Psychology2016

The Biological Perspective 49

Each action potential sequence takes about one thousandth of a second, so the neural
message travels very fast—from 2 miles per hour in the slowest, shortest neurons to 270
miles per hour in other neurons. (See Figure 2.2.)
Now the action potential is traveling down the axon. When it gets to the end of
the axon, something else happens: the message will get transmitted to another cell (that
step will be discussed momentarily). Meanwhile, what is happening to the parts of the
cell that the action potential has already left behind? How does the cell get the “fans”
back outside? Remember, the action potential means that the cell is now positive inside
and negative outside at the point where the channel opened. Several things happen to
return the cell to its resting state. First, the sodium ion channels close immediately after
the action potential has passed, allowing no more “fans” (sodium ions) to enter. The cell
membrane also literally pumps the positive sodium ions back outside the cell, kicking
the “fans” out until the next action potential opens the ion channels again. This pumping
process is a little slow, so another type of ion gets into the act. Small, positively charged
potassium ions inside the neuron move rapidly out of the cell after the action potential
passes, helping more quickly restore the inside of the cell to a negative charge. Now the
cell becomes negative inside and positive outside, and the neuron is capable of “firing
off” another message. Once the sodium pumps finish pumping out the sodium ions, the
neuron can be said to have returned to its full resting potential, poised and ready to do it
all again.
To sum all that up, when the cell is stimulated, the first ion channel opens and
the electrical charge at that ion channel is reversed. Then the next channel opens and that
charge is reversed, but in the meantime the first ion channel has been closed and the
charge is returning to what it was when it was at rest. The action potential is the sequence
of ion channels opening all down the length of the cell’s axon.

Figure 2.2 The Neural Impulse Action Potential
Voltage is graphed at a given axonal node over 2 to 3 milliseconds (thousandths of a second). From an initial resting state, enough stimulation is received that
the threshold of excitation is reached and an action potential is triggered. The resulting rapid depolarization, repolarization, brief hyperpolarization, and return to
resting potential coincide with movement of sodium and potassium ions across the cell membrane.

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Electrical charge (millivolts)-70

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Hyperpolarization

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Action potential

Time

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otential^

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