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birth of the neurosystem

  • the neurosystem is born at 15-20 days of gestation
  • gestation: period between conception and birth
    • the embryo grows and develop inside the womb
  • the embryo is a few dozen cells
  • the ectoderm in the embryo becomes the nervous system and the skin

  • neurosystem and skin share developmental origin

  • a piece of the ectoderm becomes special and turns into the neuroectoderm
  • the neuroectoderm forms the entire nervous system
    • the CNS and PNS are formed separately
    • CNS: comes from the central special ectoderm portion called neural tube
      • forms the brain and spinal cord
    • PNS: from the neural crest, the side wing portions of special ectoderm
      • the neuro crest has migratory cells of the neurosystem

neural crest

  • the migratory cells go and develop into three systems of the PNS
    • sensory cells that input info to CNS from the
    • autonomic ganglial neurons innervate internal organs
    • enteric nervous system lining the gastrointestinal track
  • the neural crest is not devoted to the nervous system
  • they also produce tissues for
    • pigment cells in the skin and hair
    • brain coverings
    • cells that are required for the development of teeth, inner ear, facial bones and muscles

neural tube

  • the CNS has to be go from the bottom of the spinal cord to the forehead
  • this is done by a tube formation
  • the neural tube part of the neural ectoderm folds up to form this tube
    • by the day of day 21, the tube invagination forms
      • this eventually curls up and joins to form a tube
    • by day 28, the tube has been completely folded
      • in the shape of an inverted omega
    • by the time the tube completely folds, other tissues fill the invagination and fill in the space to protect the neural system in the tube
  • the first place the neural tube closes completely to form a tube is the nexk
    • then it zips up and zips down from that one point
  • it zips up until a point at the top
    • there is one last opening at either ends
    • top one is called an anterior neural pore
    • bottom one is called a posterior neural pore
  • when both cords complete zipping, there is one complete cord from the bottom of the spinal cord to the forehead
    • if both the neural pores close properly

neural tube defects

  • many things can go wrong when the neural tube tries to close
craniorachischisis
  • the situation when there is no neural tube closure at all
  • extremely uncommon

craniorachischisis

fig: craniorachischisis

spina bifida
  • condition when the posterior pore doesn’t close
  • open spina bifida
  • it is an open defect

  • the non closure of the lower end of the spinal cord gets externalized

  • not covered by skin or bones
  • interferes with spinal cord function
    • patient may lose leg function
    • may have conditions of spinal cord injury
  • not always fatal
  • affect patient to varying degrees
  • does not necessarily define a person

spina-bifidia

fig: spina bifida

  • the extra loop of the externalized portion pulls down on the brain
  • this pulls the cerebellum out
  • a spinal cord defect thus produces brain problems
exencephaly
  • the anterior neural pore does not close
  • the brain is not covered
    • from day 28
  • through the rest of gestation, the brain essentially disintegrates

  • if the embryo comes to term, they have very little brain left

  • through time, this either turns into anencephaly or encephalocele
anencephaly

anencephaly

fig: anencephaly

  • anencephaly is lethal
  • less than a quarter of them make it alive
  • not a survivable condition
    • death occurs before birth
    • during birth
    • after birth
encephalocele
  • after day 28, other things may happen to the tube
  • an externalized cyst forms at the back of the head
    • the cyst contains cerebrospinal fluid (CSF) and or a part of the brain
prevention of neural tube defects
  • in-vitro surgery for spina bifida is a treatment
  • neural tube defects happen in the fourth week of gestation when many women don’t know they are pregnant
  • all grains are supplemented with folate in most countries to prevent neural tube defects
    • started in 90s
    • prevents about 70% of neural tbe defects
  • problems with this is gluten free foods are not supplemented with folate (or folic acid)
  • some individuals do not take sufficient grains
  • some neural defects are not sensitive folate supplements

brain vesicles

  • this is after a healthy tube has been formed
  • the tube becomes the spinal cord

  • a part of the tube also becomes the brain

  • at the from the neural tube there are three swelling called brain vesicles
    • the hind brain swelling
    • the mid brain swelling
    • the fore brian swelling
  • these will go and form the brain

  • the hind brain is right above the spinal cord and the other two are stacked on top of it as labelled

  • at the beginning there are only three of them
  • very quickly, the forebrain vesicle splits into two vesicles
    • diencephalon (end brain): becomes the thalamus and the hypo thalamus
    • telencephalon (interbrain): becomes the cerebral cortex, core basal ganglia, amygdala

brain-vesicles

fig: brain vesicles

telencephelon

  • begins invaginating as soon as it forms
  • to form two different swellings that separate across the mid line
    • these are called the telencephalic hemispheres
  • they lobe over the other three vesicles

diencephalon

  • optic nerves pouches come out of the diencephalon
    • everything other than these sit inside the cranium
    • in the embryonic stage, this out-pouching is called the optic vesicle
  • the bottom of the cranium cavity is a hole for the spinal cord to pass through
    • this hole is the foramen magnum
    • above this is the brain
    • below is the spinal cord
  • there are two convexities in the inside of the cranium
    • this is the roof of the globe of the eye
  • the out pouches of the optic nerve sit in these convexities
    • they form a cup, called the retina
  • the optic nerve is continuous with the diencephalon
    • it is contained with the same meninges
    • and covered by CSF
  • when you look at someones eyes, it is directly a part of CNS
    • they are windows to the brain
  • looking into the eye is useful
    • helps to diagonize the symptoms of brain conditions
    • changes in pressure within the brain can be detected by observing the retina

expansion of cerebral cortex

  • a shark and alligators have fully developed brains that look like below

shark-brain

fig: shark brain growth extent

  • sufficient processing capabilities for their behavioral repertoires
    • smell scanning for food
    • go to food
    • eat food

