Nervous System Development

Question 1

What induces formation of the neural tube

Answer 1

The notochord (composed of endoderm)

Question 2

Describe Neurulation in steps

Answer 2

• Elevation of the neural folds
• Fusion of the folds in the midline with reconstitution of the surface ectoderm
• When the folds fuse, neural crest cells detach and migrate to their target destinations
• Neural tube zips up Rostrally and Caudally (to form the Brain and Spinal Cord respectively)

Question 3

What results from failure of the neural tube to zip up/ failed neural fold fusion Rostrally and Caudally

Answer 3

Rostrally- Anencephaly

Caudally- Spina Bifida

Question 4

The Rostrally Neural Tube displays 3 ‘swellings’.

List these and state what part of the adult brain they will form

Answer 4

Prosencephalon: Becomes the forebrain

Mesencephalon: Becomes the midbrain

Rhombencephalon: Becomes the hindbrain

Question 5

Describe the further division of the Prosencephalon and the structures they develop into

Answer 5

Telencephalon: Becomes most of cerebral hemisphere

Diencephalon: Becomes Thalamus, Hypothalamus and Optic nerve/ retina

Question 6

Describe the further subdivision of the Mesencephalon

Answer 6

Remains as the boundary between the forebrain and hindbrain

Question 7

Describe the further subdivision of the Rhombencephalon and state the structures these develop into

Answer 7

Metencephalon: Forms the Pons and Cerebellum

Myelencephalon: Forms the Medulla

Question 8

Where is the Midbrain/ Cephalic Flexure?

Answer 8

Just above the Midbrain

Question 9

Where is the Cervical Flexure?

Answer 9

Between Brainstem/ Hindbrain and Spinal Cord

Question 10

Describe the general organisation of Motor and Sensory structures in the nervous system

What causes this pattern of development?

Answer 10

Motor- Sit anteriorly
Sensory- Sit posteriorly

• Notochord induces the ventral portion of the neural tube to become the Basal Plate, which gives rise to Motor neurones
• The Alar (roof) Plate forms in the absence of signals from the Notochord, and gives use to Inter- and Sensory neurones

Question 11

In the spinal cord give 3 examples of the Dorsal-Ventral organisation of Sensory and Motor neurones

Answer 11

1: Dorsal roots are sensory, Ventral roots are motor
2: Dorsal horn contains sensory neurons, Ventral horn contains motor neurons
3: Dorsal columns sit posteriorly, Corticospinal tract sits anteriorly

Question 12

In the Medulla give an example of the Dorsal-Ventral organisation of motor and sensory structures

Answer 12

• Lemnisci are sensory and sit posteriorly

- Medullary pyramids are motor and sit anteriorly

Question 13

In the Midbrain give an example of the Dorsal-Ventral organisation of motor and sensory structures

Answer 13

• Coliculi are sensory sit posteriorly

- Cerebral peduncles are motor and sit anteriorly

Question 15

In the Cerebral cortex give an example of the Dorsal-Ventral organisation of motor and sensory structures

Answer 15

• Primary motor cortex is anterior the Central Sulcus

- Primary sensory cortex is posterior to the Central Sulcus

Question 19

Describe the development of the Cauda Equina

Answer 19

• Initially, there is a one-to-one correspondence between vertebral level and spinal cord level
• However, the spine grows faster, especially the Lumbar region
• Therefore the lower portions of the cord are stretched thus drawing out the Cauda Equina

Question 20

Why are lumbar punctures safer if done below L1?

Answer 20

Below here, there is no spinal cord, but only cauda equina

Question 22

How can Neural Tube defects lead to Hydrocephalus (Fluid buildup in the cavities/ ventricles of the brain)

Answer 22

• Tethering of cord at site of defect
• Cord can’t move as the spine grows so the brainstem is ‘pulled’ downwards through Foramen Magnum
• 4th ventricle can be compressed, becoming occluded

Question 23

What are 3 commonly used terms to describe Neural tube defects

Answer 23

• Spina Bifida
• Spinal Dysraphism
• Neural Tube Defects/ Disorders

Question 24

What do all disorders involving failed neural fold fusion have in common?

