1.3. Development of Nervous System

what are the 3 stages of human development?

1. pre-embryonic

2. embryonic

3. fetal

describe the pre-embryotic stage of development in utero (include timeline)

1. egg fertilized in uterine tube; cells develop

2. cells divide and form sphere (inner/outer cell layers)

3. Cavity in sphere forms (outer cells/inner cells)

4. Implants into endometrium

5. Embryonic disc:(Ectoderm, mesoderm, endoderm)

conception- day 14

the inner cell mass forms the

embryo

the outer cells form the

placent

the inner cell mass becomes the

3 primary germ layers

describe the embryonic stage of development in utero (include timeline)

organ formation starts in etoderm, mesoderm, and endoderm

day 15- week 8

what forms in the ectoderm (outer)?

sensory organs, epidermis, nervous system

what forms in the mesoderm (middle)?

dermis, muscles, skeleton,

excretory & circulatory systems

what forms in the endoderm (inner)?

gut, liver, pancreas, respiratory system

the organs developed in the embryonic stage _______ here

BEGIN (are not fully developed)

describe the fetal stage of development in utero (include timeline)

nervous system specific... 2 things occur

1. NS continues to develop

2. myelination begins

week 9-birth

what stage(s) does the nervous system form? from germ layer?

- embryonic and fetal stages

- ectoderm

what are the 2 phases of nervous system formation?

neural tube formation and brain formation

when does neural tube formation occur? what stage of development?

day 18-day 26

embryonic stage

what is the neural plate?

long, flat thickening of the ectoderm on the surface of the

embryo from head to tail

the edges of the neural plate fold in forming the

neural groove

the neural tube is formed when the

edges of the neural plate meet (edges curl towards each other)

the edges first close in the

cervical region

the edges continue to close from cervical region and on in both directions and form openings called....

what are the directions called?

neuropores (will eventually close)

rostral and caudal

when the neural tube is developing it begins to look like the

spinal cord (early foundation)

what is also formed when the edges meet and neuropores are formed?

neural crest

the neural tube and crest are surrounded by what layer? what does this layer become?

ectoderm

eventually becomes epidermis

two rings are layers are formed during the formation of the neural tube. the inner layer is ______ matter and the outer layer is ______ matter

gray; white

what structure has an inner gray matter and outer white matter?

spinal cord

the adjacent germ layer of the neural tube is the

mesoderm (skeletal system, muscular system, and dermis layer develop here)

the mesoderm layer divides to form

somites

what are somites?

clusters of cell bulges

(foundation blocks that build dermatomes and myotomes)

what are the 3 parts to the somite and what do they form?

1. sclerotome: vertebrae + skull

2. myotome: skeletal muscle

3. dermatom: dermis

what do myotomes and dermatomes represent?

single spinal nerve contribution to patch of skin or group of muscles with common action

the ventral part of a neural tube is the

motor plate

the dorsal part of a neural tube is the

association plate

motor neurons are typically

multipolar

sensory nerves are typically

pseudounipolar

what comes immediately off of the spinal cord?

dorsal and ventral roots

Somas from motor plate extend axons to their respective ________ region of the somite

myotome

the nerves from the motor plate become

lower motor neurons

what does the neural crest separate into during dermatome development?

two columns on each side of neural tube

the neural crest forms segments during dermatome development. what do these areas correspond to?

dermal areas of somites (appear as bulges that represent spinal levels of SC)

Crest cells at each segment form

peripheral sensory neurons

peripheral sensory neurons are what type of neuron? soma location?

pseudounipolar; sit in dorsal root ganglion

what do the ends of peripheral sensory neurons connect to?

One end connects to spinal cord and the other end connects to somite (dermis)

what is the definition of a dermatome and myotome during development?

The area of a somite that will become skeletal muscle / dermis

what is the definition of a dermatome and myotome in a fully developed adult?

A group of muscles / area of dermis innervated by a single spinal nerve

the entine PNS develops from tissue of the

neural crest (exept LMN)

what does the CNS/LMN develop from?

cells from neural tube

Spinal cord segments are _________ to corresponding vertebrae until the

_____ fetal month

adjacent; 3rd

the _______ _________ grows faster than the ________ __________

vertebral column; spinal cord

what is the significance of the vertebral column growing faster than the spinal cord?

mainly due to efficiency purposes. want to continue to develop skeletal system whereas spinal cord fully develops earlier in development. would rather conserve that "energy" to focus on developing the function of nervous tissue cells

the spinal cord ends at what level vertebrae?

L1/2 (conus medullaris)

what is the cauda equina? where do nerves do?

spinal nerves that come off spinal cord. nerves go to their respective vertebral level/opening

when does the brain begin forming?

around day 28 (end of 4th week)

what structure closing tiggers the neural tube to form 3 enlargements? what are those enlargements?

superior neuropore

• Hindbrain

• Midbrain

• Forebrain

what does the hindbrain consist of?

medulla, pons, cerebellum

(medulla/pons are anterior, cerebellum is posterior)

what is the central canal?

inner part of neural tube where CSF is found. sits between the brainstem and cerebellum

the central canal of the neural tube becomes what in the hindbrain?

4th ventricle

the central canal of the midbrain becomes

cerebral aqueduct

role of cerebral aqueduct?

connects the 4th ventricle to the 3rd ventricle

what does the forebrain consist of?

diencephalon, cerebrum (telencephalon)

what structure forms in the diencephalon of the forebrain?

3rd ventricle

where do the lateral ventricles form in the forebrain?

telencephalon (cerebrum)

as the brain develops through the fetal stage it fold in on itself. what are these folded sheets of tissue called?

sulci and gyrus

what makes neurons and glia?

epithelial cells in the neural tube (migrate to final destination)

nervous system tissue develops up to a certain point. what happens after it develops?

the rest of the time is used to develop function of nerve tissue

when do neurons differentiate?

after they reach final destination

what does neuron function depend on?

which region of the brain they migrate to. as neurons put out to their respective regions, this is where nervous tissue figures out its function

what helps "link" neurons with target cells?

growth cones

where are growth cones found? how do they work?

end of neurons. they communicate with different structures and tell nervous tissues where to/not to go (either attracted to or repelled by tissue)

when does myelination begin? completed?

4th fetal month (4 months after fertilization; week 9)

completed in 3rd year of life

what are critical periods?

- times when neuronal projections compete for synaptic sites (period of nervous system optimization)

- expected level of exposure to stimuli that allow neurons to develop properly

what must be adequate for adequate development? exmaples?

Peripheral and central nervous system experiences

- Retinal cells, optic nerves, visual cortex & proper vision

- Cerebral cortex hearing centers, speech sounds & language development

must receive proper exposure during critical period for proper function to develop