Brain Development

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

Tube with 3 bulges

Forebrain

Cerebral cortex
Basal ganglia
Limbic system
Thalamus
Hypothalamus

Midbrain

Superior colliculi = vision
Inferior collicui = hearing
Homeostasis & reflexes

Hindbrain

Medulla oblongata
Cerebellum
Pons

Two 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)

Five Stages of Neurons

Proliferation

Production of new cells
Cells along the ventricles divide to become neurons and glia.

Migration

Primitive neurons find their spots
Chemicals guide cells

Differentiation

Neurons get axon & dendrites
Makes them different
Axon grow before dendrites
During migration

Myelination

Glia cells produce myelin sheaths
first in spinal cord
Then in brain
Lasts til about 30

Synaptogenesis

Continues throughout life
Forming synapses

Brain Anatomy

2 Fists; Cross your arms

Occipital Lobes

Temporal Lobes

Parietal Lobes

Frontal Lobes

Primary Motor Cortex
Pre-Motor Cortex

Supplemental Motor Cortex

Mirror neurons
Connect directly to spine
Help, don’t know how
balance?
coordination?

Pre-Motor Cortex

Connects direct to spine
Not fully understood
Planning?
Spatial guidance?
Actions of others?

Posterior Parietal Cortex

3-D view of world

Pre-Frontal Cortex

Dorsolateral

fairly new
develops til 30
connects to basal ganglia+
rules about rules
working memory
damaged in schizophrenia
drug abuse
alcohol
?

Orbitofrontal

above the eyes
Gambling
Alzheimer’s
Drug addiction
Also rules about:
- decision making
- inhibit bad behavior
OCD
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

Ventromedial

regulation of emotion
decision making
rules about
risk
fear

Other Structures

Anterior Cingulate Cortex
Corpus Callosum
Hippocampus

Connections

When neurons reach home, connect with each other
Grow dendrites & axons
Synapse formation
Synapse elimination

Pathfinding

Getting axons to their spots
Chemical Path-finding

Weiss (1924)

grafted extra leg to a salamander
axons grew, moved in sync with other legs
theory:
- nerves attach to muscles randomly
- variety of messages are sent
- each one tuned to a dif. muscle

Sperry (1943)

Severed optic nerve axons
Rotated them 180°
Grow back to their original target locations in midbrain

Chemical Gradients

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

NGF

Muscles & synapse survival produce & release NGF (nerve growth factor)
Not enough NGF, axons degenerate and cell bodies die
Neurons automatically dieif don’t make synaptic connection
Apoptosis = cell death

Neurotrophin

promotes survival & activity
similar to NGF

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

Cortex Differentiation

Different parts of cortex have different shapes
Shape and functions depend on input received
If transplant immature neurons
Become like neighbors
If transplant later
Some new, some old attributes
Redesign our brain to fit (within limits)
Experience fine tunes
Enriched environments
Thicker cortex
More dendritic branching
Best enrichment = activity

Age & Neurons

Stem cells

Nose cells always undifferentiated
Periodically divide & make new olfactory cells
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

Why need BBB?

Keeps most chemicals out of brain
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

Semi-permeable

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

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

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

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 (Violinist’s Cramp)
- Spend all day writing
- Fingers get jerky, clumsy & tired
- Practice too much
- expanded representation of each finger overlaps neighbor