S1) Embryology of the Nervous System

Question 1

Describe the five steps involved in the formation of the neural tube in early embryonic development

Answer 1

⇒ Gastrulation produces the notochord

⇒ Notochord induces neurulation

⇒ Neurulation induces the neural plate

⇒ Elevation of lateral edges of neural plate

⇒ Neural folds gradually approach each other in the midline and fuse, producing the neural tube

Question 2

What is the role of the notochord during neurulation?

Answer 2

The notochord directs the conversion of the overlying ectoderm to neurectoderm

Question 3

What is a neuropore?

Answer 3

A neuropore is a region corresponding to the opening of the embryonic neural tube in the anterior/posterior portion of the developing prosencephalon

Question 4

Defects in closure of the neuropores underlie serious and common birth defects of the nervous system.

What are neural tube defects?

Answer 4

• Neural tube defects are defects which result from failure of the neutral tube to close
• Failure can occur caudally or cranially

Question 5

What are the results of the following:

• Cranial neural tube defect
• Caudal neural tube defect

Answer 5

• Cranial neural tube defect results in anencephaly
• Caudal neural tube defect results in spina bifida

Question 6

What is spina bifida?

Answer 6

• Spina bifida is a type of neural tube defect occurring when the vertebrae don’t form properly around part of the baby’s spinal cord
• It arises from the failure of neural tube closure caudally

Question 7

Spina bifida can occur anywhere along the length of the spine.

What is the most common location?

Answer 7

Spina bifida most commonly occurs in lumbosacral region

Question 8

What is anencephaly?

Answer 8

- Anencephaly is a neural tube defect resulting in the absence of cranial structures, including the brain

• It results from the failure of neural tube closure cranially and is incompatible with life

Question 9

What is rachischisis?

Answer 9

Rachischisis is a neural tube defect occuring due to the failure of neural fold elevation

Question 10

How can one diagnose a neural tube defect?

Answer 10

• Raised maternal serum α-fetoprotein
• Ultrasound

Question 11

How can a neural tube defect be prevented?

Answer 11

Folic acid pre-conceptually (3 month) and for the first trimester reduces incidence by 70%

• given especially to woman who are obese, diabetic and epileptic ( increase 300mg to 400mg daily)

Question 12

Most of the length of the neural tube gives rise to the spinal cord.

In four steps, explain how the cauda equina forms

Answer 12

⇒ A 3 months, the spinal cord is the same length as the vertebral column

⇒ Thereafter, the vertebral column grows faster

⇒ The spinal roots must elongate in order to exit at their intervertebral foramen

⇒ Cauda equina is formed

Question 13

During neural fold formation three primary brain regions can be distinguished.

Identify these primary brain vesicles

Answer 13

• Embryonic forebrain (prosencephalon)
• Embryonic midbrain (mesencephalon)
• Embryonic hindbrain (rhombencephalon)

Question 14

At 5 weeks of development, the three primary brain vesicles become five secondary brain vesicles.

Identify these

Answer 14

Question 15

Identify the mature derivatives of the following secondary brain vesicles:

• Telencephalon
• Diencephalon
• Mesencephalon
• Metencephalon
• Myelencephalon

Answer 15

Question 16

How are flexures formed in the embryological development of the nervous system?

Answer 16

Growth & development at the cranial neural tube exceeds available space linearly, so it must fold up to form flexures

Question 17

Which two flexures are formed in the embryological development of the nervous system?

Answer 17

• Cervical flexure
• Cephalic flexure

Question 18

Where are the cervical and cephalic flexures located respectively?

Answer 18

• Cervical flexure – hindbrain junction
• Cephalic flexure – midbrain region

Question 19

What is the role of the ventricular system?

Answer 19

The ventricular system cushions the brain & spinal cord within their bony cases

Question 20

Compare and contrast the ventricular system in development and adults

Answer 20

• In development, it is a tubular structure of the developing CNS persisting as development proceeds
• In the adult, it is comprised of interconnected “reservoirs” filled by CSF, produced by cells of ventricular lining

Question 21

Relate the secondary brain vesicles to their corresponding ventricle in the ventricular system

Answer 21

• Telencephalon → lateral ventricle
• Diencephalon → third ventricle
• Mesencephalon → cerebral aqueduct
• Metencephalon & myelencephalon → fourth ventricle

Question 22

What is hydrocephalus?

Answer 22

• Hydrocephalus is a condition characterised by excessive accumulation of fluid in the brain
• Is is most common in newborns with spina bifida occur due to a blockage of the ventricular system e.g. tumour, infection

Question 23

How can hydrocephalus be treated?

Answer 23

Hydrocephalus is readily treatable by use of shunt

Question 24

Explain the early organisation of the neural tube by describing its three layers

Answer 24

• Inner: neuroepithelial layer
• Intermediate: mantle layer (neuroblasts)
• Outer: marginal layer (processes)

Question 25

What is the function of the roof and floor plates of the neural tube?

