neural development

Pre-embryonic time period

conception to day 14

single cell begins cell division as it moves down uterine tube into uterus

pre-embryonic

during implantation, inner cell mass develops into embryonic disk, consisting of 2 layers: ectoderm and endoderm

pre-embryonic

embryonic time period

day 15 to end of week 8

fetal time period

9th week to birth

organs form

embryonic stage

ectoderm develops into sensory organs, epidermis, and nervous system

embryonic

mesoderm develops into dermis, muscles, skeleton, and excretory and circulatory systems

embryonic

endoderm differentiates and becomes gut, liver, pancreas, and respiratory system

embryonic

nervous system develops more fully and myelination begins

fetal stage

formation of the nervous system occurs during the

embryonic stage

formation of the nervous system involves 2 phases

1. neural tube formation

2. brain formation

neural tube formation time period

day 18-26

tissue that becomes the nervous system comes together and forms a tube running along the back of the embryo

neural tube formation

brain formation begins on day

28

occurs when the ends of the tube close

brain formation

neural plate forms on the surface of the embryo; extending from head to tail region, in contact with amniotic fluid. this region fold in to create the

neural groove

when the edges of the fold touch, the neural tube is formed by

day 21

closure of neural tube first happens in

cervical region

Open ends of the neural tube

neuropores

neuropores close by

day 30

superior neuropore closes by day

27

Inferior neuropore closes by day

30

neural crest forms when cells adjacent to neural tube

separate from the tube and remaining ectoderm

neural tube and crest move

inside embryo

By day 26, neural tube differentiates into

two concentric rings

-inner layer contains cell bodies, will become gray matter

-outer layer contains processes of cells located in inner layer, will become white matter

Axial mesoderm forms

notochord (~16 days gestation)- complete by week 4- becomes nucleus pulposus of disc

becomes brain, spinal cord, PNS

neuroectoderm

notorchord triggers the overlying ectoderm to

form neural plate= longitudinal thickening of ectoderm

as the neural tube grows, the adjacent mesoderm divides into

somites

anteromedial part of somite

Sclerotome- becomes vertebrae and skull

posteromedial part of somite

myotome; becomes the skeletal muscle

lateral part of somite

dermatome; becomes the dermis

as the cells of the inner layer proliferate in the neural tube, grooves form on each side, separating tube into

ventral and dorsal sections

ventral section of neural tube

becomes the motor plate

-myotome

-cells become lower motor neurons

-in mature spinal cord called ventral horn

dorsal section of neural tube

association plate; cells become interneurons and projection neurons

-sensory

-dorsal horn in mature spinal cord called

Until the 3rd fetal month, spinal cord segments are...

adjacent to corresponding vertebrae

-then, VC grows faster than cord

-results in adult spinal cord ending at L1-L2

CNS develops from the

neural tube

The PNS develops from the

neural crest

cauda equina

L3-S5

conus medullaris

L1-L2

passage of the nerve roots through specific vertebral foramina is

established early in development

lower nerve roots elongate w/in the

vertebral canal to reach their passage

1st pair of somites

day 20

limb bud appears around end of

week 4 as a condensation of mesenchyme along the ventrolateral body wall

initial position of limb bud

early stages of UE and LE development are similar

the limb bud grows

ventrally out of embryo

UE limb bud

thumb is cranial

LE limb bud

great toe is cranial

UE limb bud grows opposite at approx

C3-T2

LE limb bud grows opposite at approx

L2-S2

Limb rotation happens during

7th or 8th week

UE limb bud rotates

90 degrees laterally (external, outward) so elbow if posterior

LE limb bud rotates

90 degrees medially (internal, inward) so knee is anterior

-thus UE extensors are posterior and LE extensors are anterior

as soon as the limb bud forms, spinal nerves corresponding to the

approximate levels of formation penetrate the bud

hindbrain becomes

medulla, pons, cerebellum

midbrain enlargement remains its name

t/o development

central canal becomes the

cerebral aqueduct in the midbrain, connecting the 3rd and 4th ventricles.

posterior region of the forebrain becomes the

diencephalon

major structures of the diencephalon

thalamus and hypothalamus

anterior part of the forebrain becomes the

telencephalon

telencephalon becomes the

cerebral hemispheres, which envelope the diencephalon

Deep inside the hemispheres are the

basal ganglia

as the hemispheres expand ventrolaterally to form the temporal lobe, they attain a

c-shape

as a result of this growth pattern, the caudate nucleus (part of the basal ganglia) and the lateral ventricles also become

c-shaped

lateral areas of hemispheres dont

grow as much as other areas; covered area is insula

surfaces of cerebral and cerebellar hemispheres begin to

fold; creating sulci (grooves into the surface), and gyri

The embryo is a fetus at

9 weeks

major CNS structures all present and recognizable by the end of the

first trimester

pregancy is divided into

3 trimesters (40 weeks)

deficits of a fetus can by caused by

-infections

-maternal illness

-environmental toxins

-ETOH

-medications/drugs

TORCH syndrome

Toxoplasmosis, Other agents, Rubella, CMV, Herpes simplex virus/HIV

neurons differentiate appropriately after

migrating to their final location

function of each neuron depends on

the area of the brain where the neuron migrates

Process emerges from the neuron cell body; the forward end expands to form a

growth cone

when the growth cone contacts its target cell

synaptic vesicles soon form, and microtubules that formerly ended at the apex of the growth cone project to the presynaptic membrane

neuronal connections sculpt

developing musculature

in early development, as many as

half of neurons die off- likely they failed to establish optimal connections with targets or were too inactive

myelination begins in the

4th fetal month; most sheaths are completed by the end of the 3rd year of life.

CNS most susceptible to major malformations between:

day 14 and week 20

formation of a rudimentary brainstem without cerebral and cerebellar hemispheres

anencephaly

results when inferior neuropore doesn't close

spina bifida

neural tissue remains inside vertebral column

spina bifida occulta

meninges protrude through bony defect

meningocele

neural tissue and meninges outside body

myelomeningocele

malformed spinal cord open to the surface of the body

myeloschisis

50-70% NTDs can be prevented by

folic acid

Cord adheres to a lower vertebra causing dermatomal and myotomal deficits in the lower limbs, pain in the saddle region and lower limbs, and bowel and bladder dysfunction

tethered spinal cord

autosomal recessive disorder: lower motor neurons that innervate skeletal muscles degenerate

spinal muscular atrophy

abnormalities of dendritic spines are found in many cases of

intellectual disability

movement and postural disorder caused by permanent, nonprogressive damage of the developing brain

cerebral palsy

NS damage isnt progressive but new problems may appear when developmental milestones are typically reached

CP

80% of cases result from events occurring before onset of labor

CP