Brain Development

Brain

 

Brain
Overview
18 days after conception
Primitive streak
Outer layer of embryo thickens Ectoderm forms a plate
Edges curl up
Make a neural tube
Neural tube
Cells inside tube become neurons & glial cells Closed tube
Tube with 3 bulges
1. Forebrain
Cerebral cortex Basal ganglia Limbic system Thalamus Hypothalamus
2. Midbrain
Superior colliculi = vision Inferior collicui = hearing Homeostasis & reflexes
3. Hindbrain
Medulla oblongata Cerebellum
Pons
Phases
1st Phase
Symmetrical Division
2 identical founder cells
Radial Glial Cells
Spread out like tree
Neurons climb tree to their proper position
2nd Phase
Asymmetrical Division
About 3 months
Divide into neuron & founder cells End of cortical development
founder cells receive signal (cell death)
Development

Choice 1:
Two fists, crossed arms

NOTES

• Prenatal
  • 18 days after conception
  • Primitive streak
    • Outer layer of embryo thickens
    • Ectoderm forms a plate
    • Edges curl up
    • Make a neural tube
      • Cells inside tube become neurons & glial cells
    • Closed tube
    • Tube with 3 bulges
  • Quick Preview
    • 1. Forebrain
      • Cerebral cortex
      • Basal ganglia
      • Limbic system
      • Thalamus
      • Hypothalamus
    • 2. Midbrain
      • Superior colliculi = vision
      • Inferior collicui = hearing
      • Homeostasis & reflexes
    • 3. Hindbrain
      • Medulla oblongata
      • Cerebellum
      • Pons
  • Phases
  • 1st Phase
    • Symmetrical Division
    • 2 identical founder cells
    • Radial Glial Cells
    • Spread out like tree
    • Neurons climb tree to their proper position
  • 2nd Phase
    • Asymmetrical Division
    • About 3 months
    • Divide into neuron & founder cells
  • End of cortical development
    • founder cells receive signal (cell death)
• Connections
  • When neurons reach home
  • Connect with each other
  • Grow dendrites & axons
  • Synapse formation
  • Synapse elimination
• 5 Steps of Neurons
  • 1. Proliferation
    • Production of new cells
    • Cells along the ventricles divide to become neurons and glia.
  • 2. Migration
    • Primitive neurons find their spots
    • Chemicals guide cells
  • 3. Differentiation
    • Neurons get axon & dendrites
    • Makes them different
    • Axon grow before dendrites
    • During migration
  • 4. Myelination
    • Glia cells produce myelin sheaths
    • first in spinal cord
    • Then in brain
    • Lasts til about 30
  • 5. Synaptogenesis
    • Continues throughout life
    • Forming synapses
• Age & Neurons
  • Neurons go from
    • undifferentiated
    • differentiated
    • dead
  • Stem cells
    • Nose cells always undifferentiated
    • Periodically divide & make new olfactory cells
• Pathfinding
  • Getting axons to their spots
  • Chemical Pathfinding (Weiss, 1924)
    • Grafted extra leg to a salamander
    • Axons grew, moved in sync with other legs
    • Nerves attach to muscles randomly
    • Variety of messages are sent
      • Each one tuned to a dif. muscle
  • Chemical Gradients (Sperry, 1943)
    • Severed optic nerve axons
    • Rotated them 180°
    • Grow back to their original target locations in midbrain
    • Axons attracted by some chemicals, repelled by others
    • TOPDV protein is 30x more concentrated in dorsal retina than ventral retina axons
    • Highest connect to highest
    • Lowest concentration axons connect to lowest
• Neural Darwinism
  • During development
    • Synapses form randomly
    • Selection process keeps some and rejects others
    • Chemical guidance
    • Neurotrophic factors
  • Muscles & synapse survival
    • produce & release NGF (nerve growth factor)
    • Not enough NGF, axons degenerate and cell bodies die
    • Neurons automatically die
      • don’t make synaptic connection
    • Apoptosis = cell death
  • Similar to NGF
    • Neurotrophin
      • promotes survival & activity
    • BDNF
      • brain-derived neurotrophic factor
      • most abundant neurotrophin in cortex
