Chapter_15_Developing_Nervous_System

Chapter 15: The Developing Nervous System

Development of the Nervous System

This chapter examines the complex processes involved in the development of the nervous system from prenatal to postnatal stages, highlighting a variety of developmental disorders that may arise during these key periods of growth.

Learning Objectives

• LO 15.1: Summarize human brain development from ectoderm plate to neural tube to chambers, illustrating the transformative stages of early brain formation.

• LO 15.2: Explain how prenatal development shapes the human brain, including influences of maternal health and environmental factors that can affect neural development.

• LO 15.3: Discuss influences on postnatal brain development, including genetics, environmental experiences, and their role in shaping cognitive functions and abilities.

• LO 15.4: Describe effects of alcohol on nervous system development, emphasizing both short-term and long-term impacts on cognitive and physical growth.

• LO 15.5: Contrast symptoms, causes, and treatments of inherited metabolic disorders, detailing the biological mechanisms that lead to these conditions.

Prenatal Brain Development

• Central Nervous System Development: The process begins around the 18th day of conception, where the ectoderm forms a neural plate, a critical precursor for nervous system development. By the 21st day, the edges of the neural plate fuse to create the neural tube, which ultimately develops into the central nervous system, encompassing both the brain and spinal cord. This early formation lays the groundwork for all subsequent neural architecture.

• Chamber Development: The rostral end of the neural tube divides into three interconnected chambers: the forebrain, midbrain, and hindbrain. Each region is responsible for specific functions:

  • The forebrain becomes the lateral ventricles and third ventricle, which play crucial roles in emotional regulation and cognitive function.

  • The midbrain forms the cerebral aqueduct, facilitating the flow of cerebrospinal fluid, necessary for cushioning the brain.

  • The hindbrain develops into the metencephalon and myelencephalon, forming structures essential for the regulation of automatic functions such as heart rate and respiration.

Postnatal Brain Development

• Brain development is a prolonged process, continuing for about two decades after birth, shaped significantly by genetic factors, personal experiences, and ongoing neurogenesis—the process through which new neurons are formed.

• Neurons and Migration: During this period, progenitor cells in the ventricular zone of the developing brain divide, leading to an expansion of the zone. Subsequent asymmetrical division produces radial glia, which are crucial in guiding the migration of neurons to their appropriate locations in the developing brain. This migration is essential for the correct formation of functional brain regions.

• Apoptosis: A significant process involves apoptosis, where approximately 50% of neurons may undergo programmed cell death due to the failure to establish successful synaptic connections. This selective elimination is vital for refining neural circuitry.

Disorders of Development

Toxic Chemicals

• Effects of Alcohol: Prenatal exposure to alcohol disrupts neural adhesion proteins, crucial for the proper development of neuronal connections. It can lead to fetal alcohol spectrum disorders (FASD), which encompass a range of physical, behavioral, and cognitive disabilities resulting from such exposure, emphasizing the importance of avoiding alcohol during pregnancy.

Inherited Metabolic Disorders

• These disorders arise from genetic mutations that adversely affect enzyme synthesis, potentially impairing brain development. Some common metabolic disorders include:

  • Phenylketonuria (PKU): Characterized by the lack of an enzyme that converts the amino acid phenylalanine into tyrosine; if untreated, it leads to severe intellectual disability. Early screening and dietary restrictions can manage this condition effectively.

  • Galactosemia: A condition resulting from the inability to metabolize galactose, necessitating a special diet to prevent complications like liver damage and cognitive impairments.

Down Syndrome

• Characteristics: This chromosomal disorder, caused by an extra 21st chromosome, presents with intellectual disability and distinct physical features such as a flat facial profile and upward slanting eyes. Notably, brain alterations include smaller frontal lobes and an increased propensity for Alzheimer-like degeneration after the age of 30, highlighting the importance of early interventions and supportive therapies for affected individuals.

Autism Spectrum Disorder (ASD)

• Symptoms: ASD is characterized by deficits in social interaction, communication, and the presence of repetitive behaviors. Early alerts may include a reduced level of eye contact and difficulty interpreting nonverbal social cues.

• Genetic and Environmental Factors: Twin studies suggest a strong heritable component, with environmental influences, such as prenatal exposure to infections like rubella, contributing to the onset of the disorder.

• Brain Changes: Children with ASD often exhibit atypical brain development patterns, such as rapid growth in early childhood, with specific regions like the amygdala showing marked differences in size. Changes in white matter connectivity have also been observed, reflecting altered neural network dynamics.

Attention-Deficit/Hyperactivity Disorder (ADHD)

• Symptoms: Individuals with ADHD manifest persistent inattention, hyperactivity, and impulsivity, negatively impacting their functioning and development, especially evident in educational settings.

• Genetic and Environmental Factors: This disorder has a high heritability rate, estimated at 75-91%, with variations in dopamine transmission identified as key underlying biological mechanisms.

• Brain Changes: Research indicates that children with ADHD may have a lower overall brain volume, accompanied by delayed cortical maturation. Functional imaging studies often reveal reduced activation in areas critical for attention and impulse control.

Summary of Key Points

• Prenatal Development: An essential period for establishing the brain’s functional architecture, underscoring the importance of minimizing harmful exposures during this timeframe.

• Toxic Exposures: Substances like alcohol can have profound and lasting effects on brain development, highlighting the need for public awareness and education regarding prenatal care.

• Genetic Disorders: Early identification and management of metabolic disorders are crucial for promoting optimal neural development and mitigating adverse outcomes.

• Disorders like ASD and ADHD: Demand a nuanced understanding of the interaction between genetic predispositions and environmental factors, along with targeted clinical interventions to support affected individuals and families.