Learn: lecture 4 (nervous system)

3 regions of nervous system

- central

- peripheral

- autonomic

central nervous system

brain & spinal cord, protected by cranium & vertebral column

peripheral nervous system

includes neurons, cranial nerves, spinal nerves which connect brain & spinal cord with peripheral structures

autonomic nervous system

parts in CNS/PNS; consists of neurons for smooth/cardiac muscles, glandular epithelium

when does nervous system development begin?

3rd week

what does nervous system originate from?

neural plate & neural groove develop on posterior aspect of trilaminar embryo

neural crest gives rise to cells that form most of ___ & ____

PNS, ANS

Neurulation

formation of neural plate & tube begins during 4th week

fusion of neural fold & formation of neural tube begins at

5th week

closure of neuropores coincides with establishment of...

vascular circulation for neural tube

ventricular zone

gives rise to all neurons & macroglial cells

marginal zone

white matter of spinal cord; axons grow into it from nerve cell bodies

intermediate zone

mantle layer; neuroblasts become neurons

glioblasts (2)

- supporting cells of CNS

- migrate from ventricular to intermediate & marginal zones

microglia (3)

- scattered throughout grey & white matter of spinal cord

- derived from mesenchymal cells

- originate in bone marrow

meninges (4)

- membranes covering the spinal cord

- external layer thickens to form dura mater

- internal layer composed of pia mater & arachnoid mater

- CSF begins to form during the 5th week of life

T/F: spinal cord positioning changes from fetal to adult life

TRUE

filum terminale

dura mater & arachnoid mater end at the end of S2 vertebra in adults, pia mater forms this long fibrous thread, which indicates the original level of caudal end of embryonic spinal cord

myelin sheaths (3)

- begin to form during the late fetal period & continue through the 1st postnatal year

- formed by oligodendrocytes & types of glial cells that originate from neuroepithelium

- plasma membranes of cells wrap around axon forming several layers

motor roots are ____ before sensory roots

myelinated

sacral dimple

indicated region of closure of caudal neuropore at the end of the 4th week

neural tube defects

result from failure of one/more neural arches of developing vertebrae during 4th week

incidence of neural tube defects

1-2/1000 live births

what countries are more common for neural tube defects

those with poorer nutrition

what can help reduce risk for neural tube defects

folic acid

are neural tube defects commonly screened for?

yes, quad screen

embryologic basis of neural tube defects

defective closure of neuropores

the two neuropores

rostral & caudal

nutritional factors for NTDs (2)

- low levels of B12 increases risk

- folic acid can decrease the risk

environmental factors that can increase risk of NTDs

- antiepileptic medicaitons

- diabetic embryopathy

do genetic syndrome & chromosomal disorders cause NTDs?

RARELY

spina bifida

defects involving embryonic neural arches

(1-2/1,000 live births)

meningocele

contains meninges & CSF

meningomyelocele

spinal cord/nerve roots are within cyst

myueloschisis

most severe type, spinal cord in affected area is completely open causing a flattened mass of nerve tissue

4 spina bifida & associated defects

- spina bifida occulta

- meningocele

- meningomyelocele

- myelochisis

spina bifida occulta

represented by a small dimple w a tuft of hair rising from it

what is pallister-killian syndrome caused by?

tetrasomy 12p

symptoms of pallister-killian syndrome (4)

- NTDs

- webbed neck

- omphalocele

- causes stillbirth or neonatal death

is tetrasomy 12p inherited?

no, it's usually a result of maternal nondisjunction

is pallister-killian syndrome compatible with life?

mosaic type can be

fusion of neural folds in cranial region + closure of rostral neuropore =

3 primary brain vesicles

3 primary brain vesicles

forebrain, midbrain, hindbrain

forebrain (prosencephalon)

during 5th week divides into 2 secondary brain vesicles

forebrain divides into...(2)

- telencephalon

- diencephalon

midbrain (mescencephalon)

does not divide

hindbrain (rhombencephalon)

partly divides into 2 vesicles, demarcated by pontine flexure

hindbrain divides into...(2)