Answer 24

Failure of development of the posterior vertebral arches at 1/ more levels

Question 25

Why may a neural tube defect lead to disordered formation of the posterior vertebral arch?

Answer 25

Neural tube has a role in inducing the migration of the Sclerotome from somites to form the posterior vertebral arch

Question 26

List 6 Neural Tube defect disorders from most to least severe

Answer 26

1. Craniorachischisis
2. Anencephaly
3. Myelocoele/ Rachischisis

The following have a normal neural tube, but failed development of the posterior vertebral arch

4. Myelomeningocoele
5. Meningocoele
6. Spina Bifida Occulta

Question 27

Describe Craniorachischisis

Cranio- head
Rachi- spine
Schisis- split

Answer 27

• Entire neural tube remains open
• Failure of brain AND spinal cord to form
• Incompatible with life

Question 28

Describe Anencephaly

Answer 28

• Cranial neural tube fails to close
• Failure of brain to form
• Children may be born alive but don’t live long

Question 29

Describe Myelocoele/ Rachischisis

Answer 29

• Spinal cord fails to develop
• Usually associated with a CSF filled cyst

Children frequently;

• Have neurological defects
• Are susceptible to meningitis

Question 30

What is the Neural Crest?

What happens to the them once the neural folds fuse and the surface ectoderm is reconstituted?

Answer 30

• A highly specialised population of cells derived from the Ectoderm, at the point where the neural folds fuse
• Detach from ectoderm and migrate to their targets

Question 31

List the cells derived from the Neural Crest

Answer 31

• Schwann cells
• Adrenal medulla cells
• Melanocytes
• Leptomeninges
• Head mesenchyme
• All neurones whose cell bodies are in the PNS (Primary sensory neurons, Enteric neurons, Autonomic postganglionic neurons)

Question 32

List 4 tissues that receive significant contribution from the Neural Crest

Answer 32

• Thymus
• Thyroid
• Parts of teeth
• Parts of heart (Spiral Septum which separates Truncus Arteriosus into Aorta and Pulmonary Trunk)

Question 33

Since Neural Crest has many contributions, disorders of these cells have a large variety of manifestations

List 2 Neural Crest disorders

Answer 33

• DiGeorge Syndrome

- Hirschsprung’s Disease

Question 34

List 4 ways DiGeorge Syndrome presents

Answer 34

• Heart anomalies
• Facial anomalies
• Immunodeficiency
• Hypocalcaemia

Question 35

How does Hirschsprung’s Disease present?

Answer 35

• Lack of enteric neurons in part of Large Intestine

- Leads to constipation and low motility

Question 36

As neural crest cells migrate Anteriolaterally from the dorsal embryo, they leave behind defined populations at certain points.

List these from Dorsal to Ventral

Answer 36

• Dorsal root ganglia
• Sympathetic ganglia
• Preaortic/ Prevertebral ganglia
• Adrenal medulla (Chromaffin cells)
• Gut wall (ENS)

Question 37

Describe Myelomeningocoele/ Meningomyelocoele

Answer 37

• CSF filled cyst containing the spinal cord
• Transilluminates poorly (solid tissue IN cyst)
• Children may have neurological deficits
• Repair necessary

Question 38

Describe Meningocoele

Answer 38

• Presence of a CSF filled cyst, which transilluminates brightly
• Spinal cord in normal position
• Children have good prognosis
• Cyst needs repair as it predisposes to infection

Question 39

Describe Spina Bifida Occulta

Answer 39

• Only abnormality is lack of a posterior vertebral arch
• May have a Tuft of hair or Large Naevus over the defect
• Not associated with significant neurological problems
• 10% of population

Question 40

How can neural tube defects be prevented?

Answer 40

• 400 micrograms of Folic Acid daily
• From 3 months before conception
• Up to Week 12 of pregnancy

Question 41

Folate deficiency also predisposes to Palatal abnormalities.

What does this suggest about its role?

Answer 41

May be important for the fusion of epithelial sheets