Lecture 26: Glial Cells

What is true of glial cells?

• they outnumber neurons by making up half of brain volume

• do not fire APs

What are the types of glial cells?

• astrocytes

• oligodendrocytes

• microglia

What is true of astrocytes?

they provide energy to neurons

What do astrocytes store in the brain?

glycogen

What do astrocytes convert?

glucose to lactate

Where is lactate shuttled?

to nearby neurons (energy reservoir)

What is the V_rest for an astrocyte?

~ -85 mV

What are the roles of astrocytes in K⁺ buffering?

• low Na⁺:K⁺ channel ratio

• with high neuronal activity leads to accumulation of extracellular K⁺, and then astrocytes remove K⁺ from extracellular space

What are the 3 mechanisms of astrocyte K⁺ uptake?

• Na-K pump

• Na/K/Cl co-transporter

• K_ir channels

How many gap junctions may couple 2 astrocytes?

100s

What do gap junctions do?

pass ions and organic molecules <1 kDa

What is the network of astrocytes?

syncytium

What are the characteristics of syncytium?

• similar intracellular concentrations of ions and small molecules

• similar membrane potentials

What are the characteristics of spatial buffering?

• K⁺ transported from areas of high [K⁺]_o to areas of normal [K⁺]_o, exits via leak channels

• current loop

  • electrical coupling of astrocytes maintain negative V_m, gradient for K⁺ entry

What does glutamate do?

mediates most excitatory synaptic effects

What is glutamine?

glutamate precursor

What happens when glutamate is released from presynaptic terminal?

• glutamate taken up by excitatory amino acid transporters (EAATs) on astrocyte membranes

• converted into glutamine by glutamine synthetase (enzyme only in astrocytes)

• glutamine transported into presynaptic terminal

What are the characteristics of oligodendrocytes?

• each oligodendrocyte has 15-30 processes connecting myelin sheaths to cell bodies

• produce myelin and induce axonal clustering of NaV channels

• contain most of carbonic anhydrase enzyme in brain

  • catalyzes hydration of CO₂

  • important for pH regulation

• iron metabolism

What are the characteristics of microglia?

• ~ 20% of glial cells

• macrophages of the CNS

• rapidly activated by injury, inflammation

• activation - proliferation, morphological changes, phagocytic

• activated microglia release free radicals and nitric oxide (NC)

What can changes in microglia homeostatic function contribute to?

AD and other neurological diseases