1. The development of the nervous system Flashcards
what is involved in gastrulation?
outer layer of the bilayer germ disc invaginates at the primitve streak to produce a trilaminar disc
what are the three layers of the trilaminar disc?
ectoderm - skin, neural tissue, eg: neural crest
mesoderm - heart, vessels and muscles
endoderm - gut including accessory organs
what is the notochord?
composed of endoderm, it inducts the overlying ectoderm to invaginate and form the neuroectoderm - the neural tube
what does neurulation form?
the neural tube
how is the neural tube formed?
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
if the neural tube fails to zip up in the rostral direction?
anencephaly
if the neural tube fails to zip up in the caudal direction?
spins bifida
what are formed on the rostral part of the neural tube?
primary brain vesicles - will become major parts of the adult brain
what becomes the forebrain?
prosencephalon
what becomes the midbrain?
mesencephalos
what becomes the hindbrain?
rhombencephalon
what is the prosencephalon made up of?
telencephalon = cerebral hemsipheres diencephalon = thalamus, hypothalamus, optic nerve/retina
what is the mesencephalon made up of?
only mesencephalon - acts as boundary between forebrain and hindbrain
what is the rhombencephalon made of?
metencephalon = pons and cerebellum myelencephalon = medulla
what is the fundamental relationship on the orientation of sensory and motor systems?
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)
how does the cauda equina develop?
the spine grows faster than the spinal cord therefore the lower portions of the cord are stretched and draw out the cauda equina
what condition does cauda equina predispose for and why is this?
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
what is spina bifida?
when the dorsal portions of the vertebrae fail to use resulting in two bony ridges being palpable either side of the midline
what is spinal dysraphism?
dys = disordered, raph = formation of a raphe (where tissues zip up in the midline)
what is craniorachischisis?
the entire neural tube remains open so failure of both the brain and spinal cord to form = incompatible
what is myelocoele?
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
what is myelomeningocoele?
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
what is meningocoele?
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
what is spina bifida occulta?
the lack of posterior vertebral arch = tuft of hair or large naevus over defect
no neurological problems
how can neurological deficits be prevented?
take 400 micrograms daily before from around 3 months before conception until 12th week of pregnancy
what cells are derived from the neural crest?
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
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?
dorsal root ganglia sympathetic ganglia preaortic ganglia adrenal medulla but wall
what are two disorders that result from neural crest cells?
Di George syndrome
Hirschsprung’s disease
