After studying this chapter, you should be able to:
Describe the location of the cell bodies and axonal trajectories of preganglionic and postganglionic sympathetic and parasympathetic neurons.
Name the neurotransmitters that are released by preganglionic autonomic neurons, postganglionic sympathetic neurons, postganglionic parasympathetic neurons, and adrenal medullary cells.
Name the types of receptors on autonomic ganglia and on their target organs and list the ways that drugs can act to alter autonomic neurotransmission.
Describe functions of the sympathetic and parasympathetic nervous systems.
Describe the composition and functions of the enteric nervous system.
The autonomic nervous system (ANS) is responsible for homeostasis via its innervation of smooth muscle (eg, blood vessels, the wall of the gastrointestinal tract, and urinary bladder), cardiac muscle, and glands (eg, sweat glands and salivary glands). It is comprised of the sympathetic, parasympathetic, enteric nervous systems. The sympathetic and parasympathetic divisions include preganglionic and postganglionic neurons. Although survival is possible without an ANS, the ability to adapt to environmental stressors and other challenges is severely compromised. The importance of understanding the functions of the ANS is underscored by the fact that so many commonly prescribed and over-the-counter drugs exert their actions on elements of the ANS or its effector organs. Some neurologic diseases result directly from a loss of preganglionic sympathetic neurons (eg, multiple system atrophy and Shy–Drager syndrome) and others (eg, Parkinson disease) are associated with autonomic dysfunction (Clinical Box 13–1).
CLINICAL BOX 13–1 Multiple System Atrophy & Shy–Drager Syndrome
Multiple system atrophy (MSA) is a sporadic, progressive, adult-onset disorder characterized by autonomic dysfunction, parkinsonism, and cerebellar ataxia in any combination. Autonomic failure is due to loss of preganglionic autonomic neurons, making it difficult to regulate body temperature, fluid and electrolyte balance, and blood pressure. Shy–Drager syndrome is a subtype of MSA in which autonomic failure dominates. The pathologic hallmark of MSA is cytoplasmic and nuclear inclusions in oligodendrocytes and neurons in central motor and autonomic areas. There is also depletion of monoaminergic, cholinergic, and peptidergic markers in some brain regions and in the cerebrospinal fluid. MSA may result from a neuroinflammatory process that activates microglia and produces toxic cytokines. Basal levels of sympathetic activity and plasma norepinephrine are normal in MSA patients, but they do not increase upon standing, leading to severe orthostatic hypotension. In addition to the fall in blood pressure, orthostatic hypotension leads to dizziness, dimness of vision, and fainting. MSA is also accompanied by parasympathetic dysfunction, including urinary and sexual dysfunction. MSA is most often diagnosed in individuals 50–70 years of age; it affects more men than women. Erectile dysfunction is often the first symptom of the disease. There are also abnormalities in baroreceptor reflex and respiratory control mechanisms. About 75% of patients with MSA experience motor disturbances.
There is no cure for MSA but various therapies are used to treat specific signs and symptoms. Corticosteroids help ...