Detailed notes for course 3.6 The Brain that is part of the specialization of Brain & Cognition. This document only includes notes and pictures of relevant chapters for theme 2: changing brain.
! Important: notes for Purves CH 26 are incomplete.
Purves CH 25: experience-dependent plasticity in the developing brain
● Critical periods are limited times of postnatal developmental change, they optimize
the brain so that circuitry is adapted to specific demands that confront that individual
throughout life
● As humans mature, cellular mechanisms that modify neural connectivity become
less effective and changes in neural circuits and related behaviors seen during
critical periods are no longer possible
● Many effects of activity depend on secreted signals, including neurotrophins and
neurotransmitters, transduced via second messengers and their effectors; these
activity-elicited changes influence local gene expression
● Experience-dependent critical periods likely influence connectivity and complex
behaviors; postnatal changes in cortical size may reflect consequence of experience
and activity for differential growth of some cortical areas but not others
NEURAL ACTIVITY AND BRIAN DEVELOPMENT
● Hebb’s postulate states that coordinated electrical activity of presynaptic terminal
and postsynaptic neuron strengthens the synaptic connection between them (will be
retained or sprout new branches)
○ Those terminals that are weakened by uncorrelated activity will eventually
lose hold on the postsynaptic cell, either leading to cell death or stabilization
and growth of synapses from cell on another target
● Hebb’s postulate provided understanding phenomena in brain development
○ Behaviors not initially present in newborns emerge and are shaped by
experience throughout early life
, ○ There is superior capacity for acquiring complex skills and cognitive abilities
during early life
○ Brain continues to grow after birth, in parallel with emergence and acquisition
of complex behaviors and addition of pre and postsynaptic processes
(dendritic and axonal branches)
○ There is subsequent decline in synapse number during adolescence
■ Synapse connections and neural growth are targets of
activity-dependent change during early postnatal life
■ Initially for progressive construction (brain gets larger due to postnatal
growth of dendrites, axons and synapse not due to new neurons) and
then selective elimination of connections (brain continues to grow
because continued elaboration of synapses that remain and neurons
that are their targets)
● Combination of activity-dependent modification of connections (suggested by Hebb),
brain growth and behavioral changes during early life underlie how each individual’s
brain develops to meet challenges of adapting to environment
● Typical experiences validate initial wiring and preserve, increase or adjust initial
arrangement established by intrinsic mechanisms (which establish general circuitry
required for most behaviors)
● Diminished experiences due to lack of sensory exposure or disrupted sensory
transduction have no adjustments, so brain connectivity and behavioral capacity can
be altered (can have adaptive advantage)
● Decline in capacity to remodel cortical connections most likely explains changes in
brain capacity to acquire new information and direct new behaviors over a lifetime
CRITICAL PERIODS
● Most behaviors for basic survival rely on patterns of connectivity established by
intrinsic developmental mechanisms; still, nervous system adapts to and is
influenced by environmental circumstances beyond innate behavioral capacities
● Environmental factors are especially influential in early life, during critical periods
which are times when experience and its neural activity have maximal effect on
acquisition or skilled execution of particular behavior
○ Some behaviors are expressed only if have experience during sharply
restricted time e.g. parental imprinting in hatchling birds
○ Other behaviors end less abruptly e.g. sensorimotor skills
● Acquisition of language in humans requires instructive influences (exposure to words
and sentences) and opportunities for mimicry and repetition for extended period to
ensure normal development of the behavior
● Key for successful completion of critical period = availability of instructive
experiences from environment (important for territorial and reproductive behaviors in
non-humans) and neural capacity to respond to them
● Basic properties of critical periods
○ Encompass time during which given behavior is especially susceptible to
specific environmental influences to develop normally
○ Environmental influence elicits neural activity; its nature (frequency,
amplitude, duration, and correlation) drives changes in synaptic connections
, ○ Once critical period ends, core features of behavior are largely unaffected by
subsequent experience
○ Cellular and molecular mechanisms influenced by experience via neural
activity also change
○ Failure to be exposed to stimuli during critical period is difficult to solve later
because biological mechanisms needed to change connections are no longer
available in older people
● In most mammals, critical periods seem to rely on changes in organization and
function of circuits in the cerebral cortex
ROLE OF OSCILLATIONS
● Elicited sensory experience (driven by external activity and resulting in action
potential) is key BUT other forms of electrical activity occur prior to experience-driven
activity, these establish a framework within which sensory experience can influence
patterns and function of connectivity
● Local oscillations are waves of electrical activity initially beneath the threshold for
action potential generation, essential for shaping circuit networks so are prepared for
optimal experience-driven activity
○ Most evidence come from analysis of mammalian visual system
○ Retinal waves is activity in the retina long before birth; presence of scaffold of
segregated inputs from lateral geniculate nucleus (LGN) due to organized
electrical activity in retinas
○ Each retina in individual fetus or newborn independently generates patterns of
waves of electrical activity (measured as Ca2+ influx) that moves across
population of retinal cells in orderly way
○ Waves are initiated in local retinal cells (amacrine cells) leading to action
potential firing by ganglion cells, then relayed to LGN
○ Waves are asynchronous between the two eyes, which establishes
competitive interaction between them for target space in LGN and then
primary visual cortex
○ Experiments suggest that nascent circuit (includes ACh, GABA and
glycinergic synaptic transmission from retinal amacrine cells) combined with
glutamatergic release from bipolar cells onto retinal ganglion cells is
responsible for generation of retinal waves
● In early postnatal humans, burst of electrical activity (spindles) in cortex indicates
that even before extensive visual experience, there is subthreshold oscillatory activity
and burst of action potential signaling, these influence synapse and circuit
maturation in cortex
○ Absence of cortical activity delays or abolishes appropriate maturation of
visual responses
VISUAL SYSTEM DEVELOPMENT
● Visual system is extremely responsive to experimental manipulations necessary to
test relationship between experience, activity and circuitry
○ Easy to deprive or increase visual experience in experimental animal
(compared to other senses)
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