Human Physiology, 14th edition (2016)

The Nervous System 191

as shown in chapter 9, figure 9.8. Dopamine is a neurotransmit-

ter; norepinephrine is a neurotransmitter and a hormone (from

the adrenal medulla); and epinephrine is the primary hormone

secreted by the adrenal medulla.

Other monoamines are derived from different amino acids

and so are not classified as catecholamines. Serotonin is derived

from the amino acid tryptophan and functions as an important

neurotransmitter. Histamine is derived from the amino acid his-

tidine and serves as a monoamine neurotransmitter, as well as

a regulator produced by nonneural tissues (discussed in later

chapters). As a monoamine neurotransmitter in the brain, his-

tamine promotes wakeful alertness; this is why some antihista-

mines cause drowsiness (chapter 8, section 8.4).

Like ACh, monoamine neurotransmitters are released by exo-

cytosis from presynaptic vesicles, diffuse across the synaptic cleft,

and interact with specific receptor proteins in the membrane of the

postsynaptic cell. The stimulatory effects of these monoamines,

like those of ACh, must be quickly inhibited so as to maintain

proper neural control. The action of monoamine neurotransmitters

at the synapse is stopped by (1) reuptake of the neurotransmitter

molecules from the synaptic cleft into the presynaptic axon ter-

minal, and then (2) degradation of the monoamine by an enzyme

within the axon terminal called monoamine oxidase (MAO). This

process is illustrated in figure 7.30.

7.5 Monoamines as Neurotransmitters

A variety of chemicals in the CNS function as neurotrans-

mitters. Among these are the monoamines, a chemical fam-

ily that includes dopamine, norepinephrine, and serotonin.

Although these molecules have similar mechanisms of action,

they are used by different neurons for different functions.

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Identify the monoamine neurotransmitters and
    explain how they are inactivated at the synapse.


13. Identify two neural pathways in the brain that use
    dopamine as a neurotransmitter, and explain their
    significance.

Monoamines are regulatory molecules derived from amino
acids. Dopamine, norepinephrine (noradrenalin), and epineph-
rine (adrenalin) are derived from the amino acid tyrosine and
placed in a subfamily of monoamines called catecholamines.
The term catechol refers to a common six-carbon ring structure,

Figure 7.30 Production, release, reuptake, and inactivation of monoamine neurotransmitters. Most of the monoamine
neurotransmitters, including dopamine, norepinephrine, and serotonin, are transported back into the presynaptic axon terminals after
being released into the synaptic gap. They are then degraded and inactivated by an enzyme, monoamine oxidase (MAO).

Presynaptic

neuron ending

Action

potentials

Ca2+

Tyrosine

Dopa

Dopamine 5. Inactivation of most

neurotransmitter by MAO

1. Monoamine produced and
   stored in synaptic vesicles

Receptor

Norepinephrine

Priming

Fusion

4. Reuptake of most
   neurotransmitter
   from synaptic cleft


5. Neurotransmitters
   enter synaptic cleft

Norepinephrine

Postsynaptic

cell

Depolariza

tion

2. Action potentials open
   gated Ca2+ channels,
   leading to release of
   neurotransmitter