Session 2 - The Role Of Neurones And Glia Flashcards
What is the purpose of glial cells?
To support, nourish and insulate neurones and remove ‘waste’.
What are the 3 main types of glial cells and what are their overall roles?
Astrocytes (several different types) - supporters
Oligodendrocytes - insulators
Microglia - immune response
Outline the roles of astrocytes.
Structural support
Help to provide nutrition for neurones (glucose-lactate shuttle)
Remove neurotransmitters (controls the concentration of neurotransmitters, which is especially important for glutamate which can be toxic)
Maintain ionic environment (K+ buffering)
Help form the blood brain barrier
Do neurones produce and store glycogen?
No, they rely on astrocytes to store glycogen.
Energy is provided to neurones through what two mechanisms?
Diffusion of glucose from the blood through the interstitial space to the neurones.
Glucose lactate shuttle - transfers lactate from astrocytes to neurones.
Explain the mechanism through which astrocytes can provide energy to neurones.
Astrocytes produce lactate from pyruvate. The lactate then exits the astrocytes through MCT1 channels, diffuses across the interstitial space, and is taken up by by neurones through MCT2 channels. Once inside the neurone, lactate is converted back to pyruvate.
How do astrocytes keep the concentration of neurotransmitters in the synapse low?
They allow re-uptake of neurotransmitters. For instance, astrocytes have transporters for glutamate. This helps keep the extracellular concentration of glutamate low, which is important because glutamate is toxic to neurones.
Why would high levels of neuronal activity lead to a rise in [K+] in brain extracellular fluid?
Lots of neuronal activity means lots of action potentials. During an action potential potassium moves out of the neurone, meaning the concentration of potassium outside the cell rises.
How is the rise in extracellular [K+] during high neuronal activity counteracted?
Astrocytes take up excess K+ to reduce the extracellular concentration.
Why is it important that astrocytes buffer K+ in the brain?
Increased extracellular potassium causes increased depolarisation (it decreases the threshold by increasing the resting membrane potential). This leads to inappropriate action potential firing in neurones which can cause things such as epilepsy.
What is the function of oligodendrocytes?
Oligodendrocytes are responsible for myelination of axons in the CNS. (In the PNS Schwann cells are responsible for myelination).
What is the function of microglia?
Immunocompetent cells. They recognise foreign material, become activated and then phagocytose. They can also present antigens to T cells. They are the brain’s main defence system.
What is the purpose of the blood brain barrier?
It limits diffusion of substance from the blood to the brain extracellular fluid. This maintains the correct environment for neurones.
What are the layers of the blood brain barrier?
Endothelial cells (tight junctions between endothelial cells)
Basement membrane surrounding the capillary
End feet of the astrocytes processes
What factors about the CNS mean that it is ‘immune privileged’?
Microglia can act as antigen presenting cells.
T-cells can enter the CNS.
CNS inhibits the initiation of the pro-inflammatory T cell response.
Does not undergo rapid rejection of allografts.
Why is it important that the CNS is able to inhibit the pro-inflammatory T-cell response?
The rigid skull will not tolerate volume expansion caused by the inflammatory response, would cause a rise in intracranial pressure.
What are the four main sections in a typical neuronal structure?
Cell soma (body)
Dendrites
Axon
Terminals
Outline the process of neurotransmitter release across a synapse.
Depolarisation in the terminal opens voltage-gated Ca2+ channels.
Influx of Ca2+ into the terminal.
Vesicles containing neurotransmitters fuse with the presynaptic membrane.
Neurotransmitter released into the synapse.
Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane.
What are the three chemical classes of neurotransmitter? Give examples of each.
Amino acids - e.g. glutamate, GABA, glycine
Biogenic amines - e.g. acetylcholine, noradrenaline, dopamine
Peptides - e.g. somatostatin, cholecystokinin, neuropeptide Y
What is the main excitatory amino acid neurotransmitter?
Glutamate (over 70% of all CNS synapses are glutamatergic)