neural development cs182/cogsci110/ling109 spring 2008 srini narayanan [email protected]
TRANSCRIPT
Lecture Overview
Summary Overview Development from embryo Initial wiring Activity dependent fine tuning Additional information/details
Principles of Neural Science. Kandel, Schwartz, Jessell, Mcgraw-Hill (2000).
How does this happen
o Many mechanisms of human brain development remain hidden, but
o Neuroscientists are beginning to uncover some of these complex steps through studies of
o the roundworm, fruit fly, frog, zebrafish, mouse, rat, chicken, cat and monkey.
o Many initial steps in brain development are similar across species, while later steps are different.
o By studying these similarities and differences, we cano learn how the human brain develops and o how brain abnormalities, such as mental retardation and other
brain disorders, can be prevented or treated.
Summary Overview
The Amoeba (ref. book) uses complex sensing molecules penetrating its cell membrane to
trigger chemical mechanisms that cause it to move its blobby body
towards food and away from harmful substances.
Neurons are also cells and, in early development, behave somewhat like Amoeba in
approaching and avoiding various chemicals. But rather than the whole cell moving, neural growth involves the
outreach of the cell’s connecting pathway (axon) towards its downstream partner neurons.
Summary Overview
The basic layout of visual and other maps is established during development by neurons each separately following a pattern of chemical markers to its pre-destined brain region and specific sub-areas within that
region.
For example, a retinal cell that responds best to red light in the upper left of the visual field will connect to cells in the
brain that are tuned to the same properties and these cells, in turn, will link to other cells that use these particular
properties giving rise to specific connectivity and cell receptive field properties (topographic maps).
Summary Overview In the course of development,
detector molecules in the growing neuron interact with guide molecules to route the connection to the right general
destination, sometimes over long distances as in the connection from the spinal cord to the knee.
This process will get neural connections to the right general area, but aligning the millions of neurons in visual and other neural maps also
involves chemical gradients, again utilizing mechanisms that are very old in
evolutionary terms.
Summary Overview
When an axon tip gets to an appropriately marked destination cell, the contact starts a process that develops rudimentary synapses.
Local competition among neural axons with similar marker profiles produces some further tuning at the destination.
Summary Overview:Activity Dependent Tuning In fact, the initial wiring is only approximate and leaves
each neuronal axon connected to several places in the neighborhood of each of its eventual partner neurons.
A second, activity dependent, mechanism is required to complete the development process.
The initial chemical wiring actually produces many more connections and somewhat more neurons than are present in adult brains.
The detailed tuning of neural connections is done by eliminating the extra links, as well as the strengthening functional synapses based on neural activity.
Summary
Neural development and learning is moving from a mystery to routine science.
We know enough to shape theories of how our brains learn skills, including language, and how we acquire and use knowledge.
Contemporary theories of learning are also heavily influenced by psychological experiments, some of which are described next week.
Lecture Overview
Summary Overview Development from embryo
Neural tube development Cell division and neuronal identity Mechanisms for cell type formation and
communication Initial wiring Activity dependent fine tuning Plasticity and Learning
Development from Embryo
The embryo has three primary layers that undergo many interactions in order to evolve into organ, bone, muscle, skin or neural tissue. The outside layer is the ectoderm
(skin, neural tissue),
the middle layer is the mesoderm (skeleton, cardiac) and
inner layer is the endoderm (digestion, respiratory).
Neural Tissue The skin and neural tissue arise from a
single layer, known as the ectoderm in response to signals provided by an adjacent
layer, known as the mesoderm. A number of molecules interact to determine
whether the ectoderm becomes neural tissue or develops in another way to become skin
Neural Tube formation
In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds.
The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up” cranially and caudally and the neural crest migrates into the mesoderm (somites differentiate to form vertebrae, muscles).
Other structuresincluding heart,Skeleton, etc.
BRAIN DEVELOPMENT. The human brain and nervous system begin to develop at three weeks’ gestation as the closing neural tube (left). By four weeks, major regions of the human brain can be recognized in primitive form, including the forebrain, midbrain, hindbrain, and optic vesicle (from which the eye develops). Irregular ridges, or convolutions, are clearly seen by six months.
