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 various target organs and list the ways that drugs can act to alter the function of the processes involved in transmission within the autonomic nervous system.
- Describe functions of the sympathetic and parasympathetic nervous systems.
- Describe the location of some forebrain and brainstem neurons that are components of central autonomic pathways.
- Describe the composition and functions of the enteric nervous system.
The autonomic nervous system (ANS) is the part of the nervous system that is responsible for homeostasis. Except for skeletal muscle, which gets its innervation from the somatomotor nervous system, innervation to all other organs is supplied by the ANS. Nerve terminals are located in smooth muscle (eg, blood vessels, the wall of the gastrointestinal tract, urinary bladder), cardiac muscle, and glands (eg, sweat glands, salivary glands). Although survival is possible without an ANS, the ability to adapt to environmental stressors and other challenges is severely compromised (see Clinical Box 13–1). The importance of understanding the functions of the ANS is underscored by the fact that so many drugs used to treat a vast array of diseases exert their actions on elements of the ANS.
Clinical Box 13–1
Multiple System Atrophy & Shy–Drager Syndrome
Multiple system atrophy (MSA) is a neurodegenerative disorder associated with autonomic failure due to loss of preganglionic autonomic neurons in the spinal cord and brain stem. In the absence of an autonomic nervous system, it is difficult to regulate body temperature, fluid and electrolyte balance, and blood pressure. In addition to these autonomic abnormalities, MSA presents with cerebellar, basal ganglia, locus coeruleus, inferior olivary nucleus, and pyramidal tract deficits. MSA is defined as “a sporadic, progressive, adult onset disorder characterized by autonomic dysfunction, parkinsonism, and cerebellar ataxia in any combination.” Shy–Drager syndrome is a subtype of MSA in which autonomic failure dominates. The pathological 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 several brain regions and in the cerebrospinal fluid. The cause of MSA remains elusive, but there is some evidence that a neuroinflammatory mechanism causing activation of microglia and production of toxic cytokines may occur in brains of MSA patients. Basal levels of sympathetic activity and plasma norepinephrine levels are normal in MSA patients, but they fail to increase in response to standing or other stimuli and leads to severe orthostatic hypotension. In addition to the fall in blood pressure, orthostatic hypotension leads to dizziness, dimness of vision, and even fainting. MSA is ...
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