Hockenbury Discovering Psychology 5th txtbk

48 CHAPTER 2 Neuroscience and Behaviorrush to the axon interior from the surrounding fluid, and then the sodium ionchannels close. Less than a thousandth of a second later, the potassium ion channelsopen, allowing potassium to flow out of the axon and into the fluid surroundingit. Then the potassium ion channels close (see Figure 2.2). This sequence ofdepolarization and ion movement continues in a self-sustaining fashion down theentire length of the axon.As this ion exchange occurs, the relative balance of positive and negative ions separatedby the axon membrane changes. The electrical charge on the inside of theaxon momentarily changes to a positive electrical charge of about 30 millivolts.The result is a brief positive electrical impulse that progressively occurs at each segmentdown the axon—the action potential.Although it’s tempting to think of the action potential as being conducted muchas electricity is conducted through a wire, that’s not what takes place in the neuron.The axon is actually a poor conductor of electricity. At each successive segment ofthe axon, the action potential is regenerated in the same way in which it was generatedin the previous segment—by depolarization and the movement of ions.AxonStimulus arrives+ +Sodium ions+ ++++Figure 2.2 Communication Within theNeuron: The Action Potential These drawingsdepict the ion channels in the membraneof a neuron’s axon. When sufficientlystimulated, the neuron depolarizesand an action potential begins. At eachprogressive segment of the axon’s membrane,sodium ion channels open andsodium ions rush into the interior of theaxon. A split second later, the sodium ionchannels close and potassium channelsopen, allowing potassium ions to flow outof the axon. As this sequence occurs, thereis a change in the relative balance of positiveand negative ions separated by theaxon membrane. The electrical charge onthe interior of the axon briefly changesfrom negative to positive. Once started, anaction potential is self-sustaining and continuesto the end of the axon. Followingthe action potential, the neuron repolarizesand reestablishes its negative electricalcharge.Sodium channelsopen and sodiumions rush into theaxon.Potassium channelsopen and potassiumions flow out of theaxon.-----+-+---++Potassium ions- ---++ +The first sodium channels haveclosed, but those farther downthe axon open, continuing theprocess of depolarization alongthe axon.+-+ + +++++-Interior of axonStimulus continues