Critical Periods of Development
Lecture 7: Critical Periods of Development
Introduction to Critical Periods
Critical periods refer to specific, time-limited windows in development when certain events and environmental exposures have a maximal and enduring impact on the shaping of brain circuitry and behavior. For example, language acquisition and vision development have critical periods.
Environmental factors, such as nutrition, sensory input, and social interactions, during infancy and early childhood can have profound and lasting effects on brain architecture, cognitive abilities, and emotional regulation, although the influence of experience on brain development extends beyond the early years.
Ongoing brain changes, characterized by plasticity (the brain's ability to reorganize itself by forming new neural connections throughout life), continue to occur throughout life, crucial for learning, memory, and adaptability, albeit often requiring more effort or different mechanisms than during critical periods.
Differences in Capacity for Change
Critical periods are characterized by a heightened capacity for change due to a surplus of synaptic connections and heightened neurochemical sensitivity, which diminishes significantly with age as synaptic pruning and myelination stabilize neural circuits.
As individuals age, the ability to develop full proficiency in various skills, like acquiring a native-like accent in a second language or recovering from extensive brain injury, becomes limited due to reduced plasticity and established neural pathways.
While changes in the brain can occur at any age, they generally require more substantial, prolonged, or intense experiences (e.g., extensive therapy, deliberate practice) to effect significant and observable change compared to early developmental stages.
Early Development Phenomenon
Rapid human development is an impressive natural feat, starting from a single cell. This period involves exponential cell division, differentiation, and the coordinated formation of complex organs and systems.
Changes begin at conception as the zygote undergoes rapid cell division and progressive differentiation, forming various specialized elements of the human body. This includes the intricate development of the brain, which eventually contains approximately billion neurons and trillions of synaptic connections, a process driven by a precisely timed genetic program and environmental interactions.
Blastocyst and Early Cellular Development
Three-Cell Conceptus
The term "conceptus" refers to a cluster of undifferentiated cells originating shortly after conception, specifically from the zygote's initial divisions. These cells are pluripotent, meaning they have the potential to differentiate into any cell type.
The conceptus begins as three identical cells, which rapidly divide and later differentiate into various cell types that will form all the tissues and organs of the body.
Blastocyst Formation
Around six days post-conception, following several stages of cleavage, a blastocyst forms, containing several dozen cells, typically between and . This structure is crucial for implantation into the uterine wall.
The outer layer of the blastocyst develops into trophoblasts, which are essential for forming the placenta and providing nourishment, while the inner cell mass differentiates into the embryo itself (us), containing the stem cells that will give rise to all body tissues.
Differentiation and Cell Development
Our body comprises at least different specialized cell types, all arising from three primary germ layers established during gastrulation: the ectoderm (which forms skin and nervous system), mesoderm (muscle, bone, blood, connective tissue), and endoderm (internal organs like the gut and lungs).
Chemical messengers, often growth factors and signaling molecules (not hormones or neurotransmitters in this early stage), guide the precise differentiation of cells by activating specific genes within the cells, directing them to become particular types.
As development progresses, we observe precise spatial and temporal layers within the ectoderm differentiating into neurons and glial cells, forming the neural plate which folds to create the neural tube.
Neuronal Differentiation and Organization
Neural Crest and Tube Formation
The neural plate folds inward to form a neural groove, which then fuses to create a tube-like structure known as the neural tube. This tube's inner area evolves into the ventricles of the brain for cerebrospinal fluid circulation, and its walls form the brain and spinal cord.
The neural crest, a transient group of cells that migrate from the dorsal part of the neural tube, forms into a variety of peripheral nervous system structures. The forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) emerge as primary brain vesicles during the early stages of development, with major brain divisions already identifiable within the first few weeks (around week to ) of gestation.
Neuron Types and Arrangement
Within weeks of development, specific types of neurons—sensory neurons (transmitting information from the body to the CNS), motor neurons (transmitting commands from the CNS to muscles and glands), and interneurons (connecting neurons within the CNS)—emerge in specific rows and layers of the neural tube walls. For example, motor neurons are typically found in the ventral horn.
