Lecture 2- Development of nervous system 2/2

Question 1

How do neural tube defects occur?

Answer 1

• Neural tube first forms in the cervical region
• Before the neural tube has ‘zipped up’ there are caudal and cranial pores at each pole
• If the cranial neural pore fails to close we can have neural tube disorders affecting the head
  
  • Anencephaly
• If the caudal pore fails to close we can have neural tube disorders affecting the spinal cord
  
  • Spina bifida

Question 2

formation of vertebra which surrounds the neural tube

Answer 2

• Formed by the dorsal, central, ventral and lateral sclerotome
• Failure of development of dorsal sclerotome can lead to open vertebral canal
  
  • Herniation of contents out of the vertebral canal

Question 3

Neural tube disorders (Spina bifida) fall along a spectrum of severity.

Answer 3

• The most severe are incompatible with life (cranial end).
• Mildest - may not even know there is defect

Question 4

most severe types of spina bifida (NTD)

Answer 4

• craniorachischisis
• anecephaly
• myelocoele (rachisisis)

Question 5

least severe types of spina bifida (NTD)

Answer 5

• Meningomyelocele
• Meningocele
• Spina bifida occulta (mildest form)

Question 6

• Craniorachischisis

Answer 6

• Failure of neural tube to close along entirety of length (not even at the cervical level)
• Incompatible with life
• Foetus usually stillborn
• Very rare
• Exposed tissue should have been the lumen of the neural tubes

Question 7

anecephaly

Answer 7

• Brain fails to develop
• Due to failure of closure of the cranioneural pore
• Therefore brain tissue doesn’t form normal
• Incompatible with life

Question 8

• Myelocoele (rachischisis)

Answer 8

• Neural folds fail to fuse together
• Neural tube fails to invaginate
• Leaving an open area, in contact with the outside world which should have become centre of the spinal cord
• Severe neurological defects involving the lower limb
• vulnerable to meningitis

Question 9

• Meningomyelocele

Answer 9

• Cystic lesion protruding through defect caused by failure of closure of the neural arch
  
  • Due to dorsal arches of the vertebrae have not formed- spinal cord has herniated along with meninges
• Cyst filled with CSF and spinal cord
• Neurological deficits not as severe as myelocoele
• Susceptible to meningitis

Question 10

Meningocele

Answer 10

• Spinal cord found in normal location, but CSF filled cyst protruding through defect in dorsal arch
• Neurologically okay
• susceptible to meningitis

Question 11

Spina bifida occulta (mildest form)

Answer 11

• No visible cyst
• May see tuft of hair or naevus sat at thew level of lumbar spine
• Cause: failure of closure of dorsal arches, caused by a problem of dorsal sclerotome
  
  • Missing dorsal arches of the some of the lumbar vertebra
• No neurological deficits (often undiagnosed)

Question 12

what is the cauda equina

Answer 12

Cauda equina is a bundle of dorsal and ventral roots, filling much of vertebral canal at the lumbar and sacral layer

• Not made up of spinal nerves or spinal cord tracts

Question 13

why does the cauda equina form

Answer 13

Spinal cords grows much slower than the vertebral column

• As the vertebral body enlarges and the cords stay the same length, the roots which emerge from the spinal cord become stretched
  
  • Still tethered at the point of the dorsal root ganglion in the intervertebral foramina
  • Forming cauda equina at LI
  • Levels below L1 do not have spinal cord- they have cauda equina
    
    • Safer to do lumbar puncture below L1

Question 14

Hydrocephalus and spina bifida

Answer 14

• In spina bifida, the developing spinal cord can become adherent to the vertebral column
• When process of growth in the vertebral column starts, the spinal cord can be pulled down, if you pull the spinal cord down this can affect structures in the brain such as the 4^th ventricle
  
