When a dendrite or cell body is stimulated, the permeability of the neuron's membrane changes suddenly. At the point of stimulation, the cell membrane becomes permeable to Na+ ions. The rush of Na+ ions into the cell opens voltage-gated channels in the membrane that allow even more Na+ ions into the neuron. As a result, the interior of the neuron's cell body becomes more positively charged than the exterior. This reversal of polarity across the membrane begins an action potential, shown in Figure 49-3. The action potential starts where the cell body joins the axon.
O At resting potential, a neuron's interior is negatively charged with respect to the extracellular fluid around the cell. Q The passage of an action potential over the membrane of the axon reverses this polarity, and the interior of the axon becomes more positively charged for a brief time.
^ At the resting potential, sodium channels are closed and some potassium channels are open.
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