Nervous System Development Flashcards
What induces formation of the neural tube
The notochord (composed of endoderm)
Describe Neurulation in steps
- 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)
What results from failure of the neural tube to zip up/ failed neural fold fusion Rostrally and Caudally
Rostrally- Anencephaly
Caudally- Spina Bifida
The Rostrally Neural Tube displays 3 ‘swellings’.
List these and state what part of the adult brain they will form
Prosencephalon: Becomes the forebrain
Mesencephalon: Becomes the midbrain
Rhombencephalon: Becomes the hindbrain
Describe the further division of the Prosencephalon and the structures they develop into
Telencephalon: Becomes most of cerebral hemisphere
Diencephalon: Becomes Thalamus, Hypothalamus and Optic nerve/ retina
Describe the further subdivision of the Mesencephalon
Remains as the boundary between the forebrain and hindbrain
Describe the further subdivision of the Rhombencephalon and state the structures these develop into
Metencephalon: Forms the Pons and Cerebellum
Myelencephalon: Forms the Medulla
Where is the Midbrain/ Cephalic Flexure?
Just above the Midbrain
Where is the Cervical Flexure?
Between Brainstem/ Hindbrain and Spinal Cord
Describe the general organisation of Motor and Sensory structures in the nervous system
What causes this pattern of development?
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
In the spinal cord give 3 examples of the Dorsal-Ventral organisation of Sensory and Motor neurones
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
In the Medulla give an example of the Dorsal-Ventral organisation of motor and sensory structures
- Lemnisci are sensory and sit posteriorly
- Medullary pyramids are motor and sit anteriorly
In the Midbrain give an example of the Dorsal-Ventral organisation of motor and sensory structures
- Coliculi are sensory sit posteriorly
- Cerebral peduncles are motor and sit anteriorly
In the Cerebral cortex give an example of the Dorsal-Ventral organisation of motor and sensory structures
- Primary motor cortex is anterior the Central Sulcus
- Primary sensory cortex is posterior to the Central Sulcus
Describe the development of the Cauda Equina
- 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
Why are lumbar punctures safer if done below L1?
Below here, there is no spinal cord, but only cauda equina
How can Neural Tube defects lead to Hydrocephalus (Fluid buildup in the cavities/ ventricles of the brain)
- 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
What are 3 commonly used terms to describe Neural tube defects
- Spina Bifida
- Spinal Dysraphism
- Neural Tube Defects/ Disorders
What do all disorders involving failed neural fold fusion have in common?
Failure of development of the posterior vertebral arches at 1/ more levels
Why may a neural tube defect lead to disordered formation of the posterior vertebral arch?
Neural tube has a role in inducing the migration of the Sclerotome from somites to form the posterior vertebral arch
List 6 Neural Tube defect disorders from most to least severe
- Craniorachischisis
- Anencephaly
- Myelocoele/ Rachischisis
The following have a normal neural tube, but failed development of the posterior vertebral arch
- Myelomeningocoele
- Meningocoele
- Spina Bifida Occulta
Describe Craniorachischisis
Cranio- head
Rachi- spine
Schisis- split
- Entire neural tube remains open
- Failure of brain AND spinal cord to form
- Incompatible with life
Describe Anencephaly
- Cranial neural tube fails to close
- Failure of brain to form
- Children may be born alive but don’t live long
Describe Myelocoele/ Rachischisis
- Spinal cord fails to develop
- Usually associated with a CSF filled cyst
Children frequently;
- Have neurological defects
- Are susceptible to meningitis
What is the Neural Crest?
What happens to the them once the neural folds fuse and the surface ectoderm is reconstituted?
- 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
List the cells derived from the Neural Crest
- 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)
List 4 tissues that receive significant contribution from the Neural Crest
- Thymus
- Thyroid
- Parts of teeth
- Parts of heart (Spiral Septum which separates Truncus Arteriosus into Aorta and Pulmonary Trunk)
Since Neural Crest has many contributions, disorders of these cells have a large variety of manifestations
List 2 Neural Crest disorders
- DiGeorge Syndrome
- Hirschsprung’s Disease
List 4 ways DiGeorge Syndrome presents
- Heart anomalies
- Facial anomalies
- Immunodeficiency
- Hypocalcaemia
How does Hirschsprung’s Disease present?
- Lack of enteric neurons in part of Large Intestine
- Leads to constipation and low motility
As neural crest cells migrate Anteriolaterally from the dorsal embryo, they leave behind defined populations at certain points.
List these from Dorsal to Ventral
- Dorsal root ganglia
- Sympathetic ganglia
- Preaortic/ Prevertebral ganglia
- Adrenal medulla (Chromaffin cells)
- Gut wall (ENS)
Describe Myelomeningocoele/ Meningomyelocoele
- CSF filled cyst containing the spinal cord
- Transilluminates poorly (solid tissue IN cyst)
- Children may have neurological deficits
- Repair necessary
Describe Meningocoele
- 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
Describe Spina Bifida Occulta
- 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
How can neural tube defects be prevented?
- 400 micrograms of Folic Acid daily
- From 3 months before conception
- Up to Week 12 of pregnancy
Folate deficiency also predisposes to Palatal abnormalities.
What does this suggest about its role?
May be important for the fusion of epithelial sheets
