C H A P T E R 2 9
Introduction to the autonomic nervous system
451
2. Postganglionic nerves of the parasympathetic system
and a few SNS nerves, such as those that re-enter the
spinal cord and cause general body reactions such as
sweating
3. Motor nerves on skeletal muscles
4. Cholinergic nerves within the CNS
Acetylcholine synthesis and storage
ACh is an ester of acetic acid and an organic alcohol
called choline. Cholinergic nerves use choline, obtained
in the diet, to produce ACh. The last step in the pro-
duction of the neurotransmitter involves choline
acetyltransferase, an enzyme that is also produced
within cholinergic nerves. Just like noradrenaline, the
ACh is produced in the nerve and travels to the end of
the axons, where it is packaged into vesicles. To be a
cholinergic nerve, the nerve must contain all of the
enzymes and building blocks necessary to produce ACh.
Acetylcholine release
The vesicles full of ACh move to the nerve membrane;
when an action potential reaches the nerve terminal,
calcium entering the cell causes the membrane to
contract and secrete the neurotransmitter into the
synaptic cleft. The ACh travels across the synaptic cleft
and reacts with very specific
cholinergic receptor
sites
on the effector cell (see Figure 29.5).
Cholinergic receptors
Cholinergic receptors or ACh receptors are found
on organs and muscles. They have been classified as
muscarinic receptors
and
nicotinic receptors
. This clas-
sification is based on very early research of the ANS that
used muscarine (a plant alkaloid from mushrooms) and
nicotine (a plant alkaloid found in tobacco plants) to
study the actions of the parasympathetic system.
Muscarinic receptors
As the name implies, muscarinic receptors are receptors
that can be stimulated by muscarine. They are found in
visceral effector organs, such as the GI tract, bladder
and heart, in sweat glands and in some vascular smooth
muscle. Stimulation of muscarinic receptors causes
pupil constriction, increased GI motility and secretions
(including saliva), increased urinary bladder contraction
and a slowing of the heart rate.
Nicotinic receptors
Nicotinic receptors are located in the CNS, the adrenal
medulla, the autonomic ganglia and the neuromuscu-
lar junction. Stimulation of nicotinic receptors causes
muscle contractions, autonomic responses such as
signs and symptoms of a stress reaction and release
of noradrenaline and adrenaline from the adrenal
medulla.
■■
TABLE 29.3 Comparing the effects of autonomic stimulation
Effector site
Sympathetic reaction
Parasympathetic reaction
Eye structures
Iris radial muscle
Contraction (pupil dilates)
—
Iris sphincter muscle
—
Contraction (pupil constricts)
Ciliary muscle
—
Contraction (lens accommodates for
near vision)
Lacrimal glands
—
↑
Secretions
Heart
↑
Rate, contractility
↓
Rate
↑
Atrioventricular conduction
↓
Atrioventricular conduction
Blood vessels
Skin, mucous membranes
Constriction
—
Skeletal muscle
Dilation
—
Bronchial muscle
Relaxation (dilation)
Constriction
Gastrointestinal system
Muscle motility and tone
↓
Activity
↑
Activity
Sphincters
Contraction
Relaxation
Secretions
↓
Secretions
↑
Activity
Salivary glands
Thick secretions
Copious, watery secretions
Gallbladder
Relaxation
Contraction
Liver
Glyconeogenesis
—
Urinary bladder
Detrusor muscle
Relaxation
Contraction
Trigone muscle and sphincter
Contraction
Relaxation
Sex organs
Male
Emission
Erection (vascular dilation)
Female
Uterine relaxation
—
Skin structures
Sweat glands
↑
Sweating
—
Piloerector muscles
Contracted (goosebumps)
—
—, no reaction or response.
