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P A R T 5
Drugs acting on the autonomic nervous system
Divisions
The ANS is divided into two parts: the
sympathetic
nervous system
and the
parasympathetic nervous
system
. These two parts differ in three basic ways:
(1) the location of the originating cells in the CNS,
(2) the location of the nerve ganglia and (3) the pregan-
glionic and postganglionic neurons (see Table 29.1 and
Figure 29.2).
THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system (SNS) is sometimes
referred to as the “fight-or-flight” system, or the system
responsible for preparing the body to respond to stress.
Stress can be either internal, such as cell injury or
cell death, or external, such as a perceived or learned
reaction to various external situations or stimuli. For
the most part, the SNS acts much like an accelerator,
speeding things up for action.
Structure and function
The SNS is also called the thoracolumbar system because
the CNS cells that originate impulses for this system are
located in the thoracic and lumbar sections of the spinal
cord. These cells send out short preganglionic fibres that
synapse or communicate with nerve ganglia located in
chains running alongside the spinal cord. Acetylcholine
is the neurotransmitter released by these preganglionic
nerves. The nerve ganglia, in turn, send out long post-
ganglionic fibres that synapse with neuroeffectors, using
noradrenaline or adrenaline as the neurotransmitter.
One of the sympathetic ganglia, on either side of the
spinal cord, does not develop postganglionic axons,
but produces noradrenaline and adrenaline, which are
secreted directly into the bloodstream. These ganglia
have evolved into the adrenal medullae. When the SNS is
stimulated, the chromaffin cells of the adrenal medullae
secrete adrenaline and noradrenaline directly into the
bloodstream.
When stimulated, the SNS prepares the body to flee
or to turn and fight (see Figure 29.3). Cardiovascular
activity increases, as do blood pressure, heart rate
and blood flow to the skeletal muscles. Respiratory
efficiency also increases; bronchi dilate to allow more
air to enter with each breath and the respiratory rate
increases. Pupils dilate to permit more light to enter
the eye, to improve vision in darkened areas (which
helps a person to see in order to fight or flee). Sweating
increases to dissipate heat generated by the increased
metabolic activity.
Piloerection (hair standing on end) also occurs. In
lower animals, this important protection mechanism
makes the fur stand on end so that an attacking larger
animal is often left with a mouthful of fur while the
intended victim scurries away. The actual benefit to
humans is not known, except that this activity helps to
generate heat when the core body temperature is too low.
Stimulation of the SNS causes blood to be diverted
away from the gastrointestinal (GI) tract because there is
no real need to digest food during a fight-or-flight situa-
tion. Subsequently, bowel sounds decrease and digestion
slows dramatically; sphincters are constricted and bowel
evacuation does not occur. Blood is also diverted away
from other internal organs, including the kidneys,
resulting in activation of the renin–angiotensin system
(Chapter 42) and a further increase in blood pressure
and blood volume as water is retained by the kidneys.
Sphincters in the urinary bladder are also constricted,
precluding urination.
Several other metabolic activities occur that prepare
the body to fight or flee. For example, glucose is formed
by glycogenolysis, to increase blood glucose levels and
provide energy. The hypothalamus causes the secretion
of adrenocorticotropic hormone (ACTH), leading to
a release of the adrenal hormones, including cortisol,
which suppresses the immune and inflammatory reac-
tions to preserve energy that otherwise might be used by
these activities. The corticosteroid hormones also block
protein production, another energy-saving activity, and
increase the release of glucose to provide energy. Aldos-
terone, also released with adrenal stimulation, retains
sodium and water and causes the excretion of potassium
in the urine. The hypothalamus also causes the release
of thyroid-stimulating hormone (TSH), which stimulates
■■
TABLE 29.1 Comparison of the sympathetic and parasympathetic nervous systems
Characteristic
Sympathetic
Parasympathetic
CNS nerve origin
Thoracic, lumbar spinal cord
Cranium, sacral spinal cord
Preganglionic neuron
Short axon
Long axon
Preganglionic neurotransmitter
Acetylcholine
Acetylcholine
Ganglia location
Next to spinal cord
Within or near effector organs
Postganglionic neuron
Long axon
Short axon
Postganglionic neurotransmitter
Noradrenaline, adrenaline
Acetylcholine
Neurotransmitter terminator
Monoamine oxidase (MAO), catechol-
O-methyltransferase (COMT)
Acetylcholinesterase
General response
Fight or flight
Rest and digest
