Glial cell types:
Astrocytes Oligodendrocytes Schwann cells Microglia Ependymal cells
Glia:
Supporting elements of the CNS and PNS
Outnumber neurons, occupy only about 50% of CNS
no synapses & no action potentials
Astorcytes are the _______ glial cells; most numerous; ___________ bodies; many long processes
largest; star-shaped
What are the two types of astrocytes?
Fibrous astrocytes:
Have long thin processes and are found in white matter
Protoplasmic astrocytes:
Have shorter, thicker processes and are found in gray matter
**both have types of “end feet” or “foot processes”
Oligodendrocytes:
Function: myelination of axons
one myelinated oligodendrocyte may myelinate many axons
Functions of astrocytes:
1) Structural support and repair:
glial fibrillary proteins (GFAP), “connective tissue of the CNS”; surround synapses
2) Potassium spatial buffering:
neuronal membrane potential is maintained
not electrically excitable
3) Neurotransmitter and metabolite removal:
aid in rapid termination of the synaptic signal
GLAST takes up glutamate, converts glu to gln, then releases gln to neurons (glutamate-glutamine cycle)
4) Neurotransmitter receptors can trigger calcium waves inside cell
5) Contribute to the BBB
6) Gap junctions
7) Glial guides (neuronal migration)
8) Can serve as stem cells to generate neurons and glia
____________ is a potent inhibitor of axon outgrowth and regeneration
central myelin
Inhibitors of CNS axonal elongation:
1) Myelin associated glycoprotein (MAG)
2) Neurite inhibitor
3) Nogo gene & proteins
**All inhibit regeneration of CNS axons
Myelin associated glycoprotein (MAG):
May be important in maintaining axon-myelin complexes (axon-glial signaling)
levels fall in mature PNS but not CNS
Schwann cells
Counterpart of oligodendrocytes in the PNS
Function: myelination of axons in the PNS; only one segment of one axon
Surround all peripheral axons, whether myelinated or unmyelinated
Schwann cells provide growth-promoting factors:
- Schwann cell basal lamina: laminin: required for regeneration of peripheral nerve
- Cell adhesion molecules: NgCAM/L1
- Some Schwann cells may secrete nerve growth factor
Microglia:
- Function: phagocytosis (macrophage-like)
- Smallest; many short processes
- Increase in number (hyperplasia) and size (hypertrophy) when neurons undergo degeneration
- Recruited during infection, injury and seizure
Ependymal cells:
- Cuboidal or columnar epithelial layer lining the inside of the neural tube
- Some are ciliated or have microvilli
- Ependymal cells lining the ventricles have desmosomal junctions; some substances in CSF can penetrate the brain (no tight junctions)
- modified ependymal cells and associated capillaries form the choroid plexus; since they have tight junctions they also form the CSF/brain barrier
Functions of ependymal cells:
- barrier b/w brain and CSF
- CSF circulation
- produce CSF (choroid plexus)
Myelination:
- In the PNS: 1 Schwann cell myelinates 1 segment of 1 axon
* In the CNS: 1 oligodendrocyte myelinates many axons
Myelin structure
- Inner mesaxon: present in both CNS and PNS
- Outer mesaxon: present only in PNS
- Major dense lines
- Minor dense lines
- Axons > 1 µm in diameter are myelinated
- High in lipid content
- Thickness increases with axon diameter
Myelin proteins:
1) Myelin basic protein (MBP)
2) Myelin-associated glycoprotein (MAG)
3) Myelin-oligodendrocyte glycoprotein (MOG)
Myelin basic protein (MBP):
- Major structural proteins in CNS & PNS
- On the cytoplasmic face of the myelin membrane
- Basis for CNS autoimmune disease
Myelin-associated glycoprotein (MAG):
In both CNS and PNS
Myelin-oligodendrocyte glycoprotein (MOG):
- CNS only
- On surface of myelin sheath and oligodendrocytes
- Target antigen in autoimmune aspects of CNS demyelinating diseases
Nodes of Ranvier:
- Axolemma exposed to ECF
- Saltatory (“jumping”) conduction
- High concentration of voltage-gated Na channels
- Internode size is directly proportional to axon size
- PNS: covered by the Schwann cell cytoplasm
- CNS: nodes are bare
Clefts (incisures):
Function: cytoplasmic nutrients to inner leaflets
- PNS: Schmidt-Lanterman incisures
- CNS: Longitudinal incisures
__________ axons in the ___________ are surrounded by __________ cytoplasm and ___________.
Unmyelinated; PNS; Schwann cell; basal lamina
Myelin function:
a. Insulation: decreases ionic flux across the axolemma; conserves cellular ATP.
b. In myelinated axons: Saltatory conduction. Conduction velocity can reach up to 120 m/sec.
c. In unmyelinated axons: action potentials pass continuously along the axolemma, causing the conduction velocity to be < 2 m/sec.
