Lecture 12: Oligodendrocytes and Myelin Flashcards
glia translates to
glue
glia intro
outnumber neurons in the human brain, involved in almost all neural functions, hold brain together and occupy the space between the neurons, usually quite small cells in comparison to neurons
brain metabolism, neuronal survival, modulate synaptic activity, communication
Glial cell types
oligodendrocytes astrocytes microglia ependymal cells Schwann cells
Neuron features
Excitable
Communication - important for integrating our learning and memory and the glia are supporting this function
10^11 neurons; 10^14 synapses
Post-mitotic (NB: stem cells) - the neurons we have now are the same ones that we have when we are 70-80 years old, some neurons become stem cells and neurogenesis occurs but the majority of the neurons are post mitotic
High metabolic rate - lots of energy required and the glia support this high energy intake that is required
Glia features
Not “excitable” cells
Support, nutrition, glue - communicate with each other and communicate with neurons
10x more glia (50% brain volume) - very significant
mitosis - can be replaces, this is a distinctive difference between neurons and glia
note - no chemical synapses, action potentials, neurotransmitters - release gliotransmitters instead
Oligodendrocytes
Greek: Oligos=few; Dendron= tree (have a tree like structure)
Myelination and metabolic support - these are the two main roles of oligodendrocytes
long thin process extending from this cell. Such processes connect the oligodendrocytes to their internodal lengths of myelin. The profiles of such processes can be recognized, and distinguished from the profiles of axons, by their electron dense cytoplasm and their closely packed microtubules.
Satellite oligodendrocytes
Oligodendrocytes in satellite locations are not infrequent and are sometimes regarded as a subset of the oligodendrocytic population
satellite oligodendrocytes are considered to be a part of the grey matter whereas the myelination oligodendrocytes are considered to be a part of the white matter, satellite oligodendrocytes are thought to support neuronal metabolism and they do not myelinated in the grey matter at all
Developmental process of myelination
Developmental process of myelination…
Foetus at 16 weeks have myelination starting, then 4 to 6 months the corpus callosum starts, 7 to 10 months then the interior of the cerebral hemispheres starts, 9 to 12 months subcortical myelination with the occipital lobes first then parietal and, then temporal lobes at 11 to 14 months and frontal lobes
Some evidence that some myelination occurs during adulthood
The deposition of myelin is …
a unique specialisation of glia in vertebrates
key driver of evolution
Why is myelination an evolutionary advantage ?
- Myelination strongly reduces energy consumption
action potentials and ion currents are restricted to less than 0.5% of the axon’s surface. - Rapid impulse propagation/ increased conduction velocity allowed complex yet compact higher nervous systems to evolve.
- Muscle control became the basis for the development of complex predatory and escape behaviour, which ultimately drove body size and vertebrate evolution
- Neurotrophic contribution
required for the long-term integrity and
survival of axon
Why is myelination an evolutionary advantage ? - myelination strongly reduces energy consumption
- Myelination strongly reduces energy consumption
action potentials and ion currents are restricted to less than 0.5% of the axon’s surface.
Why is myelination an evolutionary advantage ? - rapid impulse propagation/increased conduction velocity
- Rapid impulse propagation/ increased conduction velocity allowed complex yet compact higher nervous systems to evolve.
Why is myelination an evolutionary advantage ? - muscle control
- Muscle control became the basis for the development of complex predatory and escape behaviour, which ultimately drove body size and vertebrate evolution
quicker mind - muscle responses faster therefore can get away from predators faster etc
Why is myelination an evolutionary advantage ? - neurotrophic contribution
- Neurotrophic contribution
required for the long-term integrity and
survival of axon
An oligodendrocyte and the myelin sheath
responsible for myelination
creates a complex feel with the axon surface
Axons in the CNS vs PNS - myelinated axons
generally greater than 1 micrometer in diameter
CNS oligodendrocyte - myelinated multiple axons (can distribute its myelin amongst multiple cells) (many internodes/axons)
PNS Schwann cell - one segment of one axon only (one internode)
Axons in the CNS vs PNS - unmyelinated axons
CNS naked
PNS Schwann cell associate/surround (protect the unmyelinated axons, the cytoplasm of the Schwann cells surrounds the axons)
Paranode =
edges of the nodes of ranvier
Internode =
myelin
The initiation of of myelination by ….
Nrg-1 Neuregulin
Nrg-1 = Neuregulin-1 type III
axonal membrane protein
axon to Schwann signalling
This is expressed on the axon and when it is expressed there are receptors on the schwann cell or the oligodendrocytes that recognises the Nrg-1 type III and once recognised it actually initiates a signal cascade that then causes an increase in myelin gene expression so then that axon starts to be myelinated
mesaxon
mesaxon is a pair of parallel plasma membranes of a Schwann cell
inner mesaxon
The inner mesaxon (Terminologia histologica: Mesaxon internum) is the connection between the myelin sheath and the inner part of the cell membrane of the Schwann cell which is directly opposite the axolemma, i.e. the cell membrane of the nerve fibre ensheated by the Schwann cell.
outer mesaxon
the connection of the outer cell membrane to the compact myelin sheath.
Myelin wrapping
thickness of myelin is proportional to axon diameter
g ratio = 0.6 - 0.7
Bigger the diameter the bigger the myelin sheath and this thickness is important therefore if thinner can come to find problems
g ratio
0.6-0.7 =d/D d= axon radius D= axon + myelin radius The myelin g-ratio is the ratio of the inner to the outer radius of the myelin sheath
