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How many variety of cells does the nervous system?
- 200 types
- greatest variety of cells of any organ system
How many neurons are in the nervous system in the adult brain?
Once neurons are innervated (leaving the neural crest or tube), they lose what ability?
their mitotic ability (potential)
How many neurons are produced each minute during gestation?
Throughout all gestation, how many neurons are produced?
Why are excess neurons produced?
to ensure that the nervous system has the potential to do all that it needs to do (like having extra players on a team)
What are 3 critical questions regarding development?
- How do cells differentiate into various types of neurons and glial cells?
- How do neurons reach target organs?
- What happens to excess neurons?
How do cells differentiate into various types of neurons and glial cells?
- begin as neuroblasts
- differentiations depends on a series of chemical signals that control transcription of specific genes
- undifferentiated neuron
- stem cell
- doesn't know what they want to do (undecided)
What can neuroblasts become?
schwann cells, oligodendrocytes, astrocytes, etc.
What regulates the signals that control transcription of specific genes for differentiation?
What are genotype factors?
inherited by cells
What are environmental factors?
- provided by other cells and chemicals in the local embryonic environment
- not outside of the body, but the internal environment of the body
What are the types of environmental factors?
- cell to cell interaction
- mesodermal cells
Cell to cell interaction:
adjacent cells produce molecules which influence transcription (up regulation of genes) such as signaling molecules, growth factors, hormones, neurotrophic chemicals, etc.
influence ectodermal cells of the neural plate
How do neurons reach target organs?
by mechanism for guidance
Mechanism for guidance:
there are 6 layers of cells in the cortex (gray matter of the brain) that communicate with each other
What are two types of mechanisms for guidance?
- radial glial cells
- guidance cues
radial glial cells:
- act like scaffolding to guide a migrating axon to its target (mechanical type guidance system--from ventricular zone to final destination)
- tells neurons which cortex layer to stop at
- distal end of axon (the growth cone) forms filopoda (feet-like branching system)
- terminal ends (very sensitive to chemical cues) are always moving to check out the environment
- receptor proteins are embedded w/in plasma membrane of filopoda
- wants to bind to something; cues come from extracellular matrix in which the growth cone comes in contact w/ and cells that it comes in contact w/ (environment around filopoda)
- filopoda search for chemical cues for guidance
receptor molecule that is sensitive to specific guidance cues
What can the receptor proteins in the plasma membrane of filopoda bind to?
- fibroblasts: immature connective tissue cells
- glial cells
- target structures (i.e., skeletal mm cells)
What are 2 categories of guidance cues?
- NCAMS: Neural Cell Adhesion Molecules
- Neurotrophic factors (neurotrophins)
Where are NCAMS found?
surface of neighboring cells and extracellular matrix
What are NCAMS?
What are the 3 generic families of NCAMS?
What is the job of NCAMS?
stabilize axon as it grows so that past position is not lost and will adhere to target cells
What are neurotrophic factors?
released by target structures which attract the growth cone; act as chemo-attractants
What are 3 types of neurotrophic factors?
- nerve growth factors (NGF)
- brain derived neurotrophic factor (BDNF)
- neurotrophin-3 (NT-3)
What is the job of neurotrophic factors?
maintain transcription and provide chemical cues to attract axon to get it to go where they want
Are all neurotrophic factors attractants?
no, some repel the axon and some can make neurons turn, rotate, etc...
What happens to excess neurons?
- programmed cell death, no inflammatory process
- cell rapidly decomposes
- small vesicles of debris formed
Is apoptosis exclusive to neurons?
What are the characteristics of apoptosis?
- neurons must reach target to survive (but does not ensure survival)
- neurotrophic factors must be in sufficient quantity to ensure maturation
- competition at target site for neurotrophic factors
- numbers of neurotrophic factors matching neurons meet physiological needs of target
- billions of neurons die this way
Due to many aspects of the evolutionary process, there is a genetic predisposition of the NS to be hardwired. What does hardwired mean?
- established motor/sensory pathways
- variability to the brain and things can change after damage
- specific parts of the brain deal w/ specific functions
- somatotypical organization of about 40 areas
- have to form billions of synapses
In order for hardwired pathways and functional areas to be properly constructed, what has to happen?
adequate stimulation from the environment must take place during embryonic and postnatal development using guidance cues
Use it or lose it!
Why are the first 3 years of life the most critical to NS development?
because of the creation of synapses
When is the most significant maturation of the NS completed?
by age 12, but there is still more work done up to age 25 (you can influence the myelination and formation of synapses, but early stuff is most important)
3 trillion plus neurons are paired down to how many?
What happen to neurons not incorporated into networks?
they are lost
Children who have not been exposed to auditory language cannot develop...
significant language skills after age 12 because language development comes from auditory input
Lack of visual stimulation at birth will cause neurons in the cortex of the occipital lobe to...
die or be diverted to other functions (visual anosmia--a type of blindness)
Mice and rats raised in enriched environments have 25% more...
synapses than those in a plain environment; also the temporal lobe doesn't develop very well
Held and cuddled premature infants are more...
mentally alert and stronger than infants in incubators
behavior problems if synaptic patterns are not established...
early in life
Quality of stimulation:
- Talking to a child correlates to vocab--number of times they hear different words is critical
- Using complex sentences increases vocab
- Live language enhances development best
- -context is important (interaction w/ people is better than putting kids in front of a TV)
- -emotional context is also important
- You cannot greatly enhance development by overstimulation
- You can retard development by understimulation