Lecture 4 Notes: Neural Development

Teaching Block 3: Infancy

This section covers neural development in infancy, focusing on brain development from the prenatal period through early infancy.

Class 4 - Neural Development

Last Class Review
  • Gesture: Understanding the development and functions of gesture, its milestones, and the use of sign language with babies. Reference Chapter 7 (pages 232-241) for 'Gestures to Communicate with Others.'
Learning Outcomes
  • Understanding the functions of gesture throughout development.
This Class: Neural Development
  • How the human brain develops.
  • Bridging the brain and behavior.
  • Transition from involuntary to voluntary behavior.
  • Environmental influences on brain development.
  • Teratology: Understanding the effects of bad environmental agents.
  • The neglected brain.
Learning Outcomes
  • Understanding the neural underpinnings of infant capabilities.
  • Understanding what can neurologically go wrong during development.
  • Reference: Chapter 4 for Prenatal Development.

The Early Brain

  • Humans are born with approximately 100100 billion neurons, roughly the same number of stars in the Milky Way.

Plasticity in the Early Brain

  • Plasticity: The brain’s ability to change its structure, function, or connections in response to stimuli.
  • Example: CT scans of a French white-collar worker with hydrocephalus ('water on the brain') as a baby. Neural plasticity allowed him to survive.

Damage to the Early Brain

  • Example: Adi Roche's Chernobyl children.
Microcephaly
  • Symptoms include below-average head size.
  • Often caused by the failure of the brain to grow at a normal rate.
  • Head circumference measuring less than 31.5−3231.5-32 cm at birth.
  • Affects 25,00025,000 children in the US each year.
  • Linked to the Zika Virus.

An Evolutionary Perspective

  • Humans have the largest brain of all primates and the longest period of brain development.
  • Infant's head is significantly large, being 25%25\% of total body length to accommodate the brain.
  • From birth to teenage years, there is a fourfold increase in the volume of the human brain.
  • The adult brain accounts for only 2%2\% of our body weight but consumes 20%20\% of oxygen.
  • Humans uniquely show 'differential development' in timing across cortical brain areas.

Three Main Prenatal Stages

  • The Zygote (Germinal Stage): First 2 weeks.
  • The Embryo (Embryonic Stage): Weeks 3-8.
  • The Fetus (Fetal Stage): 3rd month to delivery.

Germinal Stage (0-2 weeks)

  • A two-week period following conception.
  • Begins when a sperm fertilizes the ovum, forming a zygote (fertilized egg).
  • The zygote proceeds down the fallopian tube, undergoing rapid cell division and implants in the wall of the uterus after about 7 days.
  • The germinal stage involves placenta formation to provide oxygen/nutrients and remove waste.

Embryonic Stage (3-8 weeks)

  • The amniotic sac, placenta, and umbilical cord develop.
  • Development follows cephalocaudal (head to tail) and proximal to distal (center to periphery) gradients.
  • The inner cell mass differentiates into three layers:
    • Endoderm: vital organs.
    • Mesoderm: muscles, skeleton, circulatory and excretory systems, inner skin layer.
    • Ectoderm: sensory cells and nervous system.
  • Neurulation: Formation of the neural tube and neural crest from the neuroectoderm – the foundation of the brain.

Neurological Development

  • Development of neural plate, neural fold, neural groove, neural crest, and neural tube.
  • Brain fissure development includes the Sylvian fissure, Calcarine fissure, and Parieto-occipital sulcus.

Neurological Development (cont.)

  • Development of the midbrain, forebrain, spinal cord, cerebral cortex, cerebellum, medulla, and hindbrain over time from 25 days to 9 months.

The Birth of Neurons

  • To reach approximately 100100 billion neurons, mass neural proliferation is necessary: About 250,000250,000 new neurons are added every minute.
  • Neurogenesis: The 'birth' of brain cells.
  • Neurons develop from neural cells and progenitor cells.
  • A baby’s brain already has most of its neurons at birth.
  • Most neurons are present after approximately 25 weeks of gestation.

A Neuron

  • Components of a neuron: Dendrite, Soma (cell body), Nucleus, Axon, Myelin sheath, Axon terminal button

Cellular Development

  • Proliferation of glial cells: These surround and protect the neurons.
  • Neural migration: Cells migrate to various parts of the brain, where they will later acquire specialized structure and function.
  • Myelination: Cells are insulated in a fatty (myelin) sheath, which improves synaptogenesis (communication between cells/transmission of nerve impulses).
  • Synaptogenesis: The forming of synapses begins early in prenatal life as soon as neurons appear.

Lissencephaly

  • Literally means 'smooth brain'.
  • Caused by defective neuronal migration at 12-24 weeks of gestation.
  • Characterized by a lack of folds (gyri) and grooves (sulci).
  • Children with lissencephaly will have severe developmental problems, seizures, muscle spasticity, and more.
  • Typically caused by viral infections, genetic anomalies, or insufficient blood supply.

The Fetal Stage (9-38 weeks)

  • Rapid development of muscles and nervous system.
  • By the end of the 3rd month, the fetus has all body parts.
  • Around 5 months, reflexes such as sucking, swallowing, and hiccupping appear.
  • By 6 months, the eyes can open and close.
  • Age of viability (22 – 26 weeks): The fetus’s physical systems are advanced enough that it has a chance to survive if born prematurely.

Sleep in Utero

  • At approximately 7 months, a fetus will spend most of its time sleeping.
  • Cycles every 20 to 40 minutes between Rapid Eye Movement (REM) sleep and non-REM sleep.
  • Yawning also occurs (Reissland, 2012).
  • Potential reasons: Priming visual systems, memory consolidation, likely a neural explanation.

