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P A R T 5  Drugs acting on the autonomic nervous system

■■ TABLE 29.2 Physiological effects of specific receptor sites in the autonomic nervous system Sympathetic system Parasympathetic system

Alpha 1

-receptors

Muscarinic receptors Pupil constriction

Vasoconstriction

Increased peripheral resistance with increased blood pressure

Accommodation of the lens Decreased heart rate Increased GI motility Increased GI secretions

Contracted piloerection muscles

Pupil dilation

Thickened salivary 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

THE PARASYMPATHETIC NERVOUS SYSTEM

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

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