N8 Glia: Astrocytes, Microglia, and Oligodendrocytes
Lecture 4 notes.
The primary cells of the nervous system gain their speciality from the morphology of their neurites and the physiology of their electrical excitability. For glia, the specialized morphology is that of oligodendrocytes and Schwann cells, while their specialized physiology is that of astrocytes: calcium waves.
Astrocytes
Astrocytes come in two types:
- Protoplasmic: grey matter.
- Fibrous: white matter.
Main functions:
- control local blood flow (neurovascular coupling);
- metabolic support (neurometabolic coupling): the lactate shuttle, glycogen;
- homeostasis (potassium buffering);
- modulate synaptic activity (the tripartite synapse);
- respond to injury.
They are critical for normal function. Example: Alexander disease, a rare leukodystrophy, is a genetic disease affecting the growth/development of myelin (mutations).
Microglia
Microglia serve the function of macrophages in the CNS. They make up 5–25% of brain cells in the mouse (they derive from myeloid progenitors of the embryonic yolk sac). They become reactive after CNS injury and activate nearby astrocytes; after injury, astrocytes develop “scars.” They be patrolling around.
Markers: GFAP for astrocytes; OX-18 for microglia.
A pasted figure shows the main glial types in silhouette: a fibrous astrocyte with long, sparsely branched processes, a bushy protoplasmic astrocyte, two small oligodendrocytes, and a small, finely branched microglial cell. The microglial cell is circled, with the note: related to OCD in mice, at the very least.
Oligodendrocytes and myelin
A pasted textbook figure (the only content on the last page) shows oligodendroglial cells myelinating axons in the CNS: an oligodendroglial cell sends out processes that each wrap a segment of a neuron’s axon in a layered myelin sheath. The cutaway labels the axon (with a mitochondrion inside it), the cytoplasm of the oligodendroglial cell, the myelin sheath, and the bare gap between two sheaths, the node of Ranvier.