Development of the Nervous System

Question 1

What does the development of the mature nervous system require

Answer 1

• requires complex & continued interactions of cell growth & division, migration, differentiation & even cell death

Question 2

Development of the mature nervous system happens as a result of what

Answer 2

• genetic & environmental influences
• needs natural genetically determined neuroplastic processes + experience-dependent neuroplastic process

Question 3

Developmental stages in utero

Answer 3

• Pre-embryonic: conception to day 14
• Embryonic: day 15 to end of week 8
• Fetal: start of week 9 to birth
• day 12 to 18 is the earliest nervous system tissue start to appear (neuroectoderm)

Question 4

Describe the main events from day 1 to day 23

Answer 4

• Day 1: fertilization
• Day 2: cleavage
• Day 3: compaction
• Day 4: differentiation
• Day 5: cavitation
• Day 6: zona hatching
• Day 7: implantation
• Day 9: cell mass differentiation
• Day 12: bilaminar disc formation & mesoderm formation
• Day 18: mesoderm spreading
• Day 23: amniotic sac enlargement

Question 5

Describe the pre-embryonic stage

Answer 5

• starts with repeated cell division, solid sphere
• cavity opens
• outer layer: implantation into endometrium of uterus, fetal contribution to placenta
• inner cell mass: embryo
• development of embryonic disk: 2 layers, the ectoderm and endoderm, & soon the mesoderm also starts to form in between
• chronic vill allows for more surface area & allows for distribution of nutrition

Question 6

Describe the embryonic stage

Answer 6

• from days 15 to 8 weeks all organs are formed
• Ectoderm: epidermis, sensory organs, the nervous system, gives rise to the visual, auditory, & ophthalmic systems
• Mesoderm: dermis, muscles, skeleton, excretory, circulatory systems, gives rise to the musculoskeletal system
• Endoderm: gut, liver, pancreas, respiratory system
• if looking at the dorsal side you are looking at the ectoderm side which starts to show an indentation –> groove –> plate

Question 7

Neural development during the embryonic stage

Answer 7

• day 18: the nervous system develops from the ectodermal layer which gets buried inside the mesoderm
• day 21: neural tube zipping completes by 1 month

Question 8

Describe the neural tube

Answer 8

• primordial of the CNS (brain and spinal cord)

Question 9

Describe the neural crest

Answer 9

• group of cells that break off of the tube, forms much of PNS (DRGs, sensory CNS, ANS, Schwann cells, endocrine organs etc.)
• once neural crest has developed, the tube moves inside the embryo, overlying ectoderm moves in to cover it forming the future epidermis

Question 10

Describe the differentiation of neural tube

Answer 10

• neural tube differentiates further into 2 layers: the inner layer becomes gray matter (neuron cell bodies), outer layer becomes white matter (neuron processes & glial cells)

Question 11

Describe somites

Answer 11

• as neural tube closes, adjacent mesoderm forms Somites
• first appear in rotral portion, then added caudally (tail end)

Question 12

Describe the differentiation of the neural tube & somite cells

Answer 12

• Neural tube: separates further into motor & association plates
• Motor plate: ventral horn includes motor neurons
• Alar/Association plate: dorsal horn includes interneurons
• Myotome = skeletal muscles
• Dermatome = dermis
• Sclerotome = vertebrae & skull

Question 13

Describe the changes in the spinal cord levels with growth

Answer 13

• until 3rd month in utero, spinal segments correspond well with the vertebral levels
• after 3rd month in utero, vertebral column grows faster than spinal cord

Question 14

Development of the brain

Answer 14

• Hindbrain: medulla, pons, cerebellum, & the 4th ventricle
• Midbrain: midbrain
• Forebrain: diencephalon & telecephalon
• Telencephalon: cerebral hemispheres & basal ganglia
• Diencephalon: thalamus & hypothalamus
• Cerebral hemispheres grow extensively

Question 15

Critical periods of development

Answer 15

• periods of time when neural projections complete for synaptic sites, optimizing connections for normal development
• time when synaptic pruning happens through competition between developing circuits
• between about day 14 & week 20 in humans most vulnerable to injury
• injuries during this period can lead to major implications in the form of lingering primitive connections
• problems with reflex integration: ATNR, STNR, TLR
• problems with reciprocal inhibition: co-contractions
• asymmetrical tonic neck reflexes: ATNR needs to go away by 6-9 months
• symmetrical tonic neck reflexes: STNR