Understanding the autonomic nervous system (ANS) is critical in anesthetic management, as many disease states have profound ANS effects. In addition, many of the pharmacological interventions commonly implemented by anesthesiologists have direct effects on the ANS—leading some to state that anesthesiology is the practice of autonomic medicine.
The ANS is instrumental in the control of many of the body’s organ systems below the conscious level, including central nervous system (CNS) and peripheral nervous system regulation of cardiac muscle, smooth muscle, and visceral functions. The ANS is predominantly an efferent system, consisting of the sympathetic nervous system (SNS) and parasympathetic nervous system (PSNS). The ANS also has an afferent component, transmitting information from the periphery to the CNS. Examples of this include the baroreceptors and chemoreceptors in the carotid sinus and aortic arch or the vasovagal response.
ANATOMY OF THE SYMPATHETIC NERVOUS SYSTEM
The SNS and PSNS innervate most organs, providing opposing yet complementary effects. The anatomy of the efferent ANS is characteristically different than the somatic and ANS afferent pathways. The ANS efferents consist of a two-neuron chain from the CNS to the ANS ganglion (via preganglionic fibers) to the effector organ (via postganglionic fibers). This organization contrasts with the somatic afferent and efferent system, which consists of one neuron from the CNS making direct contact with the effector organ.
The preganglionic fibers of the SNS (thoracolumbar division) originate in the intermediolateral column (lateral horn) of the gray matter in the spinal cord between the first thoracic and second lumbar vertebrae (Figure 128-1). The myelinated axons of these nerve cells leave the spinal cord with the motor fibers to form the white (myelinated) communicating rami. The rami enter one of the paired 22 sympathetic ganglia at their respective segmental levels. The preganglionic fiber enters the rami and proceeds by either synapsing with postganglionic fibers in the ganglia at the level of exit, traversing cephalad or caudad in the SNS chain to synapse in ganglia at other levels, or tracking through the SNS chain at variable distances and exiting without synapsing to terminate in an outlying, unpaired SNS collateral ganglion. The exception is the adrenal gland, where preganglionic fibers pass directly into the organ without synapsing in a ganglion.
The sympathetic nervous system. (Reproduced with permission from Butterworth JF, Mackey DC, Wasnick JD, Morgan and Mikhail’s Clinical Anesthesiology, 5th ed. McGraw-Hill; 2013.)
The SNS postganglionic neuronal cell bodies are located in ganglia of the paired lateral SNS chain or unpaired collateral ganglia (such as the celiac and inferior mesenteric ganglia). The SNS ganglia are typically closer to the spinal cord than the effector organ; thus, the SNS postganglionic neuron can originate in either the paired lateral paravertebral SNS ganglia or one of the unpaired collateral ganglia. The ...