450
P A R T 5
Drugs acting on the autonomic nervous system
THE PARASYMPATHETIC NERVOUS
SYSTEM
In many areas, the parasympathetic nervous system
works in opposition to the SNS. This allows the auto-
nomic system to maintain a fine control over internal
homeostasis. For example, the SNS increases heart
rate, whereas the parasympathetic system decreases it.
Thus, the ANS can influence heart rate by increasing
or decreasing sympathetic activity or by increasing or
decreasing parasympathetic activity. This is very much
like controlling the speed of a car by moving between
the accelerator and the brake or combining the two.
Whereas the SNS is associated with the stress reaction
and expenditure of energy, the parasympathetic system
is associated with activities that help the body to store
or conserve energy, a “rest-and-digest” response (see
Table 29.3).
Structure and function
The parasympathetic system is sometimes called the
craniosacral system because the CNS neurons that
originate parasympathetic impulses are found in the
cranium (one of the most important being the vagus or
tenth cranial nerve) and in the sacral area of the spinal
cord (see Figure 29.2). It has long preganglionic axons
that meet in ganglia located close to or within the organ
to be affected. The postganglionic axon is very short,
going directly to the effector cell. The neurotransmitter
used by both the preganglionic and the postganglionic
neurons is ACh.
Parasympathetic system stimulation results in the
following actions:
• Increased motility and secretions in the GI tract to
promote digestion and absorption of nutrients.
• Decreased heart rate and contractility to conserve
energy and provide rest for the heart.
• Constriction of the bronchi, with increased secretions.
• Relaxation of the GI and urinary bladder sphincters,
allowing evacuation of waste products.
• Pupillary constriction, which decreases the light
entering the eye and decreases stimulation of the
retina.
These activities are aimed at increasing digestion,
absorption of nutrients and building of essential
proteins, as well as a general conservation of energy.
Cholinergic response
Neurons that use ACh as their neurotransmitter are
called cholinergic neurons. There are four basic kinds of
cholinergic nerves:
1. All preganglionic nerves in the ANS, both
sympathetic and parasympathetic
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TABLE 29.2 Physiological effects of specific receptor sites in the autonomic nervous system
Sympathetic system
Parasympathetic system
Alpha
1
-receptors
Muscarinic receptors
Vasoconstriction
Pupil constriction
Increased peripheral resistance with increased blood pressure
Accommodation of the lens
Contracted piloerection muscles
Decreased heart rate
Pupil dilation
Increased GI motility
Thickened salivary secretions
Increased GI secretions
Closure of urinary bladder sphincter
Increased urinary bladder contraction
Male sexual emission
Male erection
Alpha
2
-receptors
Sweating
Negative feedback control of noradrenaline release
Nicotinic receptors
from presynaptic neuron
Muscle contractions
Moderation of insulin release from the pancreas
Release of noradrenaline from the adrenal medulla
Beta
1
-receptors
Autonomic ganglia stimulation
Increased heart rate
Increased conduction through the atrioventricular node
Increased myocardial contraction
Lipolysis in peripheral tissues
Beta
2
-receptors
Vasodilation
Bronchial dilation
Increased breakdown of muscle and liver glycogen
Release of glucagon from the pancreas
Relaxation of uterine smooth muscle
Decreased gastrointestinal (GI) muscle tone and activity
Decreased GI secretions
Relaxation of urinary bladder detrusor muscle
