1.3: development of the nervous system

everything starts as a

• flat sheet of cells that turn into our nervous system

  • starts with a tube

stages of neural tube development

1. pre-embryonic stage

2. embryonic stage

3. fetal stage

pre-embryonic stage

• conception to second week

• after sperm and egg join and egg is fertilized

embryonic stage

• 2nd week → 8th week

• disc of cells (musculoskeletal, bone, and visceral) suspended in a giant water balloon

fetal stage

• 8 weeks → birth

• differentiation occurs during this stage

  • when neural tube starts to differentiate (to bones, muscles, everything)

• develops into recognizable parts of the nervous system

embryonic disk

turns into the neural tube

• endoderm (inferior - forms digestive tract and respirayory system)

• ectoderm (superior - will form NS and skin)

embryonic disk structure

• neural plate: most dorsal part of the disk

  • made up by nerve cells

  • nerve cells start posterior (dorsal) and grow forward (ventral)

• endoderm: most ventral part of the disk

embryonic disk (dorsal to ventral)

• neural plate

• ectoderm

• mesoderm

• endoderm

neural plate

most dorsal part of embryonic disk

• made up of nerve cells

  • nerve cells start dorsal and grow forward

ectoderm

• includes neural plate

  • most dorsal part of embryonic disc

• becomes sensory organs, epidermis, and nervous system

what does the ectoderm become

• sensory organs

• epidermis

• nervous system

mesoderm

• somites live here

• becomes dermis, muscles, skeleton, and circulatory

  • (support system)

where do somites live

mesoderm

endoderm

• gut, liver, pancreas, and respiratory system

neural tube formation

• the neural plate buckles in the middle to form the neural groove

• the sides of the groove (the folds) come up and bend toward each other; eventually they fuse at midline

• somites appear and develop along the neural tube

initially, the neural tube is __

• open at both ends like a cannoli so amniotic fluid can go in and out

• eventually it will close at both ends

• the somites (bumps) that are lined up on either side

somites in the neural tube

• lined up on either side

• turn into the part of us that isn’t neural

• they can turn into dermatomes/myotomes to help with movement and sensation

why is the neural tube initially open at both ends

so amniotic fluid can go in and out

neuropores

• the temporary openings at the top and bottom of the neural tube

• should close around end of week 4

superior vs inferior neuropore

superior affects brain (higher) - anencephaly

inferior affects spinal cord (lower) - spina bifida

layers of inside of neural tube

• marginal layer (outside) - becomes white matter

• mantle layer (inside) - becomes gray matter of CNS

eventually flips, as white matter tends to be deep to gray

mantle layer

• middle, thick cellular layer that develops into the gray matter of the CNS

  • further differentiates into motor and association (sensory) plates

mantle layer splits to

(inner layer)

2 plates:

• association plate

  • houses sensory cells (not al sensory cells are housed here, but theya re sensory in nature)

  • dorsal

• motor plate

  • ventral

somites differentiate to

3 different parts:

• dermatome - part of somite that becomes the skin

• myotome - part of somite that becomes muscle

• sclerotome - part of somite that becomes mucle

developing vs mature somite parts

• developing (in utero) dermatomes, myotomes, and sclerotomes are the skin, muscle, and bone.

• mature ““ have to do with innervation

somite vs neural tube

each somite is a neighbor to a SEGMENT of the neural tube

somite segments

each level has a somite on each side

dermatomes in development

• a part of the somite that becomes the skin

dermatomes once developed

• refers to a part of the skin that is innervated by a singular spinal nerve

neural tube & dermatomes

• arms and legs but out of neural tube

• start in a stack, but as it turns into fetus, the dermatomes pull each other out (why arms and legs have uneven/warped dermatomes

myotomes in development

• part of the somite that becomes muscle

myotomes once developed

• represents all muscles innervated by a singular spinal nerve

spinal nerves vs muscle innervation

• a single spinal nerve can innervate multiple muscles

• multiple spinal nerves can innervate the same muscle

why do almost all muscles have innervation from more than one “spinal level”

• ex. C5 and C6 somites both become the biceps muscle (and eventually innervates it)

  • C5 and C6 still become other muscles in the myotome as well

• the “bicep” cells from C5 and C6 somites start migrating away from the neural tube toward their final destination on the anterior humerus

• if one of the spinal nerves for biceps is completely cut, then the biceps has only lost part of its connection to the neural tube (nervous system) - it will be weak

spinal vs peripheral nerves

• spinal nerves live close to the spinal cord

• as they move away they come together to form a peripheral nerve

• if the peripheral nerve is cut/gone, then the muscle WON’T work because the peripheral nerve is both, therefore there is no innervation to that muscle

spinal cord in 3rd month of utero

• spinal cord lines up with end of vertebrae

• as we get older, the spinal cord ends around L1 but the bone continues to grrow

  • leaving a part of the spinal cord you can draw CSF fluid from (spinal tap/lumbar puncture)

the brain starts as

• a simple forebrain, midbrain, and hindbrain

  • then those develop into parts of the brain that we know

hindbrain becomes

• pons

• medulla

• cerebellum

• 4th ventricle

PONS =

coffee machine - sends messages

belly

forebrain becomes

• diencephalon (thalamus, hypothalamus, 3rd ventricle)

• telencephalon (cerebral hemispheres, basal ganglia, cerebral cortex, and lateral ventricles)

telencephalon

• cerebral hemispheres

• basal ganglia

• cerebral cortex

• lateral ventricles

midbrain

brainstem

stays the same

ventricles of the brain develop from

• the central cavity of the embryonic neural tube

  • these later form the choroid process to crease CSF

  • this cavity expands to form the ventricles, eventually lining them with chorid plexi to produce CSF

spina bifida

• incomplete closure of the posterior (caudal/inferior ) neuropore

• can happen anywhere on the spine

• ranges from mild to severe(spina bifida occulta, meningocele, or myelomeningocele)

• when the neural tube does not close all of the way, the backbone is uable to fully develop to protect the spinal cord

• this can cause damage to the spinal cord and nerves

spina bifida myelomeningocele

a neural tube defect where the backbone and spinal canal fail to close before birth, causing a fluid-filled sac containing parts of the spinal cord and nerves to protrude through the baby's back

3 types of spina bifida

• occulta: small gap in spine - spinal cord and nerves remain in normal place and not exposed

• meningocele: rarest type -

anencephaly

• incomplete closure of the superior (rostral) neuropore during development

• results in lack of skull and parts of brain

• unfortunately, there is no cure, fatal

cerebral palsy

• permanent, non-progressive damage to developing brain

  • in utero or very shortly after birth

  • similar to stroke

• can affect movement, posture, and coordination

• brain injury doesn’t worsen over time