13. Nervous System - CNS Flashcards
1
Q
When does the nervous system develop and what are the implications of this?
A
- It is one of the first systems to develop
- Also one of the last to complete
- Implications = system most susceptible to insult during pre-natal development
2
Q
How is the neural tube formed?
A
- Gastrulation produces the notochord
- The notochord induces neurulation in the neural plate above (notochord directs conversion of overlying ectoderm to neuroectoderm)
- Elevation of lateral edges of neural plate
- The depressed mid-region is the neural groove
- Neural folds gradually approach each other in the midline and fuse, producing the neural tube
3
Q
What is the origin and fate of the notocord?
A
- The notocord arises via gastrulation and is composed of cells derived from the mesoderm
- It later becomes the nucleus pulposus in the middle of intervertebral discs
4
Q
How does folding and closure of the neural plate occur?
A
5
Q
What are the neuropores?
A
- Openings of the embryonic neural tube
- One cranially and one caudally
- Cranial = anterior neuropore
- Caudal = posterior neuropore
6
Q
How do neural tube defects occur?
A
- Failure of the neural tube to close - can occur cranially or caudally
7
Q
What does a caudal neural tube defect result in?
A
- Spina Bifida
8
Q
What is Spina Bifida?
A
- Can occur anything along the length, most common in lumbosacral region
- Neurological deficits occur but not associated with cognitive delay
- Hydrocephalus nearly always occurs
- 3 types: spina bifida occulta (mild, hidden, splits in spinal cord so small that spinal cord does not protrude), meningocele(meninges to herniate between the vertebrae), myelomeningocele (meningeal membranes that cover the spinal cord also protrude through the opening, forming a sac enclosing the spinal elements, such as meninges, cerebrospinal fluid, and parts of the spinal cord and nerve roots)
9
Q
What does a cranial neural tube defect lead to?
A
- Anencephaly
- Absence of cranial structures, including brain
- Incompatible with life
10
Q
What is Rachischisis?
A
- Neural Tube Defect
- As a consequence the vertebrae overlying the open portion of the spinal cord do not fully form and remain unfused and open, leaving the spinal cord exposed.
- Splayed nervous tissue across embryonic fetal back
- Incompatible with life - very rare
11
Q
How do we diagnose NTD?
A
- opening on surface of embryo
- therefore embryonic proteins are going to leak into the amniotic fluid and therefore into maternal circulation
- relatively easy way to screen for NTD
- blood test for a lot of serum alpha-fetoprotein
- as well as USS of spinal cord and column
12
Q
How do we prevent NTD?
A
- Multifactorial can contribute to development of NTD (environment, diet, lifestyle) BUT main:
- pre-conceptual and first trimester folic acid = reduced incidence of NTD by 70%
13
Q
How does the spinal cord develop?
A
- Length of the neural tube gives rise to spinal cord
- 3rd month, spinal cord is same length as the vertebral column. Thereafter, vertebral column grows faster.
- Important side note: spinal roots must elongate because they still exit at their intervertebral foramen and forms the cauda equina
14
Q
How does the brain develop?
A
- During the neural fold, three primary brain dilations at cranial end distinguished: prosencephalon (forebrain), mesencephalon (midbrain), rhombencephalon (hindbrain)
- After neural tube closure in the 4th week, dilations become three primary brain vesicles
- At 5th week, five secondary brain vesicles formed: telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon
15
Q
What are flexures?
A
- During the fourth week, embryonic brain grows rapidly and bends ventrally producing:
- Cephalic flexure: in the midbrain region
- Cervical flexure: at junction of hindbrain and spinal cord
16
Q
How is the ventricular system developed?
A
- The tubular structure of the developing CNS persists
- Interconnected ‘reservoirs’ form filled by CSF produced by cells of the ventricular lining (choroid plexuses)
18
Q
Discuss the early organistation of the neural tube
A
- The wall of the neural tube contains ‘neuroepithelial cells’
- these differentiate into ‘neuroblasts’ forming the ‘mantle layer’ i.e. the grey matter
- nerve fibres emerge from these neuroblasts to form the ‘marginal layer’ i.e. the white matter
- The alar plate is formed = sensory = dorsal side of neural tube
- The basal plate is formed = motor = ventral side of neural tube
19
Q
What is the neural crest and what is its function?
A
- Temporary group of cells
- Cells of the lateral boder of the neuroectoderm tube
- Become displaced and enter the mesoderm and undergo epithelial mesenchymal transition into a number of other structures
20
Q
What are some 1) nervous system 2) head, neck and midline 3) miscellanous, neural crest cell derivatives?
A
23
Q
What is Hirschsprung’s disease?
A
- Birth defect where nerve cells are missing from parts of the intestine and rectum i.e. the myenteric and submucousal plexuses helping the movement of food
- Stay in default contracted position, blocking movement of faeces
- Faeces builds up proximal to blockage leading to megacolon, constipation which is at risk of rupture
- Babies fail to pass meconium
- Down syndrome associated with hirschsprung’s
24
Q
Describe DiGeorge’s syndrome
A
- deletion of a gene that usually helps normal development of pharyngeal pouches 3 and 4. pouch 3 = thymus, inferior parathyroid gland. pouch 4 = superior parathyroid gland. these are underdeveloped = deficiency in mature T cells, decrease PTH = low Ca2+ in blood
- Congenital heart defects: truncus arteriosus, tetralogy of fallot
- Facial abnormalities: cleft palate, facies (long face, small teeth, broad nose)
