McKenna's Pharmacology for Nursing, 2e

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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 Neurotransmitter terminator

Noradrenaline, adrenaline

Acetylcholine

Monoamine oxidase (MAO), catechol- O-methyltransferase (COMT)

Acetylcholinesterase

General response

Fight or flight

Rest and digest