32 PSYCHOLOGY
At the end of the axon there is anend foot,a slightly bulging structure that
releases neurotransmitters. A neurotransmitter is a chemical messenger that
allows a neuron to communicate with either other neurons or muscle fibers. The
neurotransmitter travels across a physical gap called thesynaptic cleft.The func-
tional connection between, for example, two neurons is called thesynapse.Note
that there is a formal distinction between a synaptic cleft and a synapse. The first
refers to a physical characteristic; the second refers to a functional characteristic. It
is common to hear people refer to the gap itself as the synapse, and this is incorrect.
In order for a neurotransmitter to work, it must find a receptor siteon an
adjacent cell. The receptor site has a physical shape that matches that of the neu-
ron. A useful analogy is a key and a lock. The neurotransmitter is like the key; the
receptor site is like the lock.(a) An axon consistently sends information.(b) A neurotransmitter is also referred to as a.(c) In order for a neurotransmitter to work, what must it find on an adjacent cell?Answers: (a) away from the cell body; (b) chemical messenger; (c) A receptor site.Neurotransmitters have received a lot of attention in recent years. Their activ-
ity often forms the basis of biological theories of mental disorders. For example,
low levels of the neurotransmitter serotoninare associated with depression. Spe-
cific drugs called selective serotonin reuptake inhibitors (SSRIs)increase the
amount of useable serotonin at the synapse. (Prozac is such a drug.)
For a second example, excessive activity of the neurotransmitter dopamineis
associated with schizophrenia, a mental disorder characterized by delusions. There
is evidence to suggest that chronic schizophrenic patients have too many receptor
sites for dopamine, and this causes excessive dopamine activity. It is this activityDendrites Cell wall Axon End footNucleus Synaptic cleft
Two adjacent neurons with selected structures.