NEUR30002 Lecture Notes - Astrocyte, Microglia, Oligodendrocyte
4 May 2018
School
Department
Course
Professor
Neurophysiology: Neurons and Circuits
Lecture Notes
Introduction (1)
What is Neurophysiology?
How the nervous system controls physiological functions
Glial Cells- Astrocytes (2)
Glial cells surround neurons and provide insulation. They are found in the peripheral and central nervous
system
• Astrocytes can play a key role in signalling.
• Glia considered as the glue of the brain. Made of many different cell types like astrocytes,
oligodendrocytes, microglia (macrophage lineage cell, respond to damage).
• Blood brain barrier formed by the interaction between astrocytes and blood vessels- transporters
determine what gets through.
• Number of Astrocytes per neuron increases as brain complexity increases→ indicates that
astrocytes do not just have a supportive function
• Astrocytes occupy spaces where synapses are absent. Surrounds neurons
• Astrocytes have territories that do not overlap much and will control their own territory.
Astrocyte Function
1. Control extracellular K+ homeostasis.
Neuronal activity causes an increase in potassium ion concentration. Astrocytes remove excess K+ by
two different mechanisms: Spatial buffering and increasing pump activity (Na+/K+ ATPase)
2. Maintaining the blood brain barrier
• Siphon processes of astrocytes will attach to blood vessels, this helps form the BBB
• Plays a role in blood flow because changes in astrocyte function result in changes of glucose
uptake→ basis for reactive hyperumia.
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• As a neuron firing increases, astrocytes provide glucose to the area and will change blood flow
to the active area.
• This is used in diagnostic medicine as FMRI. Blood flow changes can indicate active and
inactive regions→ changes in blood flow result from neuronal and astrocytic activity as
glucose activity changes.
• Increased activity of neurones triggers Ca2+ signals in astrocytes
• This astrocyte activity leads to the release of vasoactive agents that regulate the local blood flow.
• astrocytes are an essential link between neuronal activity and blood flow
• Interestingly astrocytes are the only cells in the brain which can synthesize glycogen and thus can serve as an
energy reservoir.
3. Synaptic maintenance
• Astrocytes occupy spaces near the synapse, prevent NT leaking out of synapse (glutamate or
GABA) and affecting other synapses.
• Maintain fidelity of info transfer of a synapse by increasing the spatial precision of synaptic
transmission
• “Tripartite Synapse”→ synapses are built from three equally important parts, the presynaptic terminal, the
postsynaptic neuronal membrane and the surrounding astrocyte. A NT released from the presynaptic terminal
activates receptors in both the postsynaptic neuronal membrane and the perisynaptic astroglial membranes.
This results in the generation of a postsynaptic potential in the neuron and a Ca2+ signal in the astrocyte. This
Ca2+ signal may trigger release of NT from the astrocyte, which in turn will signal onto both pre- and
postsynaptic neuronal membranes.
4. Maintain ion homeostasis of extracellular fluid.
• Astrocytes take in K+ from extracellular space to maintain ion homeostasis and keep neuron excitability
to normal levels.
• K+ moves through gap junctions in astrocytes.
• Gap junctions are transmembrane spanning proteins, made of 4 membrane spanning units forming a
connexin. 6 connexins make up a channel. These together form a pore. 6 connexins make up 1
connexon. 2 connexons join to make up a gap junction. Junctions are regulatable, they form a pore that
allows small molecules in. Not directional (molecules can enter and leave if they are of right size).
• Ions can fit through gap junction. Change in potential in one neuron can change in another neuron (no
directionality)
5. Astrocytes are responsive
• NT going between neurons (glutamate) can regulate the astrocytes that are surrounding the area.
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Blocking gap junctions in Astrocytes
• Increase in Calcium
• Calcium made to trigger release of other chemicals (ATP).
• Astrocytes have the components needed to release NT. In response to stimulation glia release
neuroactive substances→gliotransmitters.
Gliotransmitter release
Astrocytes maintain (take up excess glutamate, extracellular ion environment-remove potassium adding
hydrogen) and respond (changes signalling process in response to glutamate→affecting the post synaptic
neuron creating stronger response→ changes responsivity of presynaptic neuron)
Summary
Why it’s important to regulate ECF Potassium concentration:
The neuron at rest is permeable to K+, and the membrane potential is sensitive to changes in extracellular K+.
Therefore the ECF K+ needs to be well regulated to prevent unnecessary depolarisation. Astrocytes regulate K+
ECF concentration through spatial buffering. When K+ increases, the K+ enters the glia through their postassium
find more resources at oneclass.com
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