Principles of Manual Medicine
Sympathetic Reflex Arc
Sympathetic pathways differ from somatic pathways in that each sympathetic pathway is composed of a preganglionic neuron and a postganglionic neuron. The cell body of the preganglionic neuron lies in the spinal cord, and its fiber passes through an anterior root of the cord into a spinal nerve and finally through the white ramus from the spinal nerve to a sympathetic
ganglion. Here the fiber either synapses with postganglionic neurons in the sympathetic ganglia or often passes on through the chain into one of its radiating nerves to synapse with postganglionic neurons in one of the outlying sympathetic ganglia. The fiber of each postganglionic neuron then travels through an additional nerve to its destination in one of the organs.
Many of the fibers from the postganglionic neurons in the sympathetic chain pass back into the spinal nerves through gray rami at all levels of the cord. These pathways are made up of type C fibers that extend throughout the entire body in the skeletal nerves. They control the blood vessels, sweat glands, and piloerector muscles of the hairs. Approximately 8 per cent of the fibers in the average skeletal nerve are sympathetic fibers, a fact that indicates their importance. A few of the preganglionic neurons enter the spinal nerves directly from the spinal cord rather than passing first through the sympathetic chain; they synapse with postganglionic neurons located in the spinal nerves themselves. Therefore, destruction of the sympathetic chain does not remove all sympathetic activity, though it does remove at least 90 per cent of it.
There are four possible routes of the pre and postganglionic nerve fibers. After entering a paravertebral ganglia via the white ramus, the preganglionic fibers may:
- Pass up or down the sympathetic chain without synapsing, ultimately synapsing in a higher or lower ganglion.
- Synapse in the ganglia, stimulating postganglionic nerve fibers which then leave the ganglia to reenter the spinal nerves via the gray ramus.
- Synapse in the ganglia, stimulating postganglionic nerve fibers which then leave the ganglia and are directly distributed to their effector organs.
- Pass without synapsing into the abdomen to synapse in one of the peripheral ganglia.
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