1 Describe the autonomic nervous system

The autonomic nervous system (ANS) is a network of nerves and ganglia that controls involuntary physiologic actions and maintains internal homeostasis and stress responses. The ANS innervates structures within the cardiovascular, pulmonary, endocrine, exocrine, gastrointestinal, genitourinary, and central nervous systems (CNS) and influences metabolism and thermal regulation. The ANS is divided into two parts: the sympathetic (SNS) and parasympathetic (PNS) nervous system. When stimulated, the effects of the SNS are widespread across the body. In contrast, PNS stimulation tends to produce localized, discrete effects. The SNS and PNS generally have opposing effects on end-organs, with either the SNS or the PNS exhibiting a dominant tone at rest and without exogenous stimulating events (Table 1-1). In general the function of the PNS is homeostatic, whereas stimulation of the SNS prepares the organism for some stressful event (this is often called the fight-or-flight response).

TABLE 1-1 Autonomic Dominance Patterns at Effector Sites

2 Review the anatomy of the sympathetic nervous system

Preganglionic sympathetic neurons originate from the intermediolateral columns of the thoracolumbar spinal cord. These myelinated fibers exit via the ventral root of the spinal nerve and synapse with postganglionic fibers in paravertebral sympathetic ganglia, unpaired prevertebral ganglia, or a terminal ganglion. Preganglionic neurons may ascend or descend the sympathetic chain before synapsing. Preganglionic neurons stimulate nicotinic cholinergic postganglionic neurons by releasing acetylcholine. Postganglionic adrenergic neurons synapse at targeted end-organs and release norepinephrine (Figure 1-1).

Figure 1-1 Neuronal anatomy of the autonomic nervous system with respective neurotransmitters. Ach, Acetylcholine; NE, norepinephrine.

(Moss J, Glick D: The autonomic nervous system. In Miller RD, editor: Miller’s Anesthesia, ed 6, Philadelphia, 2005, Churchill Livingstone, p 618.)

3 Elaborate on the location and names of the sympathetic ganglia. Practically speaking, what is the importance of knowing the name and location of these ganglia?

Easily identifiable paravertebral ganglia are found in the cervical region (including the stellate ganglion) and along thoracic, lumbar, and pelvic sympathetic trunks. Prevertebral ganglia are named in relation to major branches of the aorta and include the celiac, superior and inferior mesenteric, and renal ganglia. Terminal ganglia are located close to the organs that they serve. The practical significance of knowing the location of some of these ganglia is that local anesthetics can be injected in the region of these structures to ameliorate sympathetically mediated pain.

4 Describe the postganglionic adrenergic receptors of the sympathetic nervous system and the effects of stimulating these receptors

There are α₁, α₂, β₁, and β₂ adrenergic receptors. The A1, A2, and B2 receptors are postsynaptic and are stimulated by the neurotransmitter norepinephrine. The A2 receptors are presynaptic, and stimulation inhibits release of norepinephrine, reducing overall the autonomic response. Molecular pharmacologists have further subdivided these receptors, but this is beyond the scope of this discussion. Dopamine stimulates postganglionic dopaminergic receptors, classified as DA1 and DA2. The response to receptor activation in different sites is described in Table 1-2.

TABLE 1-2 End-Organ Effects of Adrenergic Receptor Stimulation