  • Make more than enough
    • Neurotrophins are also used in adult brains
    • More axon & dendrite branching
    • Deficiencies of neurotrophins lead to cortical shrinking and brain diseases
• Developing brain vulnerable
  • Toxic chemicals
    • Malnutrition
    • Infections
• Teratogens
  • Environmental factor
  • Interfere with development
  • Medication, drug, alcohol or substance
  • Disease
• Critical Periods
  • Implantation = common blood supply
    • Whatever’s in mother’s blood crosses
  • 10 to14 days after conception
  • 3.5 to 4.5 weeks
    • closure of the neural tube
  • Central nervous system vulnerable throughout pregnancy
• 3 Major Substances
  • Alcohol
  • Phenytoin
  • Chickenpox
• 1. Fetal alcohol syndrome
  • Best known non-genetic cause of mental retardation
    • (3 in 1,000)
  • Infant brains are especially sensitive to alcohol
  • Suppress release of glutamate
    • brain’s main excitatory
    • neurons receive less excitation and undergo apoptosis
  • Alcohol broken down more slowly
    • immature liver
  • Alcohol levels remain high longer
  • Worse when born to alcoholic mothers
    • drink more than four to five drinks/day
  • No amount of alcohol is safe
• 2. Phenytoin (or Dilantin)
  • Anti-convulsive
    • used to treat epilepsy (seizure disorder)
  • 10% chance of birth defects
  • Fetal Hydantoin Syndrome
  • If taken in the first trimester
• 3. Varicella (chickenpox)
  • Highly infectious disease
  • 95% of Americans have had it
  • 90% of pregnant women are immune
  • 1 out of 2,000 develop during pregnancy
  • A. If in pregnancy (week 1-20)
    • 2% chance of defects
    • “congenital varicella syndrome“
    • Scars
    • Malformed and paralyzed limbs
  • B. Newborn period
    • 5 days before to 2 after birth
    • About 25 % newborns become infected
    • About 30% of infected babies will die if not treated
• Parental use of:
  • Cocaine or cigarettes
  • ADHD
  • Antidepressant drugs
  • Heart problems
• Birth Defects
  • 3-5% of newborns
  • Leading cause of infant mortality
  • Majority have no known cause
• Cortex Differentiation
  • Different parts of cortex, different shapes
  • Shape and functions depend on input received
  • Transplant immature neurons
  • Become like neighbors
  • Transplant later
  • Some new, some old attributes
• Experience fine tunes
  • Redesign our brain to fit
    • (within limits)
  • Enriched environments
  • Thicker cortex
  • More dendritic branching
  • Best enrichment = activity
• Transfer
  • Far transfer = do well in one, do well in other tasks
  • Near transfer = practice task, do better on that task only
• Train the brain – doesn’t work
  • Blind from birth
  • Better at discriminating objects by touch
  • Increased activation in occipital lobe (vision) doing touch tasks
  • Use occipital cortex for Braille; sighted people don’t
  • Concept of straight
• Learning to read
  • Learn to read as adults
  • More gray matter in cortex
  • Thicker corpus callosum
• Music Training
  • Pro musicians
    • Bigger temporal lobe (30%)
    • 2x greater response to pure tones (in auditory cortex)
  • Violin players
    • larger area devoted to left fingers in the postcentral gyrus
  • Writer’s Cramp
    • Spend all day writing
    • Fingers get jerky, clumsy & tired
  • Musician’s Cramp
    • Practice too much
    • Fingers get jerky, clumsy & tired
    • Expanded representation of each finger overlaps neighbor
• Overruling reflexes
  • Antisaccade task
    • Object appears in periphery
    • Must look in opposite direction
  • Top-down processing overruling reflex
  • Improves with age unless
    • Very young
      • hard to look away from attention getter
    • ADHD
• Age & Neurons
  • At 30
    • Frontal cortex begins to thin
    • Much individual variation
  • 60+
    • Synapses alter more slowly (learn)
    • Hippocampus gradually shrinks
    • Compensate by using more brain areas
• Blood-Brain Barrier
  • (Paul Ehrlich, 1800’s)
  • Injected blue dye into animals
