452
P A R T 5
Drugs acting on the autonomic nervous system
Termination of response
Once the effector cell has been stimulated by ACh,
stimulation of the receptor site must be terminated and
destruction of any ACh must occur. The destruction of
ACh is carried out by the enzyme a
cetylcholinesterase
.
This enzyme reacts with the ACh to form a chemically
inactive compound. The breakdown of the released ACh
is accomplished in 1/1000 second and the receptor is
vacated, allowing the effector membrane to repolarise
and be ready for the next stimulation.
■■
The parasympathetic system, when stimulated,
acts as a rest-and-digest response. It increases the
KEY POINTS
digestion, absorption and metabolism of nutrients
and slows metabolism and function to save energy.
■■
The parasympathetic system comprises CNS cells
that arise in the cranium and sacral region of the
spinal cord, long preganglionic axons that secrete
ACh, ganglia located very close to or within the
effector tissue and short postganglionic axons that
also secrete ACh.
■■
ACh is made by choline from the diet and packaged
into storage vesicles to be released by the cholinergic
nerve into the synaptic cleft. ACh is broken
down to an inactive form almost immediately by
acetylcholinesterase.
■■
Parasympathetic system receptors are classified as
muscarinic or nicotinic, depending on what response
they have to these plant alkaloids.
Nerve
terminal
Synaptic
vesicle
Acetic acid + Choline
to blood vessel
AP
Ca+
1
2
3
4
5
A
ACh
ACh E
ACh E
Acetylcholinesterase
Acetyl CoA + Choline (from diet)
Enzyme
Synaptic
cleft
Cholinergic
receptor
Neuron or
effector cell
FIGURE 29.5
Sequence of events at a
cholinergic synapse. 1. Synthesis of
acetylcholine (ACh) from choline (a
substance in the diet) and a cofactor
(the enzyme is choline acetyltransferase,
CoA). 2. Uptake of neurotransmitter into
storage (synaptic) vesicle. 3. Release of
neurotransmitter by an action potential
(AP) in the presynaptic nerve. 4. Diffusion
of neurotransmitter across the synaptic
cleft. 5. Combination of neurotransmitter
with receptor. The events resulting
from ACh’s occupation of receptor sites
depend on the nature of the postsynaptic
cell. ACh excites some cells and inhibits
others. An enzyme, acetylcholinesterase
(AChE), found in the tissues and on the
postsynaptic cell, inactivates ACh
(A)
.
Some of the products diffuse into the
circulation, but most of the choline formed
is taken up and reused by the cholinergic
neuron.
