40 CHAPTER 2
happens at the synapse, what neurotransmitters and neuromodulators do, the nature
of receptors, and what can go wrong with chemical communication among neurons.The Synapse
When a neuron fi res and chemicals are released at the
terminal button, those chemicals usually contact an-
other neuron at a synapse, which is the place where the
tip of the axon of one neuron nestles against another
neuron (usually at a dendrite) and sends signals to it.
Most of the time, the sending neuron is not physically
connected to the receiving neuron, though. Instead, the
chemicals carry the signal across a gap, called the syn-
aptic cleft, shown in Figure 2.5. Events at the synapses
can go awry, which can underlie a variety of types of
psychopathology.Neurotransmitters and Neuromodulators
The chemicals that are released at the terminal buttons
are called neurotransmitters. Neurotransmitters can be
distinguished fromneuromodulators, which are chemi-
cals that modulate (alter) the way neurotransmitters
affect the receiving neuron. Some neuromodulators
affect the function of neurotransmitters by altering
events at the synapse, for instance, by slowing down
the rate at which molecules of a neurotransmitter are
removed from the synaptic cleft after having been re-
leased. However, researchers have discovered that some
chemicals that act as neurotransmitters can also act as
neuromodulators in certain circumstances, and vice
versa (Dowling, 1992). Thus, what originally seemed
to be a sharp distinction, between neurotransmitters
and neuromodulators, has become blurred. Research-
ers now often use the term neurotransmitter broadly,
to include both sorts of substances, or they refer to both sorts as neurotransmit-
ter substances.
It is worth looking briefl y at the major neurotransmitter substances that play
roles in psychological disorders. However, keep in mind that no neurotransmitter
substance works in isolation and that no psychological disorder can be traced solely
to the function of a single neurotransmitter substance. Nevertheless, imbalances in
some of these substances have been linked, to some extent, with certain psychologi-
cal disorders.
Dopamine is involved in reward and motivation, and it also plays roles in
executive functions in the frontal lobe, including those that orchestrate body
movements. Too little dopamine is thought to play a role in attention-deficit/
hyperactivity disorder and depression (Bressan & Crippa, 2005). Too much do-
pamine is thought to play a role in inappropriate aggression and schizophrenia
(Buchsbaum et al., 2006).
Serotonin is largely an inhibitory neurotransmitter involved in mood and
sleep, as well as motivation. Too little serotonin may play a role in depression
and obsessive-compulsive disorder (Mundo et al., 2000); too much of it may reduce
a person’s motivation.
Acetylcholine plays a particularly important role in the hippocampus,
where it is involved in the processes that store new information in memory.
It is also found in the ANS and is involved in the fight-or-flight response
(which plays a role in panic responses, as we discuss in Chapter 7). Too little
acetylcholine is apparently involved in the production of delusions (Rao &
Lyketsos, 1998), and too much can contribute to spasms, tremors, and convulsions
(Eger et al., 2002).Synapse
The place where the tip of the axon of one
neuron sends signals to another neuron.
Neurotransmitters
Chemicals that are released at the terminal
buttons and cross the synaptic cleft.
Neuromodulators
Chemicals that modulate (alter) the way
neurotransmitters affect the receiving neuron.
Some chemicals that act as neurotransmitters
can also act as neuromodulators in certain
circumstances, and vice versa.
2.5 • The Synapse
Figure 2.525 h
g 5Sending
neuronReceiving neuronNeural impulseAxon of sending
neuronTerminal buttonSacs containing
neurotransmitter
moleculesNeurotransmitter
molecules crossing
the synaptic cleftReceptorsSynaptic
cleftDendrite of
receiving neuronNeura
limpul
se