  • in humans, the brain develops much further than that

  • the telencephalon is a hungry beast
    • it takes up a lot of neural territory
    • majority of the brain is the telencephalon (forebrain)
    • only a small part of the cranium is the diencephalon (hindbrain)

forebrain

fig: telencephalon(T) vs diencephalon(H); B - back of the brain

  • in the below left image, there is one hemisphere with the
    • front lobe
    • temple lobe
    • cerebellum (fruit like geometry)
    • spinal cord cut below the cerebellum

forebrain

fig: brain with dura (left); midsagital cut cross section view of brain (right)

  • below is a midsagital cut cross section view
    • in the back is the hind brain
      • with the cerebellum
      • the medulla (the top of the spinal cord)
      • pons (right above the medulla)
    • a small piece in the center is the mid brain
    • still above it is the diencephelon
  • from the formative stages, the diencephalon expands to the front, to the sides respectively (does not cross the midline) , down
    • and expands a lot in the back
    • it envelops the rest of the brain
    • this enormous expansion is the fuel for behavioral flexibility in humans and other mammals

forebrain

fig: midsagital cut cross section view of brain with parts marked

  • the telencephalon expands to form the cerebral cortex
    • this is called the neo cortex
    • has six layers
  • only mammals have neo cortex, allowing mammals to have behavioral flexibility
    • cortex literally means bark
    • it is just the very outer rind of the brain
  • more hte surface area of this rind, the better
    • to increase this, the diencephalon develops into the front and then all the way to the back of the cranium
    • and then comes to the front on the sides as the frontal lobes
    • it is in the shape of a ram’s horn

  • some mammals like rodents do not have temporal lobe

  • to expand the territory that the cerebral cortex can take up
    • there are ridges in the brain
    • gyrus - hills (gyri - pl)
    • sulcus - valleys (sulci - pl)
    • this increases the surface area of the cerebral cortex
  • by day 28, the neural tube is completely formed
    • over the next 7-8 months of human gestation, the cerebral cortex continues to develop to increase surface area

defects

  • microcephaly:
    • when the brain doesn’t fully develop after day 28 to 8 months after
    • this is because of zika virus that is carried by mosquitoes
    • bite pregnant human who then give birth to offspring with varying degrees of microcephaly
      • depending on how late the infection occurs in the gestation
      • the head only develops enough to wrap around how much the brain develops
      • so media propaganda labels this small head disease
    • the zika virus directly affects the size of the telencephalon expansion

forebrain tracts

  • there are two forebrain tracts
  • one joins the two hemispheres of the triencephalon
    • corpus collosum
    • these make it appear as though the two hemispheres are working together
  • the other is a pair of tracts on the left and the right side
    • they connect each hemisphere of the triencephalon to the diencephalon
    • these are responsible for motor function activation
    • cortico-spinal tract
    • is contained within the internal capsule

  • the brain is one piece of tissue because of the connection via these tracts

  • there is empty space between the diencephalons and triencephalon islands
    • it has veins and arteries
    • they sit deeper than the brain parts in the cranium

block diagram

CNS-block-diagram

fig: CNS - block diagram

cerebral palsy

  • development does not end after a structure that looks brainish is formed
  • development continues for quite a while
    • more cells need to form
    • telencephalon cells are continued to be made until birth
  • in the hind brain, more than half the neurons of the brain the contained
    • these are primarily born after birth
    • several years after birth, cells are made for the cerebellum
  • even when the brain has fully developed structurally, the physiological development is not complete
    • so during development, a lot (LOT) of connections are made
    • exuberent connections
  • then synpatic pruning kicks in to keep only the ones needed
    • decide which are good and useful, then delete the ones that are not needed
  • disorders of synaptic pruning
    • autism
    • cerebral palsy
  • cerebral palsy is pretty common
    • affects 2/1000 children
  • the cortico-spinal cortex connects the telencephalon to the diencephalon which then actuates the muslce as needed
    • these are motor neurons
  • after full development, the left telenchepalon hemisphere actuates right side muscles
    • but at the start, both hemispheres actuate the skeletal muscle through a motor neuron
    • during development, the opposite side connection is kept and the same side is waned off by the time the child is toddler
      • there is only input after synaptic pruning
  • in a cerebral palsy situation
    • specifically spastic palsy, the motor neuron receives signal from both sides and no same side pruning takes place
    • the result is the whole pathway from the neuron up until the muscle is affected
    • the damages are permanent
  • it is not a progressive disorder, it is a permanent disorder
    • deficit in the motor circuits
    • damage to the motor cortex
    • damage to basal ganglia
    • damage to cerebellum
  • palsy can range from very mild to very challenging situations
    • mild: scissoring gait - the two legs are crossed
      • no necessary impairment in intellectual ability in mild palsy
    • debilitating: person has to be plopped on a wheelchair for mobility
      • does not define a person’s life

references