Larynx and Pharynx - LO5

5. How is the sympathetic nervous system structured? What are the four basic pathways of pre-ganglionic sympathetic neurons? What are the major sources of sympathetics to the head & neck?

The sympathetic division of the autonomic nervous system consists of pre- and postganglionic fibers which typically (but not always) synapse in ganglia of the sympathetic trunk. Preganglionic fibers travel from the intermediolateral (IML) nucleus (lateral horn) of the spinal cord, out via the ventral roots of spinal nerves to the ventral primary rami (VPR), and then leave the spinal nerves via white rami communicantes (at levels T1-L2) to the sympathetic trunk. Once in the trunk, preganglionic fibers may:

• • synapse at that level of the sympathetic trunk and return to the VPR via a gray ramus communicans (or leave the trunk without synapsing as a splanchnic n.),

  • ascend the sympathetic trunk (and then either synapse and return to the VPR via a gray ramus communicans, or leave the trunk without synapsing as a splanchnic n.), or

  • descend the sympathetic trunk (and then either synapse and return to the VPR via a gray ramus communicans, or leave the trunk without synapsing as a splanchnic n.).

From the synapse, postganglionic fibers carry signals back to the VPR of the spinal nerve via gray rami communicantes. These fibers can then be distributed by the VPR. Note that some postganglionic fibers will 'backtrack' from the VPR in order to reach the DPR (as these will also have sympathetics associated with it). While white rami communicantes are restricted to the T1-L2 levels, the sympathetic trunks extend the entire length of the spinal cord, and gray rami communicantes conduct postganglionic fibers back to VPRs at every level of the cord. In the neck, there are typically 2-3 sympathetic ganglia, whereas in the thorax there are typically sympathetic ganglia at every spinal level (T1-T12). 

Splanchnic nerves (collections pre-ganglionic fibers that pass through the sympathetic trunk without synapsing), typically synapse in the pre-aortic ganglia of the abdominopelvic cavity.