1.2 Development Of The Nervous System

Question 1

When does gastrulation occur?

Answer 1

17 - 19 days after fertilisation

Question 2

What is gastrulation?

Answer 2

Gastrulation is the process whereby the outer layer of the bilayer germ disc (epiblast) invaginates at the primitive streak to produce a three layered germ disc

Question 3

What are the 3 germ disc layers?

Answer 3

Ectoderm
Mesoderm
Endoderm

Question 4

What is the function of the notochord?

Answer 4

To induce a change in the ectoderm to form the neural tube and the CNS
Forms the nucleus propulsus

Question 5

What does the ectoderm go on to form?

Answer 5

Becomes skin and neural tissue as well as neural crest

Question 6

What does the mesoderm go on to form?

Answer 6

Heart, vessels, muscles etc.

Question 7

What does the endoderm go on to form?

Answer 7

Gut including accessory organs. The notochord is a crucial structure composed of endoderm. It inducts the overlying ectoderm to invaginate and form the neural tube

Question 8

What is neurulation?

Answer 8

Process of formation of the neural tube

Question 9

Describe the process of neurulation

Answer 9

Induced by the notochord Elevation of the neural folds
Fusion of the folds in the midline at mid cervical level with reconstitution of the surface ectoderm
When the folds fuse, neural crest cells detach and migrate their ultimate destinations
Neural tube zips up rostrally and caudally

Question 10

What are the primary brain vesicles?

Answer 10

Prosencephalon
Mesencephalon
Rhombencephalon

Question 11

What are the first developments of the brain?

Answer 11

3 swellings develop called the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).

Question 12

Where is the cephalic flexure?

Answer 12

Between the forebrain and the midbrain

Question 13

Where is the cervical flexure?

Answer 13

Between the developing hindbrain and the spinal cord

Question 14

The prosencephalon subdivides into which structures?

Answer 14

Telencephalon

Diencephalon

Question 15

What does the telencephalon become?

Answer 15

becomes most of cerebral hemisphere

Question 16

What does the diencephalon become?

Answer 16

becomes thalamus, hypothalamus and optic nerve/retina

Question 17

What does rhombencephalon subdivide into?

Answer 17

Metencephalon

Myelencephalon

Question 18

What does the metencephalon become?

Answer 18

Forms the pons and cerebellum.

Question 19

What does the myelencephalon go on to become?

Answer 19

Forms the medulla (the prefix myelo- refers to spinal cord, which the medulla joins on to)

Question 20

What is the function of the notochord?

Answer 20

To induct the ventral (anterior) portion of the neural tube to become the basal plate

Question 21

What is the basal plate?

Answer 21

A portion of the neural tube ventral to the sulcus limitans. Gives rise to the motor neurones. Development of basal plate induced from the ventral half of the neural tube by chemical factors released by the notochord

Question 22

What is the alar plate?

Answer 22

The alar plate, or the roof plate is the dorsal half of the neural tube that forms in the absence of influences from the notochord.
Gives rise to the inter and sensory neurones

Question 23

Describe how the pattern of distribution of sensory and motor systems is seen in the midbrain

Answer 23

The colliculi (sensory) sit posterior to the
cerebral peduncles (motor)

Question 24

Describe the distribution of the motor and sensory pathways in the medulla

Answer 24

The lemnisci (sensory) sit posterior to the pyramids of the medulla (motor)

Question 25

What is the cauda equina?

Answer 25

A collection of spinal nerve roots descending from the lower spinal cord. Contains the spinal nerve roots from L2 to Co1

Question 26

Describe the development of the cauda equina

Answer 26

1. Initially, there is a one to one correspondence between the cord levels and the vertebrae levels 2. However, the spine grows faster than the spinal cord, particularly at the lumbar levels (lumbar vertebrae are larger than thoracic and cervical ones) 3. Therefore, the lower portions of the cord are stretched, drawing out the cauda equina

Question 27

Suggest why neural tube defects predispose to hydrocephalus

Answer 27

This may be caused by tethering of the cord at the site of the defect Thus, as the spine grows the cord cannot move within the vertebral canal, resulting in the brainstem (containing the fourth ventricle) being pulled down through foramen magnum and becoming occluded There are other mechanisms at play as well

Question 28

What is spina bifida?

