Zoology 10th Edition by Stephen Miller,John Harley

Exercise 11

Neurons maintain a high concentration of sodium ions outside the cell and a high concentration of potassium ions inside the cell. Diffusion of potassium ions to the outside of the cell leads to a resting potential of about 270 mV. Opening of gated channels can depolarize the neuron membrane or hyperpolarize the membrane, causing a graded potential. Action potentials are triggered when the membrane potential exceeds the threshold value. Gated sodium channels open and depolarization occurs. Electrical synapses involve direct cytoplasmic connections between two neurons; chemical synapses involve chemicals that cross the synaptic cleft, which separates two neurons.
How can the movements of only positive ions result in depolarization and repolarization of the neuron membrane during an action potential?