Feb 24, 2024	PSY 340 Brain and Behavior Class 16: Development of the Brain (OUTLINE)

 

        

"Prenatal Surgery May Be Preferable for Spina Bifida" (USN&WR Feb 10, 2011)

I. Maturation of the Vertebrate Brain

• Human Embryology
  
  
• Embryo: Day 1 to Day 60
• Fetus: Day 61 to Day 252 (Weeks 9 to 36).
  

Week 1: Conception -> migration to uterus

Week 2: Implantation of organism

Week 3: Differentiation of embryonic nervous system begins

• Stages of Development of the Embryo

Early Embryonic Development

   

• The ectoderm thickens to form the neural plate which in term folds and forms the neural tube around a fluid-filled cavity.
   
  
• At birth, the human brain weighs ca. 350 grams (vs. 1000 grams at the end of year 1 and 1200-1400 grams for the adult brain)

A. Growth and Development of Neurons

  

1. Proliferation = production of new cells - pluripotent stem cells become neural precursor cells

2. Migration

• guided by the concentration of chemicals (immunoglobulins & chemokines) in a process called chemotaxis to various locations in the developing brain (see below "chemical pathfinding" researched by Roger Sperry).
  
• Deficits may lead to mental retardation or decreased brain size while excesses may possibly lead to some cases of schizophrenia.
  

3. Differentiation = neural precursor cells develop dendrites and an axon

4. Myelination = glial cells produce fatty sheaths to insulate (myelinate) axons

5. Synaptogenesis = formation of synapses

B. New Neurons in Later Life?

• Even though the adult brains of other animals DO grow new neurons, the evidence for such growth in human adults is weak and many researchers reject that the human adult brain grows new neurons.
  

• Some researchers suggest that there is adult neurogenesis (new neurons produced) in a few areas of the brain

• Olfactory (smell) receptors in the nose (≠ olfactory bulb)

• Hippocampus of adult mammals, particularly the dentate gyrus
  
• new learning
  
• pattern separation
  

• Other researchers strongly deny adult neurogenesis and claim previous studies had major flaws.
  

• What about adult neurogenesis in the cerebral cortex? NO
  

II. Pathfinding by Neurons

A. Chemical Pathfinding by Axons

• Chemical gradients

• Roger Sperry in experiments on newts in the 1940s.
  

B. Competition Among Axons

• "neural darwinism," i.e., random synaptic connections made in the brain during development are followed by a process of selection: some connections are kept for their utility and others are discarded.

III. Determinants of Neuronal Survival

1. Nerve Growth Factor (NGF)

• Rita Levi-Montalcini (d. 2012),
  
• nerve growth factor (NGF) in sympathetic system axons linking to muscles
  
• apoptosis: "suicide program" if not exposed to NGF
  

2. Neurotrophins = NGF or BDNF (brain-derived neurotrophic factor)

• Prevention of apoptosis
• Increased axonal branches
• Increased regrowth of axons after brain damage

3. Surplus Neuron Production

• The brain produces 2 to 3 times as many neurons as needed in adulthood
  • Some of excess neurons are eliminated during early childhood
    
  • Adolescent brain shows decrease in number of neurons but growth of white matter in parietal & temporal cortex, i.e., more connections & myelination
     
    

IV. The Vulnerable Developing Brain

The pattern of brain development in most animal species is directed by a set of genes known as homeobox genes.

What are the threats?

• malnutrition
• toxic chemicals (alcohol, cigarette smoke, others)
• infections

   

  Toxic Chemicals

Alcohol Use in Pregnancy ==> Fetal Alcohol Spectrum Disorder (FASD) including Fetal Alcohol Syndrome (FAS) How does alcohol damage the developing brain? interferes with neuron proliferation, migration, & differentiation by inhibiting signals (alcohol increases GABA & dampens glutamate), neurons do not receive signals and, thus, die by apoptosis. But, remaining neurons up the release of glutamate, become overstimulated, and die because of excess sodium & calcium ions (which damage mitochondria). Significantly under-diagnosed in American children Old estimate 0.5 to 2-3 FAS per 1,000 children New estimate: 11-50 FASD per 1,000 children (1.1 to 5%) [note Autism Spectrum Disorder estimate today is ~ 1.5% of children] NO SAFE LEVEL OF DRINKING IN PREGNANT WOMEN

Cigarette Smoking: prenatal exposure to maternal cigarette smoking may lead to a variety of disorders low birth weight & early life illness: multiple news studies are confirming this. Low birth weight babies are more likely to die in infancy. skull deformities (craniosynostosis) [from CDC.gov] Down syndrome (esp. with oral contraceptive use) [from CDC.gov] Sudden Infant Death Syndrome (SIDS) long-term intellectual deficits Attention Deficit Hyperactivity Disorder (ADHD) immune system impairment (more sickness in infancy & childhood) delinquency & criminal behavior in later life (esp. for males)

(Omit: Differentiation of the Cortex)

V. Fine-Tuning by Experience

1. Experience & Dendritic Branching

• In rats, environmental "enrichment" 
• due to exercise which causes the release of neurotrophins. 
• Exercise also appears to be helpful for older people in maintaining brain's functioning.
• Studies of computer-guided "mental exercise" programs have found NO benefit for children, adolescents, or adults.
  

2. Effects of Special Experiences

Blindness

Musicians

• Musicians: auditory cortex responds more forcefully to pure tones than among non-musicians. Part of the temporal cortex is 30% larger in professional musicians.
   
  
• Musicians who must listen for key sounds are able to recognize differences in tonal languages such as Chinese: nián (rising tone = year) vs. niàn (falling tone = study)
   
  
• Postcentral gyrus (somatosensory strip): larger area devoted to left fingers among violin players vs. non-musicians.

Focal hand dystonia

VI. Brain Development and Behavioral Development

1. Adolescence

• Antisaccade Task: Ability to control attention only emerges between 7 and 11 years old.
  

• Adolescents more frequently choose immediate rewards than later larger rewards compared to adults. 
  

• Competition between reward and cognitive control systems in brain.
  

• Prefrontal cortex (cognitive control system) of adolescent brains tend to be less responsive in situations requiring  inhibiting behaviors when social peers are present. Higher levels of impulsivity occur mostly in social settings where impressing peers becomes the dominating influence, e.g. adolescent driving with peers in car vs. alone/adults present.

2. Old Age

• Memory and reasoning begin to weaken after age 60. Very gradual loss of brain tissue in temporal cortex and hippocampus. (Frontal lobe starts declining after age 30!)

• Older people are very vulnerable to brain decline after illness or injury because of inflammatory processes.

• However, the majority of individuals in positions of executive authority (CEOs, college presidents, political leaders) are over 60 years old.

• Overall cognitive losses are relatively small, knowledge and experience base of older people is usually larger, and there are multiple compensation mechanisms that make up for many losses.