C H A P T E R 2 9
Introduction to the autonomic nervous system
449
Termination of response
Once noradrenaline has been released into the synaptic
cleft, stimulation of the receptor site is terminated and
disposal of any extra noradrenaline, as well as the
neurotransmitter that has reacted with the receptor
site, must occur. Most of the free noradrenaline mole
cules are taken up by the nerve terminal that released
them in a process called reuptake. This neurotrans-
mitter is then repackaged into vesicles to be released
later with nerve stimulation. This is an effective recyc
ling effort by the nerve. Enzymes are also in the area,
as well as in the liver, to metabolise or biotransform
any remaining noradrenaline or any noradrenaline
that is absorbed into circulation. These enzymes are
monoamine oxidase (MAO)
and catechol-
O
-methyl
transferase (COMT).
■■
The autonomic nervous system (ANS), which is
divided into two parts—the sympathetic nervous
system (SNS) and the parasympathetic nervous
system—works with the endocrine system to regulate
internal functioning and maintain homeostasis.
■■
The SNS is responsible for the fight-or-flight
response.
■■
The SNS is composed of CNS cells arising in the
thoracic or lumbar area of the spinal cord and long
postganglionic axons that react with effector cells.
The neurotransmitter used by the preganglionic cells
is acetylcholine (ACh); the neurotransmitter used by
the postganglionic cells is noradrenaline.
■■
SNS adrenergic receptors are classified as alpha
1
-,
alpha
2
-, beta
1
- or beta
2
-receptors.
KEY POINTS
Nerve
terminal
Presynaptic
neuron
Synaptic
cleft
Neuron or
effector cell
Inactive product
to blood vessel
COMT
AP
Into blood vessel
Return to presynaptic cell
Ca+
receptor
Dopa
Tyramine (from diet)
Tyramine hydroxylase
Dopa decarboxylase
Dopamine
inactive product
NE
MAO
Adrenergic
receptor
α β
1
or
β
2
α
2
cAMP
1
2
3
4
5
6
7
A
B
C
C
FIGURE 29.4
Sequence of events at an
adrenergic synapse. 1. Dopamine, a
precursor of noradrenaline (NA), is
synthesised from tyrosine in several steps.
2. Dopamine is taken into the storage
vesicle and converted to NA. 3. Release of
neurotransmitter by an action potential
(AP) in the presynaptic nerve. 4. Diffusion
of neurotransmitter across synaptic cleft.
5. Combination of neurotransmitter
with receptor. The events resulting from
NA’s occupying of receptor sites depend
on the nature of the postsynaptic cell.
6. Interaction of NA with many beta-
receptors leads to increased synthesis of
cyclic adenosine monophosphate (cAMP).
7. Feedback control at alpha2- receptor
leads to decreased NA release from
presynaptic neuron. Deactivation of
NA occurs by breakdown of NA by the
enzyme COMT
(A)
or most importantly by
reuptake into the presynaptic neuron
(C)
,
where it may be reused or inactivated by
another enzyme, monoamine oxidase
(MAO). Some of the neurotransmitter
may also diffuse away from the synaptic
cleft
(B)
.
