The Developing Brain (Chapter 6) Flashcards
Neuron Development
- while embryo gorws
- singaling molecules “turn on” certain genes and “turn off” others
- this intiates the formation of immature nerve cells
- cell division (proliferation), brain cells increase by billions
- migration= newly formed neruons travel to final destinations
three layers emerging during embyronic development
- the ectoderm(outer-most layer), the mesoderm(middle layer), the endoderm(inner-most layer)
cells in each of the three layers
- contain identical DNA instructions for development
- these layers give rise tot he rich variety of tissue types that make up the human body
what is the cause behind the diversity of these tissue types?
- singals produced by surrounding tissues
- signals turn certain genes on, others off to induce the development of specific cell types.
nerve tissue is formed…
- signals from the mesoderm trigger some ectoderm cells to be come it
- called nerual induction
- after, subsequent signalling interactions regine the nerve tissue into basic categories (neurons/glia), and then subclasses of each cell type
what is the fate of a developing cell largely dependent on
- its proximity to various sources of signaling molecules
- ## concentration of each signalling moleucel type decreases farther from the source to create gradietns in the brain
sonic hedgehog
- secreted from mesodermal tissue beheant the developing spinal cord
- a signaling molecule
- exposure to the signal causes adjacent nerve cells to be converted into a special class of glia
what happens when cells are farther away from the singal
- exposed to lower ceoncentraitions of sonic hedegog, so they become motor neruons that control the movement of muscles
even lower concentraition (farther awar)
- they become interneruons which do not relay messages to muscles but to other neurons
where do neruons arise from
- afarily small pool of nerual stem and proentior cells
progentior cells
special cells that an divide and become a variety of mature cell types.
when does the human brain begin to form
- after three week’s gestation
- first stage is nerual tube,
- by four weeks, the individual sections of brain can be recognized
- by 6 months, the ridges of the brain can be observed.
before cells become a variety of mature cell types, what has to happen>
- before achieveing their mature cell fate, this pool of cells must undergo a series of divisions to increase the number of cells that will ultimately form the brain.
early cell dicision
- are symmetrical- reuslts in two identical daughter cells that have the capacity to continue to divide
what happens as cell divisions begin to rpogress
- cells begin to divide asymmetrically
- give rise to one daughter cell that continues to rapidly divide
- second daughter cell progresses towards its final cell fate as a nerual or glial cell
proliferative process
permits rapid growth during early development of the brain
- billions of cells are produced in a matter of weeks
what happens after that series of cell divisions is complete?
- onyl few nerual stem and progenitor cells remain within the brain
neurogensis in adulthood
- is limited to a few regions of the brain, such as those involved in mmeory
protein defects
- cause premature switch from symmetric to asymmetric divisions
- this may be the cause of microcephaly
microcephalt
- severe reduction of rbain size,
- associated with serious neurological disabilityies
sometimes death in infancy
excessive proliferation of brain cells
- can lead to a disorder called megalencephaly
- brain is abnormally large and heavy
- associated with a variety of neurodevelopmental complications
induction
- those signals that turn certain genes on and others off induce the developemtn of specific cell types
waht happens after 1) induction and 2) proliferation
- migration
- new neurons journey from inner surface of embryonic brain to their long-term locations in the brain
- begins three to four weeks after a human baby is conceived
- ectoderm starts tot chicken and build up along the midline of the emrbyo
what happens afger the ectoderm starts to thicken
- cells continue to divide
- flat heural plate grows
- formation of parallel ridges (like a paper airplane) that rise along either side of the midleine
ridges on the sides of the midline
- extend from the “head end” where the future brain will form
- they are along the elenth of the embryo where the future spinal cord will develop
- few days, the ridges fold toward each other and fuse into a hollow nerual tube
waht happens to the head end of the neural tube
- it startes to thicken
- tforms three bulges
- they are the hindbrain,t he midbrain, and the forebrain
7 weeks affter gestation>
- first signs of the eyes and the brain’s hemispheres appear
what hpapens after new neruons are produced (after eyees, and hemispheres start to appear)
- they move from the nerual tube’s ventricular zone (inner surface of the tube) towards the border of the marginal zone (outer surface)
what happesn after neruons stop diciding
- they form an intermediate zone where they gradually accumulate as the brain develops
how do neurons know wehre they must go? (final destination)
- guidance cues
- radial glia
- they project radially from the intermediate zone to the cortex
- neruons inch along glial projections until they reach their final destinations
