There are three basic modes of selection on a quantitative trait.

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#Unit 7. System Physiology – Animal

The basis for the relative refractory period is the higher K+ conductance than is present at rest. 

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The spatial distribution of ion channels along the axon plays a key role in the initiation and regulation of the action potential

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Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. 

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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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#Unit 7. System Physiology – Animal

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