Answer 28

A neural tube disorder where the dorsal portion of the vertebrae fail to form

Question 29

What is spinal dysraphism?

Answer 29

Failure of the neural tube to fuse in the midline. General term for neural tube disorder.

Question 30

Why do all neural tube disorders result in the failure of the development of the posterior vertebral arches?

Answer 30

appears that the neural tube is at least partly responsible for inducing the migration of the sclerotome from somites to form the posterior bony arch. Hence an anomaly in the neural tube may lead to disordered formation of the posterior arch.

Question 31

List the neural tube defects going form most sever to least

Answer 31

``` Craniorachischisis Anencephaly Myelocoele Myelomeningocoele Meningocoele Spina bifida occulta ```

Question 32

What is craniorschischisis

Answer 32

Entire neural tube remains open Failure of both the spinal cord and the brain to form Incompatible with life

Question 33

What is anencephaly?

Answer 33

Cranial neural tube fails to close Failure of the brain to form Short term life expectancy

Question 34

What is a myelocoele?

Answer 34

Also known as rachischisis, this occurs when the spinal cord fails to develop. There is no posterior bony arch of the vertebrae and a CSF filled cyst with the open spinal cord is exposed to the outside world. Frequent neurological deficits and susceptibility to meningitis due to the presence of exposed neural tissue

Question 35

What is a myelomeningocoele?

Answer 35

A CFS filled cyst containing the spinal cord Transilluminates relatively poorly Children may have neurological deficits but less than associated with a myelocoele Repair is necessary

Question 36

What is a meningocoele?

Answer 36

Presence of a CSF filled cyst. The cord is sited within the vertebral canal. Transilluminates brilliantly. These children tend to have a good neurological prognosis. However, the cyst will need repair as it does predispose to infection

Question 37

What is spina bifida occulta?

Answer 37

The only anomaly is the lack of the posterior vertebral arch. May manifest a sign such as a tuft of hair or a large naevus over the defect. Not associated with significant neurological problems. Occurs in about 10% of the population

Question 38

What is used in the prevention of neural tube defects

Answer 38

Folic acid is prescribed to all pregnant women - also reduces risk of palatal anomalies 400 micrograms daily, to be taken before from around 3 months before conception and until week 12 of pregnancy

Question 39

What are the neural crest cells?

Answer 39

Part of the ectoderm in a vertebrae of an embryo that lie on either side of the neural tube. Multifunctional

Question 40

Where do the neural crest cells derive from?

Answer 40

The point at which the neural folds fuse when the surface ectoderm is reconstituted

Question 41

What cells are completely derived from the neural crest cells?

Answer 41

All neurones whose cell bodies are in the peripheral nervous system • Primary sensory neurones • Autonomic postganglionic neurones • Enteric neurones Schwann cells Cells of the adrenal medulla (these are after all specialised sympathetic postganglionic neurones) Melanocytes The leptomeninges (arachnoid and pia) Head mesenchyme, which contributes to many tissues such as pharyngeal arches

Question 42

What tissues receive a significant contribution from the neural crest?

Answer 42

Thymus Thyroid Parts of the heart Parts of the teeth

Question 43

During their migration from the dorsal wall, where are some of the neural crest cells left?

Answer 43

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 gut wall (enteric nervous system)

Question 44

What is Di George Syndrome?

Answer 44

A disorder of the neural crest cells due to a small deletion in chromosome 22. Resulting in • Immunodeficiency (due to involvement of the thymus) • Facial anomalies (due to contribution of neural crest to facial development) • Heart anomalies • Hypocalcaemia (involvement of parathyroids More frequent cleft lip/palate

Question 45

What is Hirschprung’s disease?

Answer 45

Lack of enteric neurones in sections of the large intestine leading to hypomotility and constipation