Glial cells
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- Name three differences between glial cells and neurons.
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1.Glia are able to divide
2.Glia do not have axons and cannot propagate action potentials
3.Glia lack identifiable chemical synapses and synaptic vesicles
4.Glia cannot perform rapid point to point signalling - How many segments of myelin can be made by a single Schwann cell? By a single oligodendrocyte?
- Each Schwann cell can make only one segment of myelin whereas an oligodendrocyte is able to make up to fifty segments.
- What happens to a Schwann cell if the axon it surrounds dies?
- The cell loses its myelin, becomes phagocytic, and forms a tube of basal lamina that serves as a path of regeneration. In addition, the cell secretes neural growth factors.
- What other structural function besides myelination do Schwann cells have?
- They also ensheath small fibers without myelinating them.
- Describe the processes and location of the two types of astrocytes.
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*fibrous astrocytes have long processes and are found in the white matter
*protoplasmic astroocytes have veil-like structures and are found in the gray matter - What is the "glia limitans"?
- The lining around blood vessels and ventricles formed by the foot processes of astocytes. It helps to regulate the passage of materials between the blood and neurons.
- How are astrocytes involved in synaptic function in the CNS?
- They play a role in the uptake of neurotransmitter from synapse and recycling of material back to the pre-synaptic neuron.
- How are astrocytes connected? What purpose does this serve?
- They are connected by gap junctions which allows formation of a syncytium that can spread small molecules such as ATP or ions.
- How do astrocytes react to CNS injury?
- They exhibit hyperplasia and hypertrophy and form astroglial scars.
- Where in the CNS can ependymal cells be found? Describe their apical surfaces and lateral borders.
- Ependymal cells line the ventricles. They have cilia and microvilli on their apical surfaces and are not connected to adjacent cells by tight junctions, thus allowing CSF to flow into neural tissue.
- What are the two forms of microglia?
- They are in a dendritic form at rest and then adopt a phagocytic form when activated.
- What is the pathological significance of microglia?
- They are the site of HIV infection and replication in the brain.
- What is the purpose of myelination?
- It prevents loss of electrotonic current and decreases capacitance to speed conduction along a nerve fiber.
- Describe the structural roles of glial cells. Which type of glial cell assumes these functions?
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*provide physical support
*keep parts of neurons compartmentalized
*isolate synapses to prevent cross stimulation
*astrocytes - How do glial cells provide nutritive and trophic support to neurons?
- Through secretion of glucose, neural growth factors, and other material. This is being explored for possible treatment of neurodegenerative diseases.
- What are radial glia and what so they do?
- These cells provide a scaffold for the migration of neurons during cortical development.
- Where might there be high levels of extracellular K+? How do glial cells rectify this problem?
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*areas where neurons are stimulated often
*K+ is taken up by nearby glial cells and then spread via gap junctions through the glial syncytium to disperse it - How do glial cells remove excess glutamate from synaptic areas? What purpose does this process serve?
- Astrocytes express a specific Glu receptor and take up the molecule. It is converted to Gln and then released for re-uptake by pre-synaptic cells. The removal of Glu prevents excess stimulation of the synapse.
- An experiment cultured RGC alone and with glial cells - what changes were observed?
- The RGC grown with glial cells had 2x as many synapses and showed post-synaptic currents that were 5x as large and 70x as frequent.
- How are some glial cells able to modulate neurotransmission?
- They express receptors for neurotransmitter.
- How are astrocytes able to modulate NMDA receptors?
- Astrocytes are unique in that they express d-serine which serves as a co-agonist for NMDA stimulation.
- Are the waves of excitement generated by glial cells the same thing as APs? Why or why not?
- They are not - they are much slower and propogate by a different mechanism.
- The movement of what molecule generates the waves of excitement seen in glial cells? How do these waves move between adjacent cells?
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*Ca++
*the wave travels through gap junctions or jumps between cells using ATP or adenosine as a signalling molecule - What is the purpose of the waves of excitement seen in glial cells?
- They change the firing rate and firing properties of associated neurons.
- Why are CNS tumors more likely to arise from glial cells than neurons?
- Neurons are post-mitotic which greatly reduces the likelihood they will mutate.