C H A P T E R 2 9
Introduction to the autonomic nervous system
453
CHAPTER SUMMARY
■■
The autonomic nervous system (ANS) works with
the endocrine system to regulate internal functioning
and maintain homeostasis. The two parts of the
ANS, the sympathetic nervous system (SNS) and the
parasympathetic nervous system, work in opposition
to maintain minute-to-minute regulation of the
internal environment and to allow rapid response to
stress situations.
■■
The SNS, when stimulated, is responsible for the
fight-or-flight response. It prepares the body for
immediate reaction to stressors by increasing
metabolism, diverting blood to big muscles and
increasing cardiac and respiratory function.
■■
The parasympathetic system, when stimulated,
acts as a rest-and-digest response. It increases the
digestion, absorption and metabolism of nutrients
and slows metabolism and function to save energy.
■■
The SNS is composed of CNS cells arising in the
thoracic or lumbar area of the spinal cord, short
preganglionic axons, ganglia located near 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.
■■
One SNS ganglion on either side of the spinal cord
does not develop postganglionic axons, but instead
secretes noradrenaline directly into the bloodstream
to travel throughout the body to react with
adrenergic receptor sites. These ganglia evolve into
the adrenal medulla.
■■
SNS adrenergic receptors are classified as being
alpha
1
-, alpha
2
-, beta
1
- or beta
2
-receptors based on
the effectors that they stimulate.
■■
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.
■■
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.
■■
Noradrenaline is made by adrenergic nerves using
tyrosine from the diet. It is packaged in storage
vesicles that align on the axon membrane and is
secreted into the synaptic cleft when the nerve is
stimulated. It reacts with specific receptor sites and
is then broken down by MAO or COMT to relax
the receptor site and recycle the building blocks of
noradrenaline.
■■
Parasympathetic system receptors are classified as
muscarinic or nicotinic, depending on what response
they have to these plant alkaloids.
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