Early experience and neural
development
Overview of neuronal development
Neuronal survival vs. apoptosis
Competition for cortical space
The critical period
Cortical plasticity in the adult
Neuronal Development
Growth and Differentiation of the Vertebrate Brain
CNS begins to form at two weeks gestation
At birth, brain weighs 350g, at one year 1,000g
Growth and Development of Neurons
Proliferation-
production of new cells
Migration-
move toward final destination
Differentiation-
form axons and dendrites
Diverse shapes of neurons
Transplantation experiments
Myelination-
addition of insulating sheath
Neuronal survival
At 11 weeks (gestational age) we have 2-3 times as many
neurons as needed
“Pruning”: As connections are formed the number drops
(until 25 weeks)
What determines which neurons survive?
Studies by Hamburger, Levi-Montalcini
Neuronal survival
What determines which neurons survive?
Studies by Hamburger, Levi-Montalcini
Competition for “trophic factor” secreted by target
examples: NGF, BDNF, GDNF; (specific receptors on neurons)
Winners survive, losers undergo “apoptosis” (programmed
cell death)
Who wins competition for connections/ trophic factors?
- Activity dependent development of primary visual cortex (V1):
Ocular Dominance Columns (ODC)
Visual system: partly crossed in human, monkey, cat
Each half of V1 receives input from both eyes
Some V1 cells respond to only one eye, some to both
Cells with similar ocular dominance are organized in columns
Changing the balance of activity between the two eyes:
Effects of Monocular Deprivation
- early vs. late deprivation; “critical period”
Binocular deprivation birth to 3 months (monkey)
Observations in Frog Visual System
Questions:
1. How would you demonstrate involvement of trophic factor?
2. What implication do the findings have for the removal of
congenital cataract?
(video clip)
Conclusions
Neuronal survival is mediated by competition for targetderived trophic factors.
Similarly, cortical organization is mediated by activitydependent competition early in life (i.e. during the critical
period).
This activity-dependent competition appears to be mediated
by trophic factors.
Normal experience during the critical period that leads to
normal neural activity is essential for normal cortical
development.
How drastically can cortical organization be influenced
by sensory inputs?
- auditory cortex responds to vision in “rewired” ferret brains
(von Melchner et al., 2000)
- visual cortex in the congenitally blind is responsive to touch
Plasticity later in life
Cortical reorganization in somatosensory cortex:
- monkey amputation studies
- phantom limbs
- string players
- effects of non-use in stroke patients
Phantom Limb Sensations:
Sensations in amputated hand
arise when face is touched.
Explanation:
Axons from face now activate
hand area in cortex.
Since neurons in cortex hand
area represent feelings in hand,
their activity is experienced as
arising from non-existent hand!