  • All tissues turned blue EXCEPT brain and spinal cord
  • Keeps most chemicals out of brain
  • Why need BBB?
    • Brain has no immune system
    • Neurons can’t replicate-replaced
    • No way to fix damage
    • Viruses that do enter kill you
      • Rabbies
    • Neural disorders last whole life
      • Chicken pox-shingles
  • How it works
    • Keeps out harmful chemicals
    • Keeps out medications
    • Cancer med
    • Dopamine for Parkinson’s
    • Astrocytes form layer around brain blood vessels
    • may be responsible for transporting ions from brain to blood
  • Semi-permeable
    • Endothelial cells line capillaries
    • Small spaces between each
    • Some things can move between them
    • Loosely joined in body, large gaps
    • Tightly joined in brain, blocking most molecules
    • Large molecules can’t easily pass thru
    • Molecules with a high electrical charge are slowed down
    • Protects the brain
  • What can cross passively
    • Small uncharged molecules
    • Oxygen & carbon dioxide
    • Molecules dissolve in fats
      • capillary walls are fats
  • What can cross actively
    • An active transport system
      • protein-mediated process
      • uses energy to pump chemicals
      • E.g., burn glucose for energy
    • Broken by:
      • Hypertension (high blood pressure)
      • Development (not fully formed at birth)
      • High concentrations of some substances
      • Microwaves & radiation
      • Inflammation
      • Brain injury
      • Infections
      • Alzheimer’s disease
        • endothelial cells shrink
        • makes gaps
        • harmful chemicals enter
  • Nourishing Neurons
    • Almost all need glucose
    • Practically only nutrient that crosses blood-brain barrier in adults
    • Ketones can also cross but are in short supply.
  • If you can’t use glucose
    • Korsakoff’s syndrome
    • thiamine (vitamin B1) deficiency
      • inability to use glucose
    • neuron death
    • severe memory impairment
• Head Injury
  • Open or Closed
    • Open head injury (penetrating)
      • Object enters brain
    • Closed head injury (skull not broke)
      • Concussion
      • Most common traumatic injury
      • Brain gets rattled
  • Causes
    • Car, train, airplane accident
    • Fall
    • Assault
    • Sports
  • Symptoms
    • Can show immediately or develop slowly
    • Unequal pupil size
    • Headaches
    • Obvious
    • Object sticking out of head
    • Fluid draining from nose-ears
    • Clear or bloody
    • Coma or unconscious
    • Paralysis
    • Seizures
  • Sort Of Obvious
    • Slurred speech
    • Blurred vision
    • Lack of coordination
    • Memory loss
    • Stiff neck
    • Vomiting more than once; children often vomit once
  • Not So Obvious
    • Irritability (especially children)
    • Mood or personality changes
    • Drowsiness
    • Confusion
    • Loss of hearing, vision, taste or smell
    • Low breathing rate
    • Memory loss
    • Symptoms improve, then get worse
  • Get immediate help if
    • Loss consciousness, even briefly
    • Severe headache or stiff neck
    • Vomits more than once
    • Behaves abnormally
    • Unusually drowsy
  • Do
    • Call 911
    • Make sure breathing
    • Assume spinal cord injury
    • If normal breathing but unconscious
    • Stabilize head and neck
    • Hands on both sides of head
  • If bleeding
    • Press clean cloth on wound
    • If soaks through, don’t remove it
    • Put another cloth over it
  • DO NOT
    • Don’t wash deep head wound
    • Don’t move or shake
    • Don’t remove helmet
    • Don’t pick up child
    • Don’t drink alcohol (48 hours)
    • If skull fracture
      • Don’t apply pressure to bleeding site
      • Don’t remove debris from wound
    • No aspirin
      • Aspirin & ibuprofen can increase risk of bleeding
    • If vomiting
      • Roll the head, neck & body as one unit
    • Sleeping
      • Wake every 2 to 3 hours, check alertness
      • ask simple questions: “What is your name?”