to this person at least two main reasons
why this is a either a good or bad strategy.