The precise organization and patterning of these neurons, dictated by genetic programs and signaling gradients, is critically underway by week two of development, laying the foundation for functional neural circuits.
Neuron Development and Migration
Neurogenesis
Neurogenesis is the process of new neuron production from neural stem cells and progenitor cells. In humans, nearly all cortical neurogenesis occurs prenatally, primarily within the first four months of development, with only limited neurogenesis (e.g., in the hippocampus) continuing postnatally.
Neuron production rates vary significantly throughout these developmental stages, peaking at approximately million per day during the second trimester of gestation, ensuring a rapid increase in brain cell number.
Neuron Migration and its Challenges
After their birth in the ventricular zone, neurons must migrate to their designated areas throughout the developing brain, a highly regulated process. Mutations affecting migration (e.g., in genes like reelin) can potentially cause severe developmental issues, such as lissencephaly (smooth brain) or periventricular heterotopia.
Neuronal migration includes somal translocation, where a neuron grows an extension that adheres to the developing surface and then pulls its cell body along. This process is largely guided by growth cones, specialized motility structures at the tip of growing axons and dendrites, that sense environmental chemical cues (chemoattractants and chemorepellents).
Long-Range Migration
Long-range migration, characteristic of cortical neurons, often utilizes radial glial cells that span from the ventricular zone to the pial surface. These cells serve as a temporary scaffold for immature neurons traveling to their final predetermined cortical layers.
This precise and extensive movement is essential for neurons to establish appropriate functional connections and form the layered structure of the cerebral cortex.
Aggregation and Communication of Neurons
Aggregation Process
Aggregation refers to the clustering of similar neuron types into specific brain nuclei or cortical layers based on intricate cell-to-cell interactions. This is largely mediated by cellular adhesion molecules (CAMs) on the surface of neurons, which help signal neuron types and facilitate their specific binding preferences, ensuring accurate circuit formation.
Gap Junctions and Signaling
Gap junctions are specialized intercellular channels that are vital for direct electrical and metabolic communication among neurons and glia, particularly during early development. They allow for the rapid cytoplasmic exchange of small molecules and ions, coordinating metabolic activities and synchronous electrical activity crucial for neural circuit maturation.
Research in stem cell applications, particularly induced pluripotent stem cells (iPSCs), seeks to restore lost connections and functional circuitry in neurodegenerative conditions like Parkinson's Disease by replacing damaged neurons or supporting existing ones, potentially leveraging the principles of early neurodevelopment.
Memory and Development of Neural Connections
Understanding Memory Formation
Historical views, notably linked to the tabula rasa (blank slate) concept, emphasized that all neuronal connections and thus behaviors are entirely experience-driven. However, this view has been refined to acknowledge innate biases and genetic predispositions in brain wiring.
Roger Sperry's groundbreaking research on salamanders and tadpoles in the 1940s and 50s challenged a purely experience-driven model. By altering eye orientation or transecting the optic nerve, he showed that regenerating retinal axons would project back to their original, genetically predetermined targets in the tectum, even if this resulted in inverted vision. This supported his "chemoaffinity hypothesis," suggesting that chemical markers guide axon guidance and synaptic specificity.
Neurotrophic Factors
Neurotrophic factors are a family of essential molecules (e.g., Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin- (NT-)) that support neuronal survival, growth, differentiation, and the maintenance of synaptic connections during development and into adulthood. They primarily exert their effects by binding to specific receptors on target neurons, initiating intracellular signaling cascades.
They guide the establishment and refinement of synapses by promoting axon and dendrite growth and by preventing programmed cell death (apoptosis) in neurons that successfully form connections, which are crucial for functional connectivity and the sculpting of mature neural circuits.
Synaptogenesis and Brain Plasticity
Early Brain Growth
Humans exhibit significant brain growth postnatally, not primarily from neurogenesis, but from synaptogenesis (the formation of new synapses), myelination, and dendritic arborization. There is a substantial synaptic overproduction, particularly during infancy and early childhood, which underlies the exceptionally high plasticity of the young brain, allowing it to adapt to a wide range of environments.
Synaptic density is highly linked to activity level; active and frequently used connections are strengthened and refined through a process called synaptic pruning (elimination of weak or unused synapses) over time, while inactive synapses are removed. This optimization process makes neural circuits more efficient.
