THE SYMPATHETIC SYSTEM
The preganglionic fibers of the sympathetic system take their origin from the lateral column of the thoracolumbar part of the spinal cord (Figure 8—74/1) and pass into the ventral roots of the thoracic and first several lumbar nerves.
They continue into the spinal nerves and then issue from the ventral rami, constituting the white communicating branches, which join the ganglia of the sympathetic trunks (Figure 8-53/5,7). The bilateral trunks run the length of the neck and back, and each have a basically segmental arrangement, although strict correspondence of the ganglia with spinal nerves is evident only in the thoracic and cranial lumbar regions.The cervical part of the trunk begins at the large, spindle-shaped, cranial cervical ganglion placed close to the base of the skull (Figure 8-74/5). The trunk is associated with the vagus within the carotid sheath and forms the vagosympathetic trunk that proceeds down the neck. The two components part company at the entrance to the chest, where the sympathetic trunk often bears a middle cervical ganglion by the first rib (Figure 8-75/7'). The trunk then continues subpleurally, over the line of the costovertebral articulations, before passing dorsal to the diaphragm to gain admission to the abdomen. Its thoracic part shows a regular arrangement of ganglia, although the first one or two are fused with a caudal cervical element to form the large cervi- cothoracic ganglion deep to the head of the first rib (Figure 8-75/7). The lumbar part of the trunk, which lies between the psoas musculature and vertebral bodies, at first also carries a regular complement of ganglia, but the arrangement later becomes more erratic in that some caudal lumbar ganglia split into two or, less commonly, fuse with their neighbors. The sacral part is even less regular and may fuse, temporarily or finally, with its fellow before extending into the tail, where it rapidly fades (Figure 8-74/5).
Figure 8-75 Distribution of sympathetic (black) and parasympathetic (dottedyellow) nervous systems, semischematic. 1, Parasympathetic oculomotor nucleus; 2, salivatory nuclei (rostral and middle parasympathetic nuclei); 3, dorsal vagal nucleus; 4, cranial cervical ganglion; 5, vagosympathetic trunk; 6, vertebral nerve; 7, cervicothoracic ganglion; 7', middle cervical ganglion; 8, ansa subclavia; 9, sympathetic outflow from spinal cord; 10, sympathetic trunk with paravertebral ganglia; 11, celiac ganglion; 12, cranial mesenteric ganglion; 13, caudal mesenteric ganglion; 14, vagus nerve with distribution to thoracic and abdominal organs; 15, sacral outflow of parasympathetic nervous system.
Because the sympathetic outflow is restricted, it follows that only the thoracic and cranial lumbar ganglia are joined by white communicating branches. However, all spinal and many cranial nerves are joined by bundles (gray communicating branches) of postganglionic fibers destined for vessels, skin glands, and so forth. It should be stressed that the body wall and limbs are innervated only by these postganglionic sympathetic fibers. Those to most cervical nerves join within a single trunk, the vertebral nerve, which runs from the cervico- thoracic ganglion through the foramina of successive cervical transverse processes (Figure 8-74/7). The postganglionic sympathetic fibers to the first two cervical nerves and to cranial nerves extend from the cranial cervical ganglion; many form the internal carotid nerve that follows the like-named artery.
Several alternative fates are open to the preganglionic fibers that enter the sympathetic chain, each to project on many ganglion cells. Some fibers synapse immediately within the local ganglion, others run crani- ally or caudally within the trunk to synapse within ganglia that are more cranial or caudal in the series, and yet others pass uninterruptedly through the trunk to proceed to a second set of (prevertebral) ganglia placed about the origin of the visceral branches of the abdominal aorta (Figures 8-74/10,11 and 8-75/11,12).
This last group constitutes the splanchnic nerves, which are rather variable in arrangement; usually one greater splanchnic nerve is formed by preganglionic fibers that leave the trunk from about the sixth to the penultimate thoracic ganglia with lesser thoracic and lumbar splanchnic nerves arising at more caudal levels (Figure 8-74/8,9).The viscera and vessels of the head receive their sympathetic innervation via the cranial cervical ganglion. The postganglionic fibers that emerge from this ganglion radiate in a number of directions that carry them into the territories of the cranial and first two cervical nerves. Though many pass through parasympathetic ganglia, they of course do so without interruption. The details are of rather limited clinical importance (although relevant to experimental work), and only a few points are presented here (see Figure 8-70).
One large group of fibers follows the internal carotid artery into the cranial cavity and there provides twigs to the intracranial vessels and fiber bundles that join various nerves, especially the trigeminal and those to the extraocular muscles. Another group of fibers passes through the ciliary ganglion to the eyeball for ultimate distribution to the dilator pupillae. At a more proximal level, the internal carotid nerve gives off the deep petrosal nerve, which combines with the greater petrosal nerve (Figure 8-70/11) in its passage through the pterygoid canal to the pterygopalatine ganglion (Figure 8-70/7). These fibers are ultimately dispersed with the various nerves that supply structures within the orbit, nasal cavity, sinuses, and palate.
Other branches concur with parasympathetic fibers in forming a plexus within the tympanic cavity from which the parotid gland is supplied after passage beyond the otic ganglion. Yet other bundles of fibers entwine the external carotid artery and its branches.
The thoracic organs—heart, trachea, and lungs— are supplied by postganglionic fibers that form cardiac and pulmonary plexuses within the mediastinum after leaving the thoracic portion of the sympathetic trunk.
These plexuses combine with the corresponding parasympathetic component (see Figure 8-75).The abdominal and pelvic organs receive their sympathetic innervation through the various splanchnic nerves that lead to the celiac, cranial mesenteric, renal, aorticorenal, gonadal, and caudal mesenteric ganglia placed on the ventral face of the aorta by the origins of the visceral arteries. The preganglionic fibers synapse in these ganglia and in the postganglionic fibers that emerge from intricate plexuses (combining vagal contributions) that enmesh, and run parallel to, the visceral arteries from which they obtain their names (Figure 8-76).
The pelvic organs are supplied with postganglionic fibers that leave the caudal mesenteric ganglion within the paired hypogastric nerves (Figure 8-76/8). These enter the pelvic cavity below the peritoneum to form a common pelvic plexus with the parasympathetic pelvic nerves (Figure 8-75). As already mentioned, the sympathetic contribution to the pelvic plexus includes preganglionic fibers that have deferred their synapses to peripheral locations within the pelvis.