Sympathetic Nervous System

Sympathetic nerve fibers originate in the intermediolateral horns of the gray matter of the spinal cord between segments T-1 and L-3. There are no sympathetic nerve fibers that originate in the cervical segments of the cord or in the lumbar and sacral segments below L-3. Because of the diffuse nature of the sympathetic trunk, nerve fibers travel upward and/or downward through the sympathetic chain to supply the head and leg regions. It is essential for the osteopathic physician to understand and have committed to memory the somatic regions that are likely to be effected by viscero-somatic reflexes resulting from visceral pathology. You may find the sympathetic drill exercise to be helpful).

The sympathetic nervous system is different from skeletal motor nerves in the following way: Each motor fiber to a skeletal muscle is composed of a single fiber originating in the cord. You may find it helpful to review the monosynaptic reflex arc. Each sympathetic pathway is composed of a preganglionic neuron and a postganglionic neuron. You may find it helpful to review the sympathetic reflex arc.

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 the sympathetic chain. 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. A comparison of in the autonomic nervous system reveals that, in general, sympathetic preganglionic neurons are shorter than sympathetic postganglionic neurons.

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, controlling blood vessels, sweat glands, and piloerector muscles of the hairs.

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