Neuronal Circuit - Autonomic Nervous System

Neuronal Circuit

The sympathetic neurons in the interme-diomedial nucleus and in the intermediolateral nucleus (lateral horn) (A1) of the thoracicspinal cord send their axons through the anterior root (A2) into the spinal nerve. They run through the white communicatingbranch (A3) to reach the sympathetic trunk ganglion (A4) as preganglionic fibers. Here,some of them terminate at neurons from where postganglionic fibers return into the spinal nerve via the gray communicatingbranch (A5). The preganglionic fibers en-tering the ganglion are myelinated so that the connecting branch appears white (white communicating branch). The post-ganglionic fibers exiting the ganglion are unmyelinated so that the connecting branch appears gray (gray communicating branch).

Other postganglionic fibers (A6) extend from the sympathetic trunk via autonomic nerves to the organs. Some preganglionic fibers (A7) pass through the ganglion without synapsing and terminate in the pre-vertebral ganglia (A8) which lie on both sidesof the aorta. Numerous small and very small terminal ganglia (A9) lie in the regions of in-ternal organs. They are part of the neu-roplexuses spreading through each organ; they are found in the sheaths (extramuralganglia) as well as inside the organs (in-tramural ganglia). Whereas preganglionicand postganglionic fibers of the parasympa-thetic nervous system are both cholinergic (B), the cholinergic preganglionic fibers of the sympathetic nervous system synapse in the ganglia with noradrenergic neurons (C).

According to their localization, we distin-guish three different types of ganglia in which preganglionic fibers synapse with postganglionic neurons:

!   The sympathetic trunk ganglia

!   The prevertebral ganglia

!   The terminal ganglia

The sympathetic trunk ganglia and the pre-vertebral ganglia are sympathetic ganglia, whereas the terminal ganglia are mostly, but not exclusively, parasympathetic ganglia.