Brain Weight
Lecture Overview
Summary Overview Development from embryo
Neural tube development Cell division and neuronal identity Cell communication
Initial wiring Activity dependent fine tuning Plasticity and Learning Development and Infant behavior
Neural cell categories
After the ectodermal tissue has acquired its neural fate,
another series of signaling interactions determine the type of neural cell to which it gives rise.
The mature nervous system contains a vast array of cell types, which can be divided into two main categories: the neurons, primarily responsible for signaling, and supporting cells called glial cells.
Factors/gradients in cell formation Researchers are finding that the destiny of neural tissue
depends on a number of factors, including position, that define the environmental signals to which the cells are exposed. For example, a key factor in spinal cord development is a
secreted protein called sonic hedgehog that is similar to a signaling protein found in flies.
The protein, initially secreted from mesodermal tissue lying beneath the developing spinal cord, marks young neural cells that are directly adjacent to become a specialized class of glial cells.
Cells further away are exposed to lower concentrations of sonic hedgehog protein, and they become the motor neurons that control muscles.
An even lower concentration promotes the formation of interneurons that relay messages to other neurons, not muscles.
Timing of Cell Differentiation
Remarkably, the final position of the neuron (its laminar position) is correlated exactly to its birthdate The birthdate is the time of final mitosis
Cells leaving later migrate past the older neurons (in deeper cortical layers) to the outermost cortex.
The layering of the cortex is thus an inside-first outside-last layering.
Illustration by Lydia Kibiuk, Copyright © 1995 Lydia Kibiuk.
Improper migration leads to diseases including childhood epilepsy, mental retardation, lack of sense of smell and possibly others.
Hiroshima Nagasaki Effects
As the brain develops, neuronsmigrate from the inner surfaceto form the outer layers. Left:Immature neurons use fibersfrom cells called glia ashighways to carry them to theirdestinations. Right: A singleneuron, shown about 2,500times its actual size, moves ona glial fiber. (10-6 m/hr)
Neural Development Time Lapse Video (Nature Neuroscience 2001)
http://www.nature.com/neuro/journal/v4/n2/extref/nn0201-143-S1.mpg
Lecture Overview
Summary Overview Development from embryo Initial Wiring details
Axon Guidance Synapse formation
Activity dependent fine tuning Plasticity and Learning Development and Infant behavior
Axon guidance mechanisms
Axonal growth is led by growth cones Filopodia (growing from axons) are able to sense the
environment ahead for chemical markers and cues. Mechanisms are fairly old in evolutionary terms.
Intermediate chemical markers Guideposts studied in invertebrates
Short and long range cues Short range chemo-attraction and chemo-repulsion Long range chemo-attraction and chemo-repulsion
Gradient effects
Axons locate their target tissues by using chemical attractants (blue) and repellants (orange) located around or on the surface of guide cells. Left: An axon begins to grow toward target tissue. Guide cells 1 and 3 secrete attractants that cause the axon to grow toward them, while guide cell 2 secretes a repellant. Surfaces of guide cells and target tissues also display attractant molecules (blue) and repellant molecules (orange). Right: A day later, the axon has grown around only guide cells 1 and 3.
Synapse formation The two cells exchange a variety of signals.
Vesicles cluster at the pre-synaptic site Transmitter receptors cluster at the post-synaptic site.
The Synaptic Cleft forms When the growth cone contacts the target cell
(immature muscle cell in the case of a motor neuron), a cleft (basal lamina) forms.
Multiple growth cones (axons) get attracted to the cleft.
All but one axon is eliminated. A myelin sheath forms around the synaptic cleft
and the synaptic connection is made.
Overall Process
Basic Process Neurons are initially produced along the central canal in the neural
tube. These neurons then migrate from their birthplace to a final
destination in the brain. They collect together to form each of the various brain structures
and acquire specific ways of transmitting nerve messages. Their processes, or axons, grow long distances to find and connect
with appropriate partners, forming elaborate and specific circuits. Finally, sculpting action eliminates redundant or improper
connections, honing the specificity of the circuits that remain. The result is the creation of a precisely elaborated adult network
of 100 billion neurons capable of a body movement, a perception, an emotion or a thought.
Nature requires Nurture
Initial wiring is genetically controlledSperry Experiment
But environmental input critical in early development Occular dominance columns
Hubel and Wiesel experiment
Sperry’s experiment
Each location in space is seen by a different location on the retina of the frog
Each different location on the retina is connected by the optic nerve to a different location in the brain
Each of these different locations in the brain causes a different movement direction.