- metencephalon

- myelencephalon

mentencepaholon (2)

pons

cerebellum

myelencephalon

medulla oblongata

2 optic vesicles that appear when rostral neuropore closes

primordia of retinae

optic nerves

telencephalon

vesicles arise more dorsally & rostrally, forming the primordia of cerebral hemispheres

cavities from telencephalon become..

lateral ventricles

cerebral hemispheres meet in the middle to form...

falx

diencephalon

3 swellings in lateral walls of 3rd ventricle

3 parts of diencephalon

thalamus

hypothalamus

epithalamus

pituitary gland

ectodermal in origin, made up of both glandular & nervous tissues

corpus callosum

extends over the roof of diencephalon

choroid plexus & CSF

sheet of pia mater invaginates 4th ventricle where it differentiates into choroid plexus

choroid plexus cysts

seen in 1-3% of normal fetuses & have no clinical significance

CP cysts + other abnl us findings

increased risk for chromosomal aneuploidy, esp trisomy 18

birth defects of the brain

abnormal development is common because of embryologic complexity

incidence of birth defects of the brain

3/1,000

cerebral palsy

central motor deficit often is the result from adverse events during birth

what is cerebral palsy caused by?

maternal infection, thyroid disorder, Rh incompatibility, RARELY genetic/hereditary conditions

population incidence of developmental delay?

3%

by end of 16th week..

most neuronal proliferation & cell migration to cerebral cortex are completed

developmental delay/intellectual disability can be caused by

exposures during 8-16 week period

examples of exposures (4)

rubella

CMV

high levels of radiation

maternal alcohol abuse

congenital _____ increases risk for DD

hyperthyroidism

agenesis of corpus callosum

complete or partial absence of corpus callosum

- may be asymptomatic

what are 2 things commonly seen in agenesis of corpus callosum?

seizures

mental deficiency

some symptoms of agenesis of corpus callosum (3)

poor motor coordination

hypotonia

delays in motor milestones

chiari malformation

structural defect of cerebellum

- projection of medulla penetrates through vertebral canal

type 1 chiari malformation

most common, usually asymptomatic, detected in adolescence

type 2 chiari malformation

arnold-chiari malformation

- hernia accompanies by occipital encephalocele & lumbar myelomeningocele

type 3 chiari malformation

most severe, herniation of cerebellum & brainstem into vertebral canal leads to serious neurological consequences

type 4 chiari malformations

cerebellum is absent or underdeveloped

LETHAL

microcephaly

caused by reduction in brain growth

can microcephaly have a genetic basis?

yes

enchephalocele

most common in occipital region

1/2,000

meningohydroenchephalocele

contains meninges, part of brain, & part of ventricular system

meningoencephalocele

contains meninges & part of brain

hydrocephaly

caused by imbalance in production & absorption of CSF

- excess in ventricular system

x-linked, mutations in L1CAM

can cause congenital aqueductal stenosis for hydrocephaly

holoproscencephaly

results from failure of cleavage of the prosencephalon

holoprosencephaly is associated with..

facial defects corresponding to severity

4 types of holoprosencephaly

alobar

semi-lobar

lobar

middle

anencephaly (meroencephaly)

resulting from failure of rostral neurpore to close during 4th week

- 1/1000 births

what is absent in anencephaly (3)

forebrain

midbrain

most of hindbrain

is anencephaly more common in boys or girls

2-4x more common in girls

anencephaly is...

multifactorial mode of inheritance

PNS develops from...

neural crest

PNS begins as..

bipolar cells with 2 processes

spinal nerves

arise from basal plates of developing spinal cord

ventral nerve root

fibers destined for a particular developing muscle group become arranged in a bundle

12 pairs of cranial nerves form during ___ & _____ weeks

5th & 6th

3 groups of cranial nerves

- somatic efferent cranial nerves

- nerves of pharyngeal arches

- special sensory nerves

somatic efferent cranial nerves (4)

- oculomotor

- trochlear

- abducent

- hypoglossal

nerves of pharyngeal arches (5)

trigeminal

facial nerve

glossopharyngeal nerve

vagus nerve

spinal accessory nerve