Understanding Psychology (10th Ed)

(a)
(b)
Neurotransmitter
Synapse
Synapse
Receptor site
Receptor
site
Dendrite
Axon
Neurotransmitter
Neurotransmitter
Step 1: Neurotransmitters
are produced and stored
in the axon.
Step 3: Neurotransmitters
travel across the synapse to
receptor sites on another
neuron’s dendrite.
Step 2: If an action potential
arrives, the axon releases
neurotransmitters.
Step 4: When a neurotransmitter
fits into a receptor site, it delivers
an excitatory or inhibitory message.
If enough excitatory messages are
delivered, the neuron will fire.
neurotransmitters
Chemicals that 
carry messages across the synapse to 
the dendrite (and sometimes the cell 
body) of a receiver neuron.
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66 Chapter 
3 
Neuroscience and Behavior
(and sometimes the cell body) of a receiving neuron. Like a boat that ferries pas-
sengers across a river, these chemical messengers move toward the shorelines of 
other neurons. The chemical mode of message transmission that occurs between neu-
rons is strikingly different from the means by which communication occurs inside 
neurons: Although messages travel in electrical form within a neuron, they move 
between neurons through a chemical transmission system. 
There are several types of neurotransmitters, and not all neurons are capable of 
receiving the chemical message carried by a particular neurotransmitter. In the same 
way that a jigsaw puzzle piece can fi t in only one specifi c location in a puzzle, each 
kind of neurotransmitter has a distinctive confi guration that allows it to fi t into a 
specifi c type of receptor site on the receiving neuron (see Figure 4b). It is only when 
a neurotransmitter fi ts precisely into a receptor site that successful chemical com-
munication is possible. 
If a neurotransmitter does fi t into a site on the receiving neuron, the chemical 
message it delivers is basically one of two types: excitatory or inhibitory.