Two fundamental concepts about the ion channels of a "resting" neuron are that the channels
A) are always open, but the concentration gradients of ions frequently change.
B) are always closed, but ions move closer to the channels during excitation.
C) open and close depending on stimuli, and are specific as to which ion can traverse them.
D) open and close depending on chemical messengers, and are nonspecific as to which ion can traverse them.
E) open in response to stimuli, and then close simultaneously, in unison.
