Understanding Psychology (10th Ed)

action potential
An electric nerve 
impulse that travels through a 
neuron’s axon when it is set off by 
a “trigger,” changing the neuron’s 
charge from negative to positive.
FIGURE 2
Movement of an action 
potential along an axon. Just before 
Time 1, positively charged ions enter 
the cell membrane, changing the charge 
in the nearby part of the axon from 
negative to positive and triggering an 
action potential. The action potential 
travels along the axon, as illustrated in 
the changes occurring from Time 1 to 
Time 3 (from top to bottom in this 
drawing). Immediately after the action 
potential has passed through a section 
of the axon, positive ions are pumped 
out, restoring the charge in that section 
to negative. The change in voltage 
illustrated at the top of the axon can be 
seen in greater detail in Figure 3.
(Source: Stevens, 1979.)
Voltage
Time 1 
Voltage
Time 2 
Voltage
Time 3 
Positive charge 
Negative charge 
Direction of impulse 
– 
– 
– 
– 
– 
– 
+ + 
+ + 
+
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64 Chapter 
3 
Neuroscience and Behavior
law—there is variation in the frequency of impulses, providing a mechanism by 
which we can distinguish the tickle of a feather from the weight of someone stand-
ing on our toes.
 MIRROR NEURONS 
Although all neurons operate through the fi ring of action potentials, there is signifi -
cant specialization among different types of neurons. For example, in the last decade, 
neuroscientists have discovered the existence of