PSY2301 - Topic 10: How Does the Nervous System Develop and Adapt?

Neurobiology of Development

  • The nervous system and behaviors develop rapidly during early stages.
  • During embryonic development, vertebrate embryos exhibit remarkable similarities.
    • Embryos across different species resemble each other more than their own adult versions.
    • Suggests evidence of evolution as embryos gain physical characteristics unique to their species over development.

Embryonic Vertebrate Nervous System

  • The developing embryonic nervous system shares species similarities.
    • By approximately 28 days, human embryos display forebrain, midbrain, and hindbrain.
    • The neural tube, which forms from the neural plate, develops into the brain and spinal cord.

Gross Development of the Human Nervous System

Neural Plate

  • A thickened region of early neural tissue leading to the formation of the neural groove, which curls to become the neural tube.

Major Events in Development

  • 7 weeks: Embryo resembles a miniature person.
  • 14 weeks: Brain takes on a distinctly human appearance.
  • 7 months: Formation of gyri and sulci begins.
  • 9 months: Brain structure resembles that of an adult.

Origins of Neurons and Glia

  • Stem cells are derived from various sources and exhibit differentiation potentials for multiple cell types.

Key Definitions

  • Neural Stem Cell:
    • A self-renewing, multipotent cell capable of giving rise to neurons and glia.
    • These cells are found lining the neural tube during development.
  • Subventricular Zone:
    • Contains a lining of neural stem cells in adults surrounding the ventricles.

Differentiations from Stem Cells

  • Neural stem cells can produce:
    • Progenitor Cells: Precursor cells from stem cells that differentiate into neuroblasts or glioblasts.
    • Neuroblasts: Results from progenitor cells that develop into various neuron types.
    • Glioblasts: Develop into different types of glial cells.

Discovery of Stem Cell Capabilities

  • Research by Weiss and colleagues (1996) indicated that stem cells not only produce neurons and glial cells during development but also in the adult brain.
    • Wang this capability could replace dying neurons remains under investigation.
    • Questions remain on how stem cells determine their lineage.

Factors Guiding Stem Cell Differentiation

Neurotrophic Factor

  • A chemical that signals cells to develop into neurons.

Growth Factors

  • Epidermal Growth Factor (EGF): Stimulates progenitor cell production.
  • Basic Fibroblast Growth Factor (bFGF): Stimulates neuroblast production.

Neuronal Growth and Development

  • The human brain requires about 10 billion cells to form a single hemisphere's cortex.
  • Approximately 250,000 neurons are produced per minute during peak prenatal brain development.
  • Brain undergoes a pruning process of unnecessary cells and connections, sculpting according to individual's experiences and needs.
  • Identified 7 stages of brain development:

Stages of Brain Development

1. Cell Birth (Neurogenesis and Gliogenesis)

  • Commences approximately 7 weeks post-conception, largely complete by 5 months.
    • The hippocampus can generate new cells throughout life, essential for learning and memory.
    • Neurogenesis helps the brain cope better with injury early, whereas after this period, neurogenesis is drastically reduced.
    • Gliogenesis, however, continues throughout life.

2. Cell Migration

  • Initiates soon after the first neurons arise.
    • Distinct six layers of the cortex (sensory vs. motor) require guiding cells.
    • Radial Glial Cell: Path-making cells for migrating neurons to follow to their destinations.

3. Cell Differentiation

  • Glial fibers extend from the ventricular zone to the cortical surface, influencing the migration of cells.
    • Cells populate inner layers before migrating to outer layers sequentially (Layers 6 to 1).

4. Neuronal Maturation

  • Starts around week 20 and persists beyond birth.
    • Post-migration, neurons mature by growing dendrites for synapse formation with other cells and extending axons to initiate neural communication.

Dendritic Growth

  • Dendritic Arborization: Growth of dendrites from a single protrusion to complex branching.
  • Dendritic Spines: Growth occurs slower than axonal growth (approx. 1000x slower).