In a normal animal, a retinal region which sees in a particular direction is connected to a tectal region which causes a movement in that direction
Innervation of the Optic tectum
Sperry’s experiment
Sperry took advantage of the fact that in amphibians, the optic nerve will regrow after it has been interrupted
Sperry cut the optic nerve and simultaneously rotated the eye 180 degrees in the eye socket.
In 'learning’ movements to catch prey, the part of the retina now looking forward (backward) should connect to the part of the brain which causes forward (backward) movement.
Sperry’s findings
After regeneration, his animals responded to prey items in
front by turning around and to prey items behind by moving forward.
and kept doing this even though they never
succeeded in reaching the prey.
Conclusion from experiment
The conclusion from this (and some supporting experiments) is
that the pattern of connections between retina and tectum, and the movement information represented is not based on experience.
It is innate based on the initial distribution of chemical markers in the brain.
Lecture Overview
Summary Overview Development from embryo Initial wiring Activity dependent fine tuning
The role of the environment
The development of ocular dominance columns Cat and later monkey (Hubel and Wiesel)
Retinal input connects to the LGN (Thalamus) LGN is composed of layers. Each layer receives input (axons)
from a single eye LGN connects to layer IV of the visual cortex
The visual cortex develops ocular dominance columns Cells that are connected to similar layers in the LGN get stacked
together in columns forming stripes.
http://neuro.med.harvard.edu/site/dh/
VISUAL CORTEX
LGN
Monocular deprivation critical period Hubel and Wiesel deprived one of the eyes of
the cat (later macaque monkey) at various times 1 week – 12 weeks (in the monkey case) 4 weeks – 4 months (for the cat).
The found that the ocular dominance cell formation was most severely degraded if deprivation occurred at 1 – 9 weeks after birth.
Deprivation after the plastic period had no long-term effect.
Critical Periods in Development
There are critical periods in development (pre and post-natal) where stimulation is essential for fine tuning of brain connections.
Other examples of columnsOrientation columns
Pre-Natal Tuning: Internally generated tuning signals In the womb, what provides the feedback to establish which neural
circuits are the right ones to strengthen? Not a problem for motor circuits - the feedback and control networks for
basic physical actions can be refined as the infant moves its limbs and indeed, this is what happens.
But there is no vision in the womb. Recent research shows that systematic moving patterns of activity are spontaneously generated pre-natally in the retina. A predictable pattern, changing over time, provides excellent training data for tuning the connections between visual maps.
The pre-natal development of the auditory system is obviously relevant Research indicates that infants, immediately after birth, preferentially
recognize the sounds of their native language over others. The assumption is that similar activity-dependent tuning mechanisms
work with speech signals perceived in the womb.
Post-natal environmental tuning
The pre-natal tuning of neural connections using simulated activity can work quite well – a newborn colt or calf is essentially functional at birth.
This is necessary because the herd is always on the move. Many animals, including people, do much of their
development after birth and activity-dependent mechanisms can exploit experience in the real world.
In fact, such experience is absolutely necessary for normal development. As we saw, early experiments with kittens showed that
there are fairly short critical periods during which animals deprived of visual input could lose forever their ability to see motion, vertical lines, etc.
Adult Plasticity and Regeneration
The brain has an amazing ability to reorganize itself through new pathways and connections rapidly.
• Through Practice:• London cab drivers, motor regions for the skilled
• After damage or injury • Undamaged neurons make new connections and take
over functionality or establish new functions• But requires stimulation (phantom limb sensations)
• Stimulation standard technique for stroke victim rehabilitation
Summary
Both genetic factors and activity dependent factors play a role in developing the brain architecture and circuitry. There are critical developmental periods where
nurture is essential, but there is also a great ability for the adult brain to regenerate.
Next lecture: What computational models satisfy some of the biological constraints.
Question: What is the relevance of development and learning in language and thought?
5 levels of Neural Theory of Language
Cognition and Language
Computation
Structured Connectionism
Computational Neurobiology
Biology
MidtermQuiz Finals
Neural Development
Triangle Nodes
Neural Net
Spatial Relation
Motor Control Metaphor
SHRUTI
Grammar
abst
ract
ion