Answer 25

Roof & floor plates regulate **dorsal & ventral patterning**

Question 26

Describe the modality of the alar and basal plates respectively

Answer 26

- Alar plate = **sensory** - Basal plate = **motor**

Question 27

Describe the location and function of neural crest

Answer 27

- **Location**: cells of the lateral border of the neuroectoderm tube - **Function**: become displaced and enter the mesoderm and undergo epithelial to mesenchymal transition

Question 28

Identify 8 neural crest derivatives in the nervous system

Answer 28

- Cranial nerve ganglia - Spinal root ganglia - Sympathetic ganglia (chain & pre-aortic) - Parasympathetic ganglia - Schwann cells - Glial cells - Leptomeninges (arachnoid & pia) - Connective tissue & bones of the face & skull

Question 29

Identify 3 neural crest derivatives in the head, neck and midline

Answer 29

- Odontoblasts - Dermis (face & neck) - C cells of the thyroid gland

Question 30

Identify 3 miscellaneous neural crest derivatives

Answer 30

- Conotruncal septum (heart) - Melanocytes - Adrenal medulla

Question 31

what is neurolation?

Answer 31

* formation of the neural tube * induced by the notochord * fusion of the folds in the midline at around the neck level * once folds fuse, neural crest cells detach and ,migrate

Question 32

what direction does the neural zip close up

Answer 32

* rostrally and caudally

Question 33

what two parts does the forebrain split into?

Answer 33

* telencephalon → becomes most of cerebral hemisphere * diencephalon → thalamus, hypothalamus an optic nerve

Question 34

what does the rhombencephalon split into?

Answer 34

* metencephalon → pons and cerebellum * myelencephalon → medulla

Question 35

what is the relationship between the sensory and motor neurones

Answer 35

* motor structures sit anteriorly and sensory sit posteriorly * due to the neural tube making the floor give rise to motor neurones and roof sensory

Question 36

give some examples of the relationship of sensory and motor neurone's

Answer 36

Medulla The medial lemniscus (sensory) sit posterior to the pyramids of the medulla (motor) o Midbrain The colliculi (sensory) sit posterior to the cerebral peduncles (motor)

Question 37

development of the caudal equina

Answer 37

* initially spinal chord and spine grow at same pace * soon spine grows faster at lumbar level

Question 38

what can happen to the caudal equine to cause hydrocephalus?

Answer 38

* tethering of chord at sight of defect * as spine grows the cord can't * 4th ventricle gets pulled down foramen magnum and becomes occluded

Question 39

what is a meningomyelocoele?

Answer 39

* failure of somite to form bone precursors around neural tube (neural tube still fully developed) * cyst filled with CSF * spinal chord gets stretched and pulled and damaged * transilluminates poorly due to solid tissue in cyst

Question 40

what is a meningocele?

Answer 40

* failure of somite to form a bone precursor around the neural tube * here spinal cord isn't being pulled and is in correct position * presence of a cyst full of CSF * transilluminates well * good prognosis

Question 41

what is a myelocoele?

Answer 41

* neural tube fails to develop * neural tube exposed to outside world * susceptible to meningitis

Question 42

what is spinal bifida occulta

Answer 42

* lack of posterior vertebral arch * manifest with hair tuft on back * no significant neurological problems

Question 43

neural crest cells

Answer 43

* highly specialised * multifunctional and give rise to wide range of tissues * derived when the neural folds fuse

Question 44

what are some of the derivatives of neural crest cells

Answer 44

* sensory neurones in the PNS * enteric neurones * Schwann cells * cells of adrenal medulla * head mesenchyme

Question 45

which tissues receive a significant contribution from neural crest cells

Answer 45

* thymus * thyroid * parts of heart and teeth

Question 46

As streams of neural crest cells migrate from the dorsal part of the embryo, defined populations become left behind at certain points

Answer 46

from dorsal to ventral: * The dorsal root ganglia (sensory neurones) * The sympathetic ganglia (sympathetic postganglionic neurones) * The preaortic ganglia (sympathetic postganglionic neurones that receive input from splanchnic nerves) * The adrenal medulla (chromaffin cells, which are homologous to sympathetic postganglionic neurones) The but wall (enteric nervous system)

Question 47

Di George syndrome

Answer 47

* disorder of the neural crest cells: * Immunodeficiency (due to involvement of the thymus) * Facial anomalies (due to contribution of neural crest to facial development) * Heart anomalies * Hypocalcaemia (involvement of parathyroids)

Question 48

Hirschprung’s disease

Answer 48

* disorder of neural crest cells Lack of enteric neurones in sections of the large intestine → no peristalsis inside parts of the colon This leads to hypomotility and constipation

Question 49

what does the term meningo refer to

Answer 49

CSF

Question 50

what does the term myelo refer to

Answer 50

cord

Question 51

what other tissue does the CNS share embryological origin

Answer 51

it derives from ectoderm same as the skin

Question 52

what organ develops from the same origin as the nervous system

Answer 52

skin, from ectoderm

Question 53

what is another name for cerebral peduncles

Answer 53

crus cerebri

Question 54

which part of the brainstem lies directly at the level of the tenotrium cerebellum

Answer 54

midbrain

Question 55

the falx cerebri is composed of which meningeal layer

Answer 55

meningeal dura