Lecture 16: Plasticity in Development Flashcards
What is netrin?
- A chemoattractant molecule that binds to the DCC receptor
Which guidance molecule is responsible for preventing developing axons from crossing midline more than once?
- Slit
If one followed a growth cone through a ‘decision point’ at which a choice about direction must be made, the growth cone…
- Extends filopodia and searches for environmental cues
What molecules support an innervating neuron, leading to its growth/survival?
- Neurotrophic factors
In general, what is one role of neurotrophic factors?
- Adjusting size of neuronal populations to an appropriate number
What type of mechanisms does neural development include?
- Fairly stereotypical cellular and molecular mechanisms
Is neurodevelopment sufficient to account for behaviour/personality differences?
- No
- Synapses can be reinforced/pruned based on activity patterns that can be heavily influenced by the environment
What is Hebb’s postulate?
- Coordinated electrical activity of a presynaptic terminal and a postsynaptic neuron strengthens the synaptic connection b/n them
- Neurons that fire together, wire together
What was Hebb’s postulate originally proposed to explain?
- cellular learning/memory
- LTP/LTD
- But it is far more versatile
- Long-term modifications in synaptic strength/distribution
What happens when inputs on a postsynaptic neuron are correlated?
- Better able to activate postsynaptic cell
- Cause postsynaptic cell to acquire similar firing pattern
What happens when inputs on a postsynaptic neuron are poorly-correlated?
- Gradually weaken
- Are eliminated
- Synapses lost and nerves may die off
What is the overall effect of Hebb’s postulate?
- Creates synchrony within circuits of the brain
What are the 3 phenomena observed with the developing brain?
- Behaviours not initially present emerge and are shaped by early experience (ex. learn to walk)
- Superior capacity for learning complex skills and cognitive abilities early in life (extra sensitivity to environmental inputs)
- Brain continues to grow after birth
What happens to neural circuits early in life?
- Growth of dendrites and axons
- Combined with synaptogenesis
What happens to neural circuits later in life?
- Pruning of weak or redundant synapses parallels strengthening of synapses that remain
- Aim for energy efficiency
What does a neural circuit look like at birth, compared to 2 and 6 years?
Birth - not many connections, fairly spread out
2 years - very dense, lots of connections
6 years - more connections than birth, but less than 2 years due to pruning
What does the progression of synapses look like over time in the human brain?
- Early rapid increase, then elimination of synapses
- Highest around 8 months
What does the elaboration of remaining synapses occur in synchrony with?
- Sensory and motor abilities
- Capacity for social interaction
- Cognitive behaviours (ex. language)
- Cellular connectivity reflects this
- All sensitive to environment
What do mechanisms need to be combined with to explain development?
- Flexibility demanded by dynamic environment
- Typical experiences validate initial wiring
- Diminished experiences can alter brain connectivity
Is the brain capacity for remodelling consistent?
- No
- Eventual decline
What is a critical period?
- Complex circuit formation during specific developmental time frames (temporal windows)
- Some are quite constrained, others are less constrained
- Although variable in duration/behaviours, all encompass time during which a behaviour requires specific environmental influences for normal development
What did Konrad Lorenz find?
- Goslings follow first large moving object they see and hear upon emergence from egg
- Critical period lasts less than a day
How can stimulus-evoked instincts be tested in a lab? What is the result?
- Chicks on running wheel imprint on red circle
- Approach red circle and avoid blue square
What is an example of critical period for ewes?
- Ewes have 2-4hr after birth to imprint on scent of their own lamb
- Otherwise will reject lamb
What is a less constrained critical period?
- Communication in songbirds and humans
- Extended period to ensure normal development
What effect does environment have on neural circuitry?
- Environmental influence elicits neural activity
- Nature of activity drives changes in synaptic connections
What happens if there is a failure to be exposed to appropriate stimuli during critical period?
- Difficult/impossible to remedy
- Ex. visual development/language development
- Deprivation of circuitry of chance to remodel during the critical period
Where does visual system development occur?
Afferents of LGN of thalamus terminate in visual/striate cortex (occipital lobe)
- Layered (1-6)
How do neurons respond to visual stimulation (except in layer 4)?
- Neurons respond to varying degrees from stimulation in either eye
What is different about Layer 4?
- There are ocular dominance columns (alternating series of eye-specific domains)
- All neurons are driven exclusively by one eye or ther other
How can you measure which cells are innervated by a certain eye?
- Injection of tracers into that eye
- Tracer is transported along visual pathway that corresponds to that eye
Is ocular dominance seen in only layer 4?
- No, seen in other layers
- Not as neatly organized
How can you find categorization of ocular dominance?
- Pass an electrode through visual cortex while responses of individual neurons to light stimulation in one or the other eye is recorded
What are ocular dominance categories?
- Defined based on frequency of AP activity elicited from visual cortical neurons following illumination in relevant eye
- How often does that neuron fire?
Describe group 1?
- Driven only by contralateral eye stimulation
Describe group 7?
- Driven only by ipsilateral eye stimulation
Describe group 4?
- Driven equally well by both eyes
What does the graph of ocular dominance groups look like in most layers?
- Normal distribution with most cells in group 4 (being driven equally well by either eye)
What happens when a cat has one eye closed b/n 1 wk and 2.5 months of age?
- Cortical cells only responsive from ipsilateral, non-deprived eye
- Recordings in retina and LGN layers indicate normal activity in response to electrical stimulation
- Cortical blindness is permanent
- Only group 7 neurons (or nt responsive)
What is the result of monocular deprivation in an adult cat?
- Outside critical period
- Neuronal responses still driven by both eyes
- Cortical activity is generally attenuated
How many days of deprivation during critical period causes shift in favour of non-deprived eye?
- 3 days
What happens after 6 days of monocular deprivation during the critical period?
- Causes a shift in favour of non-deprived eye nearly to extent of 2.5 months of deprivation
What did Hebbian discover?
- There is a competitive interaction for post-synaptic space b/n afferent axons driven by each of the 2 eyes during the critical period
What is the result of competitive interaction that occurs from monocular deprivation?
- Active eye gains competitive advantage
- Replaces many of the synaptic inputs that would have remained innervated from closed eye
What happens to axons terminating in layer 4 from LGN after monocular deprivation during critical period?
- Greatly reduced branching
What is the result of binocular deprivation in the critical period?
- Results in ocular dominance arrangement that, though attenuated, is similar in distribution to that of no deprivation (normal)
- Indicates that balance of inputs shapes normal pattern of connections
- Both eyes retain territory in cortex
What does language development look like at 4 months?
- Can distinguish all sounds measured by responsivity to novelty
What does language development after 6 months look like?
- Show preference for native language
After about 12 months, what level of language development will the infant be at?
- No longer respond robustly to phonetic elements particular to non-native language
When does the critical period for new language fluency decline?
- After age 7
What does the cellular basis of language development in humans look like?
- Evidence for shifted mode of processing
- Neural regulation changes
- Dominant brain structures used shifts
