Notes

Glial Cells

• Central Nervous System
  • Microglial cell
  • Astrocyte
  • Oligodendrocyte
• Peripheral Nervous System
  • Satellite cells
  • Schwann cells

Astrocyte There exist two different main morphologies of astrocytes in grey and white matter.

• Grey Matter Astrocytes (Protoplasmic) They have a spongy look and are difficult to represent. They touch on blood vessels as they have control over blood regulation.
• White Matter Astrocytes (Fibrous)

Astrocytes Form a Network Astrocytes form networks through gap junctions: gap junctions form a channel between 2 membrane, they allow ions and metabolites to pass between cells and are made of connexin (Cx) proteins. Astrocytes: Cx30 and Cx43. They form sort of synapses between each other in a network even though they are not excitable cells.

Rodent vs. Human Astrocytes

• Structural Complexity: human astrocytes are bigger than rodents ones and the number of the main processes is way bigger in humans.
• Domains are way bigger in humans.
• Mice Implanted with Human Astrocytes: about 10 years ago, human astrocytes were implanted into mice's and they found that LTP was enhanced.
  • Neurons displayed enhanced long-term potentiation (LTP).
  • Animals learned faster.

Astrocyte Structure

• The Astrocyte connects three parts in the Neurovascular Unit: Neurons, Astrocytes and Blood Vessels.
• Tripartite Synapse: Astrocytes surround synapses. Glia transmission, they are releasing neurotransmitters in the synaptic cleft.

They release transmitters and influence synapses thorough a morphological change in synapse coverage. It is a really plastic system with a lot of different spatial and temporal scales of how astrocytes modulate synaptic strength.

Astrocyte Calcium Signaling When a neuron spikes there is a concurrent increase in calcium concentration. Calcium concentrations in astrocytes is mirroring neuronal activation. Two pathways:

• Direct calcium influx through ion channels.
• Release from the ER via GPCR and second messengers ($IP_3$). The cell shows local domain increases due to its complex morphology, i.e., there are many subdomains. Calcium waves between astrocytes has also been reported:
• Gap junctions.

Long-Term Two-Photon Imaging of Identical Cell Populations Mice cortex has been exposed, genetically encoded viruses are injected to visual calcium concentrations with two-photon imaging. Through whisker stimulation neurons shows spiking activity, sorting these neuronal traces by time they show to be pretty fast. On the contrary, astrocytes respond much slower to the whisker stimulation. However, there are neuronal subdomains that listen to the neuron activation. The correlation in time between the neuron and the astrocytes surrounding it, they show that the activation of the neuron likely elicits the activation of surrounding astrocytes.

Functional Roles of Astrocytes

• Energy Metabolism
• Neurotransmission
• Biosynthesis
• Waste Recycling
• Local Blood Flow Regulation