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Anatomy of the Nervous System

Neurulation!!navigator!!

  • neural plate = CNS originates as a plate of thickened ectoderm on the dorsal aspect of the embryo
  • neural crest = elevation of the lateral margins of the neural plate; forms the peripheral nervous system
  • neural tube = invagination between the 2 neural crests; its wall forms the brain + spinal cord; its lumen forms the ventricles + spinal canal
  • 4.6 weeks MA: formation of neural tube
  • 5.6 weeks MA: rostral neuropore closes
  • 5.9 weeks MA: caudal neuropore closes
  • 6.0 weeks MA: 3 primary brain vesicles develop (prosencephalon, mesencephalon, rhombencephalon) + development of cervical flexure
  • 6.5 weeks MA: prosencephalon cleaves into telencephalon anteriorly + diencephalon posteriorly
  • 7.0 weeks MA: 2 additional primary brain vesicles form out of rhombencephalon (pontine flexure divides into myelencephalon, metencephalon)
  • 15 weeks MA: dorsal portion of alar plates bulging into 4th ventricle have fused in midline to form cerebellar vermis

Brain Growth!!navigator!!

= increase in thickness of brain mantle with relative constant ventricular width

Most rapid brain growth from 12 to 24 weeks MA!

Sulcal Development!!navigator!!

  • 10 weeks MA: interhemispheric fissure longitudinal cleavage
  • 14–19 weeks sylvian fissure
  • 16–22 weeks calcarine, parieto-occipital, cingulate sulci
  • 20–25 weeks rolandic sulcus
  • 23–26 weeks superior temporal + intraparietal sulci
  • 26–28 weeks superior frontal, pre- and postcentral, middle temporal sulci
  • 34 weeks all primary + most secondary sulci present

Neuronal Migration!!navigator!!

7th week subependymal neuronal proliferation = germinal matrix depicted as low SI band along ventricular wall on T2WI

8th week radial migration to cortex along radial glial fibers

Germinal Matrix

= highly vascular gelatinous subependymal tissue adjacent to lateral ventricles in which the cells that compose the brain are generated; has its largest volume around 26 weeks GA; decreases in size with increasing fetal maturity; usually involutes by 32–34 weeks of gestation

Location: greatest portion of germinal matrix above caudate nucleus in floor of lateral ventricle, tapering as it sweeps from frontal horn posteriorly into temporal horn, roof of 3rd + 4th ventricle

Arterial supply: via Heubner artery from ACA, striate branches of MCA, anterior choroidal a., perforating branches from meningeal a.

Capillary network: persisting immature vascular rete = large irregular endothelial-lined channels devoid of connective tissue support (collagen and muscle)

Venous drainage: terminal vv., choroidal v., thalamostriate v. course anteriorly + feed into internal cerebral v. which has a posterior course

Myelination!!navigator!!

Progression: caudal to rostral; posterior to anterior; center to periphery

MR: T1WI if <7 months of age; T2WI if >7 months of age

Milestones:

  • term birth: brainstem, cerebellum, posterior limb of internal capsule
  • 2 months: anterior limb of internal capsule
  • 3 months: splenium of corpus callosum
  • 6 months: genu of corpus callosum

Occipital white matter:

  • central at 5 months (T1WI), 14 months (T2WI)
  • peripheral at 7 months (T1WI), 15 months (T2WI)

Frontal white matter:

  • central at 6 months (T1WI), 16 months (T2WI)
  • peripheral at 11 months (T1WI), 18 months (T2WI)

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