Fetal Brain Development Milestones

  • 25 days: Brain weight
  • 40 days
  • 100 days
  • 5 months
  • Migration, Neuron proliferation, Myelination, Reorganization of synapses, Programmed death of cells, Branching, synaptogenesis

Milestones

  • Twitches
  • Moves arms/legs
  • Moves head
  • Breathing movements
  • Opens jaw, yawns
  • Sucks, swallows
  • Walking movements, sleep cycles
  • Pain reactions, cry expressions
  • Reactions to sound
  • Habituation
  • Blink reflex
  • Visual fixation
  • Imitates
  • Babbling

Prenatal Summary

  • Animation shows brain development from conception through birth.

Some More Fine Tuning

  • Two processes reduce the number of neurons and connecting fibers:
    • Synaptic pruning: Axons and dendrites are disposed of if a particular neuron is not receiving much stimulation.
    • Programmed cell death (apoptosis): When new synapses are formed, some surrounding neurons die to provide more space. This elimination of excess brain cells helps achieve more efficient functioning.

The Potting Metaphor

  • Neurogenesis/Synaptogenesis: Clay formation.
  • Cell specialization: Sculpting.
  • Synaptic pruning/cell death: Sculpting.
  • Perceptual narrowing: Unused clay trimmed away.
  • Sensitive periods: The clay does not dry all at once; parts may harden before others.

Brain Maturation in Infancy

  • As the cerebral hemispheres continue to mature, early motor reflexes disappear and are replaced by more voluntary motor behaviors (e.g., reaching, crawling, walking, pointing).
  • By around six months, hearing and vision modalities are mature.
  • The cerebellum continues to develop in Y1 to help muscle tone and balance.
  • The last areas to develop are the prefrontal areas (e.g., planning action, inhibition – executive function).

The Timecourse of Development

  • Illustrates the developmental course of human brain development from conception to adolescence, highlighting neurulation, cell proliferation and migration, synaptogenesis and synaptic pruning, myelination, experience-dependent synapse formation, and dendritic arborization in different brain regions.

The ‘Body Story’ Video

  • The video follows Robert, a baby going through the first few months of life.
  • It describes the changes that occur in the brain as the infant grows and learns.
  • In the first 6 months of life, a major transformation occurs: from reflex, involuntary behavior to voluntary behavior that is under the infant’s control.

Returning to Reflexes

  • Involuntary reactions to external stimuli.
  • First forms of human movement.
  • Primarily serve as protection or nourishment.
  • Development of future movement.
  • Diagnostic tool.
  • Appear at birth, many disappear.

Reflexes Correlate with Brain Development

  • Moro reflex: (<3 months)
    • Baby is 'dropped' or hears a loud noise.
    • Extends limbs and fingers, draws back head.
  • Grasping (Darwinian reflex): (<4 months)
    • Palm of baby’s hand is stroked.
    • Makes a strong fist; can be raised to standing if both are grasped.

Reflexes Correlate with Brain Development (cont.)

  • Stepping: (1-4 months)
    • Baby is held under arms, bare feet touching a flat surface.
    • Makes step-like motions.
  • Rooting reflex: (<~4 months)
    • Turns face towards cheek touching/stroking.
    • Aids breastfeeding.

Risks in the Prenatal Environment

  • Examples: Agent Orange, Thalidomide.
Thalidomide
  • Causes Phocomelia (malformation of the limbs).

Teratogens Can Be Hard to Understand

  • An environmental agent that may cause developmental deviations.
  • Impact may depend on many factors, some of which are very hard to measure.
  • Typically, most threatening in the embryonic stage but may also harm the fetus.
  • Each organ system has a different critical period (e.g., the heart is 20 - 40 days after conception).
  • Teratogens often have specific effects.
  • Exposure length, time, and intensity are important factors.
  • Maternal or fetal genotypes may counteract a teratogen’s effects.

Teratogens: Specific Examples

  • Drugs, illegal and legal: e.g., thalidomide, warfarin, tetracycline, heroin, cocaine.
  • High caffeine dosage: miscarriage, low birth weight, prematurity.
  • Parental factors: e.g., age, diet, emotional state.
  • Disease: e.g., rubella, herpes simplex, chickenpox.
  • Metals: e.g., lead, mercury.
  • Radiation: e.g., cell death, chromosome injury.

Fetal Alcohol Syndrome

  • Physical defects:
    • Flattened midface.
    • Thin upper lip.
    • Indistinct/absent philtrum.
    • Short eye slits.
  • Growth suppression:
    • Lower birth weight.
    • Height and/or weight low.
  • Central Nervous System:
    • Impaired fine motor skills.
    • Learning disabilities.
    • Behavior disorders.

Postnatal Environment

  • Brain plasticity: Changes in the brain as a result of experience.
  • Diamond (1988) observed the cortical structure of the brains of rats raised in either an enriched or an impoverished environment.
  • Cortical thickness differed according to the environment.

The Neurological Impact of Neglect

  • CT scan comparison between a healthy 3-year-old's brain and a girl who has suffered severe global neglect.
  • Although much of the brain’s structure is genetically determined, the brain’s plasticity means that the amount and type of stimulation received also plays a crucial role.

Recovery from Neglect

  • Perry and Pollard (1998) examined the brains via neuroimaging of 122 neglected children, some of whom also had pre-natal drug exposure.
  • Frontal Occipital Circumference (FOC): Brain growth and organization were all worse than a comparison group of non-neglected children.

Class Summary

  • Brain development during the pre- and post-natal period.
  • Cell proliferation and migration.
  • Synaptogenesis and pruning.
  • Behavioral correlates of early brain development indicate a broad transition from involuntary reflexes to voluntary movement.
  • The developing brain responds to pre- and post-natal environmental influences on development, including:
    • Enrichment.
    • Teratogens.
    • Plasticity.