Understanding Psychology (10th Ed)

 Module 
7 
Neurons: The Basic Elements of Behavior 
63
When a message arrives at a neuron, gates along the cell membrane open 
briefl y to allow positively charged ions to rush in at rates as high as 100 million 
ions per second. The sudden arrival of these positive ions causes the charge 
within the nearby part of the cell to change momentarily from negative to posi-
tive. When the positive charge reaches a critical level, the “trigger” is pulled, and 
an electrical impulse, known as an
 
action potential
 
, travels along the axon of the 
neuron (see Figure 2).
The
 action potential 
moves from one end of the axon to the other like a fl ame 
moving along a fuse. As the impulse travels along the axon, the movement of ions 
causes a change in charge from negative to positive in successive sections of the axon 
(see Figure 3) on page 64. After the impulse has passed through a particular section 
of the axon, positive ions are pumped out of that section, and its charge returns to 
negative while the action potential continues to move along the axon. 
Just after an action potential has passed through a section of the axon, the cell 
membrane in that region cannot admit positive ions again for a few milliseconds, 
and so a neuron cannot fi re again immediately no matter how much stimulation it 
receives. It is as if the gun has to be reloaded after each shot. There then follows a 
period in which, though it is possible for the neuron to fi re, a stronger stimulus is 
needed than would be if the neuron had reached its normal resting state. Eventually, 
though, the neuron is ready to fi re once again. 
These complex events can occur at dizzying speeds, although there is great 
variation among different neurons. The particular speed at which an action poten-
tial travels along an axon is determined by the axon’s size and the thickness of 
its myelin sheath. Axons with small diameters carry impulses at about 2 miles 
per hour; longer and thicker ones can average speeds of more than 225 miles per 
hour. 
Neurons differ not only in terms of how quickly an impulse moves along the 
axon but also in their potential rate of fi ring. Some neurons are capable of fi ring 
as many as 1,000 times per second; others fi re at much slower rates. The intensity 
of a stimulus determines how much of a neuron’s potential fi ring rate is reached. 
A strong stimulus, such as a bright light or a loud sound, leads to a higher rate of 
fi ring than a less intense stimulus does. Thus, even though all impulses move at 
the same strength or speed through a particular axon—because of the all-or-none