1. The development of the nervous system

Question 1

what is involved in gastrulation?

Answer 1

outer layer of the bilayer germ disc invaginates at the primitve streak to produce a trilaminar disc

Question 2

what are the three layers of the trilaminar disc?

Answer 2

ectoderm - skin, neural tissue, eg: neural crest
mesoderm - heart, vessels and muscles
endoderm - gut including accessory organs

Question 3

what is the notochord?

Answer 3

composed of endoderm, it inducts the overlying ectoderm to invaginate and form the neuroectoderm - the neural tube

Question 4

what does neurulation form?

Answer 4

the neural tube

Question 5

how is the neural tube formed?

Answer 5

induced by the notochord, the neural folds elevate and the folds fuse in the midline at the mid cervical level. when the folds fuse, the neural crest cells detach and migrate. the neural tube zips up rostrally and caudally

Question 6

if the neural tube fails to zip up in the rostral direction?

Answer 6

anencephaly

Question 7

if the neural tube fails to zip up in the caudal direction?

Answer 7

spins bifida

Question 8

what are formed on the rostral part of the neural tube?

Answer 8

primary brain vesicles - will become major parts of the adult brain

Question 9

what becomes the forebrain?

Answer 9

prosencephalon

Question 10

what becomes the midbrain?

Answer 10

mesencephalos

Question 11

what becomes the hindbrain?

Answer 11

rhombencephalon

Question 12

what is the prosencephalon made up of?

Answer 12

telencephalon = cerebral hemsipheres
diencephalon = thalamus, hypothalamus, optic nerve/retina

Question 13

what is the mesencephalon made up of?

Answer 13

only mesencephalon - acts as boundary between forebrain and hindbrain

Question 14

what is the rhombencephalon made of?

Answer 14

metencephalon = pons and cerebellum
myelencephalon = medulla

Question 15

what is the fundamental relationship on the orientation of sensory and motor systems?

Answer 15

motor structures tend to sit anteriorly and sensory structures tend to sit posteriorly
the notochord inducts the ventral portion of neural floor to become basal plate and the basal plate gives rise to motor neurones 
the alar plate forms in the absence of influences from notochord which gives rise to inter and sensory neurones 
dorsal roots are sensory and ventral roots are motor
dorsal horn contains sensory neurones whilst the ventral horn contain motor neurones
dorsal column (sensory) sit posteriorly whilst CST (motor) sits anteriorly
the lemnisci (sensory) sit posterior to the pyramids of medulla (motor)
the colliculi (sensory) sit posterior to the cerebral puduncles (motor)
primary sensory cortex (sensory) sits posterior to primary motor cortex (motor)

Question 16

how does the cauda equina develop?

Answer 16

the spine grows faster than the spinal cord therefore the lower portions of the cord are stretched and draw out the cauda equina

Question 17

what condition does cauda equina predispose for and why is this?

Answer 17

hydrocephalus (build up of fluid in brain)
tethering of the cord to the site of the defect so as the spine grows the cord cannot move within the vertebral canal resulting in the brainstem being pulled down through foramen magnum and becoming occluded

Question 18

what is spina bifida?

Answer 18

when the dorsal portions of the vertebrae fail to use resulting in two bony ridges being palpable either side of the midline

Question 19

what is spinal dysraphism?

Answer 19

dys = disordered, raph = formation of a raphe (where tissues zip up in the midline)

Question 20

what is craniorachischisis?

Answer 20

the entire neural tube remains open so failure of both the brain and spinal cord to form = incompatible

Question 21

what is myelocoele?

Answer 21

the spinal cord fails to develop and is associated with a CSF filled cyst
children have neurological deficits and are susceptible to meningitis as the neural tissue is exposed

Question 22

what is myelomeningocoele?

Answer 22

a CSF fluid filled cyst containing the spinal cord due to failure of the development of the posterior vertebral arch
transilluminates poorly
may have neurological deficits

Question 23

what is meningocoele?

Answer 23

presence of CSF filled cyst due to failure of the development of the posterior vertebral arch
cord is within the vertebral canal
transilluminates brilliantly
good neurological prognosis
cyst will need repair as it predisposes to infection

Question 24

what is spina bifida occulta?

Answer 24

the lack of posterior vertebral arch = tuft of hair or large naevus over defect
no neurological problems

Question 25

how can neurological deficits be prevented?

Answer 25

take 400 micrograms daily before from around 3 months before conception until 12th week of pregnancy

Question 26

what cells are derived from the neural crest?

Answer 26

neurones whose cell bodies are in PNS - primary sensory neurones, autonomic postganglionic neurones, enteric neurones
schwann cells, cells of adrenal medulla, melanocytes, leptomeninges, head mesenchyme
tissues such as thymus, thyroid, parts of heart and teeth

Question 27

the neural crest cells migrate from dorsal part of the embryo, but some populations of cells are left behind at certain points, what does this form?

Answer 27

dorsal root ganglia
sympathetic ganglia
preaortic ganglia
adrenal medulla
but wall

Question 28

what are two disorders that result from neural crest cells?

Answer 28

Di George syndrome

Hirschsprung’s disease