Axonal Growth

  • Axons may need to grow meters in the developing brain.
    • Growth Cone: The leading tip of an axon that extends and navigates through the environment.
    • Filopodia: Protrusions at the axon tip searching for targets or signaling messages.

5. Synaptic Development

  • The adult human brain contains over 100 trillion synapses.
    • By the 5th gestational month, simple synaptic connections begin forming; refined by the 7th month as deep cortical neurons develop.
    • Rapid synaptic development occurs post-birth, especially within the first year, with increased synaptic density in the visual cortex doubling within the initial two months.

6. Cell Death and Synaptic Pruning

  • Infants possess an excess of neurons and synapses which is metabolically taxing.
  • Neural Darwinism: The hypothesis that cell death and synaptic pruning result from competition among neurons for resources, akin to natural selection.
  • If resources are insufficient, cells undergo apoptosis (programmed cell death).
    • Surviving connections are pruned based on experience, optimizing brain efficiency by maintaining only the most functional neurons/synapses.

7. Myelinogenesis

  • Myelination begins after birth, where oligodendroglia form myelin in the CNS.
    • Simpler function areas myelinated first prior to those serving more complex functions.

Unique Aspects of Frontal Lobe Development

  • The frontal lobe is the last to mature fully, particularly the dorsolateral prefrontal cortex (DLPFC).
  • Maturation can extend beyond age 20, with dendritic spines continuing to be eliminated past this age, stabilizing around age 30.
    • This prolongation may explain impulsive behaviors common in adolescence.

Cognitive Development

  • Piaget (1952) proposed that children's strategies for exploring and understanding their world evolve as the brain matures.
  • Four Stages of Cognitive Development:
    • Sensorimotor: Birth to 24 months
    • Preoperational: Ages 2 to 6 years
    • Concrete Operational: Ages 7 to 11 years
    • Formal Operational: Ages 12 and up

Changes Underlying Cognitive Development

  • Neural growth spurts represent irregular periods of rapid expansion lasting finite times.
  • Epstein (1979) identified five specific growth spurts, with an average weight gain of 5-10% of brain mass during each.
    • The initial four spurts correspond with the onset of Piaget's stages. The last occurs around ages 14 to 16.
    • Increased mass results from significant growth of glial cells, blood vessels, myelin, and synapses rather than neuron count change.

Problem-Solving in Children

  • Overman and colleagues (1992) observed performance variances in problem-solving tasks based on children's abilities, indicating regional brain maturation timing.
  • Concurrent Discrimination Task: Children (around 12 months) learn to connect objects with food.
  • Nonmatching-to-Sample Task: Object recognition based task mastered by children around 18 months involves the temporal lobe.

Brain Development and Environmental Impact

  • Environmental experiences significantly mold brain structure and development as related to neuroplasticity.
  • Hebb (1947): Environments rich in cognitive stimulation optimize intellectual development, yielding larger dendrites and increased synapses.

Abnormal Experiences and Their Impact

  • Early deprivation of sensory and social experiences can negatively affect later cognitive and social behaviors.
    • Short-term deprivation might allow for recovery due to neuroplastic effects.

External and Genetic Factors in Abnormal Development

Genetic Contributions

  • Genetics significantly influence both normal and abnormal development.
    • Spina Bifida: A spinal cord abnormality caused by incomplete closure of the neural tube, often leading to severe motor issues.
    • Importance of Folic Acid: Critical for preventing neural tube defects.
    • Anencephaly: A severe condition where the anterior neural tube fails to close, resulting in underdevelopment of the forebrain, usually leading to infant mortality shortly after birth.

Migration and Differentiation Issues

  • Abnormalities in cell migration or differentiation can yield various problems.
    • Schizophrenia: Associated with disorganization of neurons in the hippocampus, potentially linked to excess synapses due to failure in pruning.
    • Seizures and Synesthesia: Possibly resulting from neural dysfunctions related to abnormal synaptic connections.