Prefrontal Cortex Development
The prefrontal cortex, the most anterior part of the frontal lobe, is pivotal in higher-order cognitive functions such as working memory, planning, decision-making, impulse control, attention, and executive functions. Its protracted maturation, which extends into early adulthood, significantly changes cognitive functionality and behavioral regulation with age.
Learning Mechanisms
Hebb's Rule
Hebb's rule, often summarized as "neurons that fire together, wire together," posits that if neuron A repeatedly or persistently takes part in firing neuron B, some growth process or metabolic change takes place in one or both cells such that A's efficiency in firing B is increased. This principle explains how associative learning can strengthen neural connections.
Conversely, connections weaken if neurons do not activate concurrently or if one consistently fails to activate the other, leading to synaptic weakening or depression.
Physiological Changes in Learning
Plastic changes in synapses can occur both physiologically (e.g., changes in neurotransmitter release, receptor sensitivity) and structurally (e.g., formation of new synapses, changes in dendritic spine morphology) through various forms of learning, predominantly through mechanisms like Long-Term Potentiation (LTP) and Long-Term Depression (LTD).
Role of NMDA Receptors
NMDA Receptor Function
The NMDA (N-methyl-D-aspartate) receptor is a crucial glutamatergic ionotropic receptor that selectively allows calcium (), sodium (), and potassium () ions to pass when two specific conditions are met: it must bind the neurotransmitter glutamate (and often a co-agonist like glycine), AND the postsynaptic membrane must be significantly depolarized to remove a magnesium ion () plug from its pore.
Understanding its unique role, especially its voltage-dependent magnesium block and calcium permeability, provides critical insight into the molecular mechanisms of synaptic plasticity, particularly Long-Term Potentiation (LTP) and Long-Term Depression (LTD), which are fundamental cellular processes underlying learning and memory formation.
Memory: Distinctions and Understanding
HM Case Study
The landmark case of H.M. (Henry Molaison), who underwent bilateral medial temporal lobe resection to treat severe epilepsy, profoundly highlighted the distinct forms of memory. He developed severe anterograde amnesia for declarative memories (inability to form new facts or events) while largely preserving his procedural (non-declarative) memory capabilities (e.g., learning new motor skills).
Research using H.M. bridged the relationship between cognitive psychology and neuroscience, demonstrating that the hippocampus and surrounding medial temporal lobe structures are essential for the consolidation of long-term declarative memories but not for their eventual storage or for the acquisition of non-declarative memories, thus enhancing understanding of memory formation and retention circuits.
Declarative and Non-Declarative Memories
Declarative memories (explicit memories), which include episodic memory (events) and semantic memory (facts, concepts), remain critically linked to hippocampal function and the medial temporal lobe for their initial formation and consolidation. However, the case of H.M. and other research illustrate that other cortical and subcortical brain structures also contribute to the long-term storage and retrieval of these memory processes.
Non-declarative memories (implicit memories) engage distinct neural pathways and brain structures, independent of the hippocampus, for skills and habits (basal ganglia), classical conditioning (cerebellum, amygdala), perceptual priming (neocortex), and habituation/sensitization (reflex pathways).
Conclusions on Long-Term Memory
Storage of Memories
While the hippocampus acts as a temporary hub for the initial encoding and consolidation of long-term memories, these memories eventually become distributed across a network of cortical structures beyond the hippocampus. This network includes the entorhinal cortex, perirhinal cortex, and parahippocampal cortex (collectively, the parahippocampal region), along with different regions of the cerebral cortex for various types of information.
Damage to these interconnected medial temporal lobe areas or their associated cortical regions results in impaired memory formation and retrieval, exemplified in conditions like Korsakoff's Syndrome. Korsakoff's Syndrome, often caused by severe thiamine (vitamin B) deficiency (e.g., due to chronic alcoholism), leads to damage in structures like the mammillary bodies, thalamus, and cerebellum, resulting in severe anterograde and retrograde amnesia, confabulation, and emphasizing the intricate interdependencies in memory systems beyond the hippocampus.