  • As the 4^th ventricle gets pulled down it can be compressed by the foramen magnum
  • This can impair drainage of CSF out of 4th ventricle and into the subarachnoid space
  • This causes increased pressure in the ventricular system = hydrocephalus- dilation of ventricular system

Question 15

Fundamental relationship between motor and sensory systems

Answer 15

Sensory info comes into the CNS through sensory neurones (afferent) processed (interneurons)  motor output (efferent)  effectors

Question 16

Formation of motor and sensory neurones

Answer 16

• Neural tube can be separated in half (asymmetry induced by the notochord- releases chemical factors which induce the cells of the ventral half to become basal cells and dorsal half to become alar cells)
• Dorsal (posterior) half of the neural tube= alar (roof plate)
  
  • Gives rise to sensory neurones and interneurons
• Ventral (anterior) half= basal (floor plate)
  
  • Gives rise to motor neurones
• Neurones of the CNS are derived from these 2 regions

Question 17

• Dorsal (posterior) half of the neural tube=

Answer 17

• alar (roof plate)
  
  • Gives rise to sensory neurones and interneurons

Question 18

• Ventral (anterior) half=

Answer 18

• basal (floor plate)
  
  • Gives rise to motor neurones

Question 19

Dorsal side of neural tube= Sensory and interneurons

Answer 19

• Spinal cord: Think dorsal horn - sensory in function connects to the sensory root
• Midbrain: Colliculus (superior or inferior) contain regions of grey matter involved in receiving visual or auditory stimuli (sensory)
• Medulla: dorsal column nuclei receiving axons from the gracilis and cuneatus fasciculus (sensory)

Question 21

Ventral side = motor neurones

Answer 21

• Spinal cord: Think ventral horn - motor in function connects to motor root
• Midbrain: cerebral peduncles containing the cortical spinal tracts (motor)
• Medulla: medullary pyramids – part of the cortical spinal tract (motor)

Question 22

Does this pattern (D= sensory, V= motor) hold for the cerebral hemispheres?

Answer 22

• Have to think about this in relation to the central sulcus
  
  • Motor structures sit in front of central sulcus (frontal lobe)- anterior
  • Sensory structures sit behind the CS – posterior

Question 23

Neural crest cells

Answer 23

• Derived from ectoderm which invaginates to form the neural tube
• Population of cells present at the point where the ectoderm heals back over after the neural tube is formed separates themselves and then migrate to many regions of the body (antero (GI) laterally (skin-melanocytes)
• As cells migrate certain populations stop in specific regions e.g. adrenal medulla cells
• Then the rest carry on anteriorly e.g. to contribute to the enteric motor system

Question 24

Derivatives of neural crest cells

Answer 24

• Sensory neurons in PNS
• Postsynaptic autonomic neurones
• Enteric neurones
• Schwann cell
• Adrenal medulla cells
• Head mesenchyme
• Melanocytes in the skin
• Arachnoid and pia mater of meninges

Contributes to

• Thymus
• Thyroid
• Parrs of the heart e.g. spiral septum
• Teeth

Question 25

important disorders related to neural crest cells

Answer 25

Di George syndrome

Hirshsprungs disease

Question 26

• Di George syndrome

Answer 26

Problems affecting the face and thymus (immunodeficiency)

• no T cell education

Question 27

• Hirschsprung’s disease

Answer 27

• Problem with neural crest cell migration to the gut wall
• Certain segments of the gut ( esp large intestine) will have no enteric nervous system (leading to constipation)

Question 28

Orientation terminology

• The embryo

Answer 28

is in the prone (face-down) position, whereby the terms ventral and dorsal correspond to the adult anterior and posterior , and rostral and caudal correspond to superior and inferior .

Question 29

Orientation terminology

• In adults

Answer 29

neuroaxis flexes at the cephalic flexure, however our orientation is based around that of other animals whos brains do not flex at the midbrain

• Rostral = anterior
• Caudal = posterior
• Dorsal= superior
• Ventral= inferior