Flashcards in Principles of Neuroscience Lecture 5, Neuronal damage Deck (23)
In what ways are neurons fragile?
They demand a constant O2 supply so that they can perform cellular respiration
Require constant glucose for same reason
Even short periods without these two things can lead to permanent damage
What part of a neuron is most likely to be damaged?
The axon ; it's so much longer than the cell body
Briefly, describe the differences between axonal regeneration in CNS and PNS
CNS cannot regenerated damaged axons
PNS, only limited success with axon regrowth (10% success)
What is the term for axonal damage
What does axotomy give rise to in terms of structures?
A proximal stump and distal segment
Describe the two possible situations of Wallerian Degeneration
I. The severed axon is phagocytosis and the whole cell dies (apoptosis)
II. If the neuron survives, axon sprouts form, trying to reconnect with the target.
Why can severed axons be painful?
Neuromas form : nerve endings form a clump in scar tissue
What is the best way to achieve reconnection of axons to their targets?
Sew them back together
Describe Henry Heads' experiment
He severed a nerve in his arm and documented the reemergence of sensation over a number of years
What are the layers of connective tissue around neurons?
Endoneurium : Myelin sheath around individual axon
What is the significance of endoneurium in axonal regrowth?
After the severed axon dies, the endoneurium tube remains. This can act as a guide for the sprouting axon; leading it back to its target.
Crush or cut?
Crush is better than cut because the endoneurium is still continuous, thus acts as a better guide for the axon sprouts
Why does the entire neuron die when axotomy occurs?
Signals are sent back up the axon to the cell body from the point of damage (retrograde) that trigger apoptosis. This is so that necrosis does not occur, affecting the neighbouring cells
Describe axonal regrowth in CNS
In CNS, there is no axonal regrowth. This is due to three inhibitory factors.
Why is there no axonal regrowth in the CNS?
Why would this have been selected for in evolution?
There are three inhibitory factors :
I. Myelin is inhibitory
II. Glial scarring
III. Lack of trophic factors
It is thought that it is better for there to be no regrowth in the CNS so that no scrambling of circuitry occurs.
The brain spends 20 years developing, after this point, we 'clamp down' and inhibit change.
Which tissues are capable of making new neurons?
Neural stem cells are present in the subventricular zone of the brain.
Which parts of the body receive new neurons?
Olfactory lobe, and nasal epithelium
What are exogenous stem cells?
Derived from somewhere else.
For example : foetal brains, iPS ( non neural stem cells), embryonic stem cells
Describe the effect of glial scarring
1. glial cells invade area where the axons were and proliferate, taking up the space
2. Glial cells have proteoglycans (GAGs) that are inhibitory
Describe how myelin affects regrowth of axons in the CNS
Myelin inhibits axon regrowth
Proteins associated with myelin that prevent axonal regrowth:
MAG1, OMgp, Nogo A
All these have the same receptor
He does a lack of trophic factors affects axonal regrowth in the CNS?
1. In the embryo, growth of axons in the CNS is possible through trophic factors that guide the axons.
In the adult CNS, these aren't present.
2. In the adults, the axons also must traverse a greater distance and the environment is more complex
What are granular cells?
These are the new neurons derived from stem cells in the hippocampus
The stem cells come from the ventricular zone