Answer: Glial cells called ASTROCYTES help maintain the blood-brain barrier, whereas SCHWANN CELLS create the myelin that envelopes axons
Explanation: Glial cells are specialized cell that are found in both central and peripheral nervous systems, that surrounds the neurones, providing them with support and insulation. There are different types of Glial cells these includes:
- Astrocytes: they maintain the blood brain barrier.
- Schwann cells: create the myelin that envelopes axons of the peripheral nervous system neurones.
- satellite cells: These cells help regulate the external chemical environment of neurones.
- Microglia: they clean up cellular debris through phagocytosis.
- Oligodendrocytes: create the myelin that envelopes axons of the central nervous system
 
        
                    
             
        
        
        
They use a formula and that formula is 
People/Land Area=Population Density
        
                    
             
        
        
        
Answer:
Doppler effect is a change in frequency and wavelength of a wave. It is caused by the change in distance between the thing creating the wave (causer) and whatever is measuring seeing or hearing the wave (watcher or observer). Another word for "causer" is "sender".
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