lecture 5

Question 1

In what way are neurons fragile cells?

Answer 1

• energy demand high
• obligate aerobic metabolism (O2 critical)
• totally dependent on glucose supply (via blood)
• Brain (2% of body) gets 15% of blood
• loss of O2 for a few minutes, glucose for 10-15 min, is fatal to neurons

Question 2

What is the most vulnerable part of the neuron?

Answer 2

the axon

Question 3

What do nervous system injuries often involve?

Answer 3

• axons: trauma, demyelination
• axons are the largest and most vulnerable part of a neuron
• 20µ diameter cell and 30cm axon more like a cortical axon that will extend from the top of the head down into the neck

Question 4

What is the response to damage of the nervous system?

Answer 4

• different outcome in peripheral or central nervous system

Question 5

What is axotomy?

Answer 5

• cutting an axon

Question 6

What happens when we cut an axon?

Answer 6

• gives a distal segment and a proximal stump

- result of cutting an axon in periphery is Wallerian degeneration (loss of peripheral (distal) part)

Question 7

What occurs in Wallerian degeneration I?

Answer 7

• severed axon degenerates and is phagocytosed (4 days)
• chromatolysis of cell body (swelling, loss of organelles
• neuron can die or survive

Question 8

What occurs during Wallerian degeneration II?

Answer 8

• if it survives, the axon sprouts (1-3 days)

- sprouts can reconnect to target (if axon is in the peripheral nervous system)

Question 9

What occurs during Wallerian degeneration III?

Answer 9

• bad trauma leads to scarring, sprouting axon may not find its way back
• painful neuroma results - sensory endings trapped in the scar tissue that generates chronic pain

Question 10

How does axon find its way back and reconnect to target cell?

Answer 10

• axon reconnects poorly across break
• best if cut nerve (nerve being whole structure/bundle of neurons and connective tissue) stitched back together
• sprout extends down surviving endoneurium and perineurium to target

Question 11

Who is Henry Heads?

Answer 11

• surgeon interested in recovery from injury
• cut own nerve in arm
• recovery over 2 years mostly successful
• ’ the art of self-experimentation’
• late 1800s

Question 12

What are the endoneurium and perineurium?

Answer 12

• outer connective tissue sheath (epineurium)
• bundles of axons wrapped in connective tissue (perineurium)
• individual axons wrapped in Schwann cells and basal lamina (endoneurium)
• hopefully when you cut a nerve you are left with the endoneurium and other connective tissue that provides a ‘runway’ or ‘track’ for a newly growing/regenerating axon to follow

Question 13

What is the role of distal nerve in neuron healing?

Answer 13

• acts as an axon guide
• sprouting axons can grow along empty tubes formed by epi- and perineurium
• leads them to target
• crush better than cut - tubes intact all the way

Question 14

What is nerve repair?

Answer 14

• sewing nerves together can misalign distal and proximal tubes
• sometimes a piece of nerve is destroyed
• need a bridge to guide sprouts to empty endoneural tubes
• • can be nerve transplant
• • can be artificial

Question 15

What are important things to consider in regards to peripheral axon regeneration?

Answer 15

• only a minority make it back to target (10% in case of cut nerve)
• functional recovery is never perfect
• bad injuries rarely recover
• but they try

Question 16

Why do cell bodies sometimes die?

Answer 16

• after losing an axon, neurons die by apoptosis
• apoptosis - programmed cell death
• internal biochemical cascade
• doesn’t damage surrounding cells (cf. necrosis)

Question 17

What is the trigger for apoptosis?

Answer 17

• signal from target cell suppresses apoptosis - no signal, apoptosis occurs
• signal carried retrogradely up the axon
• cutting axon interrupts signal
• outcome depends on neuronal size and age

Question 18

How does central regeneration differ from peripheral?

Answer 18

• cut sensory, motor and autonomic axons in the periphery can often regrow
• Axons in the CNS never regrow

Question 19

What do rat spinal grafts show us?

Answer 19

1. normal spinal projections.
2. create spinal lesion in adult rat - no recovery
3. graft sciatic nerve bridge across lesion, axons regrow down graft, stop at spinal cord

• this shows that it is the central nervous system environment that prevents regrowth of nerves

Question 20

What are the inhibitors of CNS axon regrowth based on current understanding?

Answer 20

Three things

• glial scar
• lack of attractive cues/trophic factors
• central myelin is inhibitory

Question 21

What is glial scarring?

Answer 21

• glial cells retain ability to divide
• will increase division at site of injury
• tend to fill damaged area (glial scar)
• non-neuronal cells invade (microglia, macrophages, fibroblasts)
• sprouts don’t like growing on glial scar

Question 22

What are some of the known inhibitory components of scar?