- cells that arrive the earlier are the oldest ones that form the deepest layer of the cortex
- late-arricing are the youngest neurons that form the outermost layer
another mechanisms for guiadance
- neurons migrate sideways or tangentially (rather than radially), moving parallel to the brain’s surface and across the radial cortical columns
what can infleunce migration
- exposure to alcohol, cocaine, or radiation can prevent proper migration, resulting in misplacement of cells
- ## misplacement can lead to intellectual ability, or epilepsy
what happens when there are mutations in the genes that regulate migration
- can cause rare genetic forms of intellectual disability and epilepsy in humans
what happens when neruons reach their final lcoations
- ## they begin making the connections that will determine how particular functions (e.g. vision and hearnign) can occur
internal processes of fetal development
- induction, proliferation, and miogration
external phases of brain development
- occur afterbirht
- factors such as watching a mobile spin, listening to a voice, and even proper nutrition can influence the connections formed by neurons
how do neurons become interconncented
- through their short branches called dendrites and long azons (can occur after birth from external factors)
axon length
- in order to reach their targets, axons can span distance many times the size of their cell body, many corsssing to the opposite side od the brain
longest human axons
- in the periphery
- they extend from the lower spnial cord all the way to muscles in the toes
how does a developing axon grow
- through the extension of its growht cone
- it actively explore the environment to seek out its precise destination
how is the gorwht cone guided to final destination
- molecular clues in its envionrments
- some molecules are on the srugace of cels while others are secreted into areas near the growh cone
- receptors no the gorwh cone enable its response to these environmental cues.
signaling molceules taht make up molecular cues
- families of proteins with names such as netrin, semaphoring, and ephrin
- these are found in many organisms such as worms, insectes and mammals although it may be of a smaller concentration in flies or worms than in mice or poeprle- but functions are still similar
what happens when axons reaches its destination
- a synpase begins to form
neurotransmittters
- can either promtore or hinder the generation of a new electrical signal in the receiving neuron
how many synapses does our brain contain
trillions
what has to happen, in order to allow the brain to process complex info
the formaion of synaptic connections must be highly specific
- specificity is partly because of the mechanisms that guides the axons to their proper targets
how is the synpase anchored
- dendrites and axons both produce proteins that span the space between them and anchor the snyapse togehter
differentation after establishment of a synapse (presynaptic)
- tiny axon termioanl that contacts the dendrite becomes specialized for releasing neurotransmitters
- stocks itself with neurotransmitter packets and proteins that enable the packets to be held in place and then realeased
differentiation post-synaptic
- receptors that repsond to those neruotransmitters begin to dot the membrane
- ensures that a synpase can transmit signals quickly and effectively.
strocytes
- type of flial cell in the brain
- previously thought to simply provide scaffolding an passive support to neurons
- they exert their own influence on synaptic development and function
- many synpases in the brain are contacted by astrocytes]
- contact thousands of synapses across multiple neruons
astrcotyes two
- neurons orm only few synapses when developing in a culture dish when astrocytes are absent
- molecules secreted by astrocytes regulate aspects of synaptic development
what defects could contribute to autism
- molecules from multiple sources work together to promote proper synpase formation
- defects in such molecules could contribute to such disorders
what could underlie the degradation that occurs udrig aging
- the loss of certain other molecules
what determines the type of neruotransmitter a neruon will use to commmunciatie
- for some cells such as motor neruons, they will always use acetlycholine
neurotransmitter
- when certain immature neruons are maintained in a culture dish w/o any other cell types, they produce NOREPINEPHRINE
- when same neruons are culkutred with specific cells e.g. cardiac tissue they produce acetlycholine
what infleunces the signal signals that engages the gene and the final determination of the chemical messenger a neruon will produce
- influenced by factors that come from the location of the synapse itself.
myelination
- the fatty wrapping of axons by extension so flia
- increases by as much as 100 times the speed at which signals can travel along axons
nodes of Ranvier
- the way they are wrapped increases the speed
- regularly spaced gaps of the sheath they interrupt the sheath
nodes of Ranvier for speed
- alternating pattern of insulation and nodes allows electrical signal to move down an axons faster, from one node to the next
saltatory conduction
- the alternating pattern of inslulation and nodes is responsible fo more rapid transmission of electrical signals
when does formation of meyelin occur
througout the lifespan
