Anatomy of the Nervous System
Neurulation
- 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
= increase in thickness of brain mantle with relative constant ventricular width
◊Most rapid brain growth from 12 to 24 weeks MA!
Sulcal Development
- 10 weeks MA: interhemispheric fissure ← longitudinal cleavage
- 1419 weeks sylvian fissure
- 1622 weeks calcarine, parieto-occipital, cingulate sulci
- 2025 weeks rolandic sulcus
- 2326 weeks superior temporal + intraparietal sulci
- 2628 weeks superior frontal, pre- and postcentral, middle temporal sulci
- 34 weeks all primary + most secondary sulci present
Neuronal Migration
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 3234 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
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)
Outline