Answer 22

• chondroitin sulfate proteoglycans (GAGs)
• remove GAGs with enzymes - glial scar no longer inhibitory
• GAGs bind signalling molecules (semaphorin 3A?)

Question 23

Why do we have a lack of attractive/trophic factors in the CNS?

Answer 23

• in embyro, many mechanisms guided growing axon
• in adult, distances much greater, environment more complex and guidance mechanisms may be lacking
  (perhaps a weaker argument for why we don’t get regeneration in CNS)

Question 24

What is the evidence for myelin being inhibitory?

Answer 24

• central axons can regrow until myelin forms in embryo
• oligodendrocytes (myelinating glial cells of CNS) can prevent axon regrowth in vitro
• destroying myelin in rat allows functional regrowth of spinal cord axons

Question 25

What about myelin is inhibitory?

Answer 25

1. Myelin associated glycoprotein (MAG1) 2. oligodendrocyte myelin glycoprotein (OMgp) 3. Nogo A - all work through the same receptor: Nogo receptor

Question 26

Why is myelin inhibitory in the first place?

Answer 26

- CNS is complicated and circuitry is crucial - uncontrolled axonal growth likely to scramble circuits - develop brain and then clamp down on change

Question 27

What are some diseases that cause death of the whole neuron?

Answer 27

- Alzheimer's - Parkinson's - Huntington's - Motor neuron disease

Question 28

Why is it hard for the brain to make new neurons?

Answer 28

- neurons are terminally differentiated cells (can't divide) - some tissues have small numbers of undifferentiated cells (stem cells) that keep dividing to generate new tissue cells e.g. skin - brain may contain neural stem cells

Question 29

What was a non-human species that provided some evidence that we have stem cells in the brain? How?

Answer 29

- song birds learn new songs each year - rebuild "song centre" in brain annually - new neurons from stem cells - migrate long distances, integrate into new neural circuits

Question 30

Do mammals have neural stem cells?

Answer 30

- yes - neural stem cells exist in subventricular zone - supply new neurons to olfactory bulb (in rats)

Question 31

How do stem cells function in the olfactory system?

Answer 31

- in rats: not something that happens in humans - stem cells in ventricular zone generate new neurons that migrate to olfactory bulb - travel via RMS (rostral migratory stream) - become new interneurons in olfactory bulb - new olfactory neurons generated in olfactory epithelium (nasal cavity)

Question 32

What role to stem cells have in the hippocampus?

Answer 32

- hippocampus involved in memory - also site of generation of new neurons from ventricular zone - new neurons are granular cells - seem to be involved in the making of new memories in rats/mice etc - sure if vital for laying down memories but somehow they contribute towards it

Question 33

What is the role of new neurons in the brain?

Answer 33

- new neurons detected in brain of experimental animal after damage - existing neural stem cells seem insufficient to repair damage - new cells hang around for weeks but not integrated into circuits new hippocampal cell in humans - hippocampus crucial for human learning - learning occurs throughout life - very difficult to prove - no experimental data - use natural experiment

Question 34

How did scientists use nuclear bombs to study stem cells?

Answer 34

- above ground nuclear bomb testing common in 50s and 60s - filled atmosphere with 14C - if no generation of neurons then 14C should reflect time of birth (low in 30s and 40s, high in 50s and 60s) - however it was seen in the brains of people who were born before the increased levels of C-14 that they had higher amounts of C-14 then was in the atmosphere at the time they were born seemingly proving that they made new cells well into their life

Question 35

What are some features of new neurons in old hippocampi?

Answer 35

- evidence that dentate neurons turn-over in human hippocampus - continues with modest decline into old age - generation of new neurons may have functional significance (unsure yet)

Question 36

What are some questions regarding the use of exogenous stem cells?

Answer 36

- can additional stem cells be added to damaged brain? - source? (fetal brains, embryonic stem cells, non-neural stem cells) - how many do you need? - - need to grow them in cell culture to increase numbers?

Question 37

What are some problems with using exogenous stem cells?

Answer 37

- tumorigenesis (teratomas: a tumour which has aspects of every single type of cell in the body) - allodynia (pain due to sprouting of sensory endings) - unwanted phenotypes - but these complications rarely reported (why?) - rejection (lifelong immune suppression)

Question 38

Give an example of a problem that occurred with exogenous stem cell therapy

Answer 38

- Donor-derived brain tumour following neural stem cell transplantation in an ataxia telangiectasia patient - ataxia telangiectasia (AT) treated with intracerebellar and intrathecal injection of human fetal neural stem cells - four years later was diagnosed with a multifocal brain tumour - tumour was of non-host origin - from the transplanted neural stem cells - first report of a human brain tumour complicating neural stem cell therapy - generally though stem cells seem to be pretty safe

Question 39

What is medical tourism?

Answer 39

- popular for stem cell therapy - often a last resort - gives access to experimental treatments prior to scientific and clinical validation