<span>a small constriction in the <em>myelin sheath</em> of axons ... The hyperpolarized membrane is in a <em>refractory period</em> and cannot fire. ... The <em>speed</em>
of conduction of an action potential along an axon is influenced by
both the diameter of ... Energy Transportation, Introduction to Work and
Energy, and The <em>Role</em> of Energy and Metabolism</span>
By trapping harmful substances I believe :)
The answer is D. on the surface of the ocean at the poles.