In some cases, the various heterochromatic regions, especially those associated with centromeres, aggregate into a densely staining chromocenter.

The common form of heterochromatin that always remains heterochromatic is called constitutive heterochromatin. 

It is permanently condensed. 

It replicates late in S phase and has a reduced frequency of genetic recombination relative to euchromatic gene rich areas of the genome.

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The number of Na + channels per square micrometer of membrane in myelinated mammalian neurons has been estimated: 

50–75 in the cell body, 350–500 in initial segment, less than 25 on the surface of the myelin, 2000–12,000 at the nodes of Ranvier & 20–75 at the axon terminals 

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The number of Na + channels along the axons of unmyelinated neurons, is about 110.

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Depolarization in myelinated axons travels from one node of Ranvier to the next, this “jumping” of depolarization from node to node is called

saltatory conduction

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Saltatory conduction is a rapid process that allows myelinated axons to conduct up to 50 times faster than the fastest unmyelinated fibers

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Lioyd and Hunt, applies only to sensory nerve fibers and uses a Roman numeral nomenclature of I, II, III, and IV

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Neurotrophins 

Necessary for survival and growth of neurons

In the CNS produced by astrocytes