Neuroanatomy and Mechanisms of Visceral Pain

Introduction to the Autonomic Nervous System

The autonomic nervous system regulates integral body functions such as heart rate, blood pressure, respiratory rate, temperature, digestion, and pupillary response. The specific targets of autonomic efferent (motor) fibers are smooth muscle, cardiac muscle, and glands ( Fig. 2.1 ). The autonomic nervous system includes three basic divisions: the sympathetic, parasympathetic, and enteric divisions. Although extrinsic sympathetic and parasympathetic input can moderate the actions of smooth muscles and glands in the gastrointestinal (GI) system, the intrinsic neurons of the enteric nervous system, which are distributed throughout the wall of the GI tract, can sustain digestive function in the absence of extrinsic input. In this chapter, we will review the autonomic nerves and plexuses responsible for normal function in the thorax and abdomen, as well as the physiological mechanisms of visceral pain.

Sympathetic Nervous System

General sympathetic nervous system functions include vasoconstriction, increased heart rate, inhibition of glandular secretion and smooth muscle contraction in organs, and contraction of smooth muscle sphincters.

Except for sympathetic nerve fibers that synapse directly on chromaffin cells in the adrenal medulla, all autonomic visceral efferent pathways consist of at least two neurons.

The nerve cell bodies of sympathetic presynaptic neurons are found in the lateral gray horn of spinal cord segments T1 through L2 ( Fig. 2.2 ). The sympathetic chain, also known as the sympathetic trunk, consists of a series of interconnected ganglia that run from the base of the skull to the coccyx lateral to the vertebral column ( Fig. 2.3 ). The sympathetic chain allows the axons of sympathetic presynaptic neurons to synapse at ganglia above or below their spinal cord segmental origin.

The myelinated axons of all sympathetic presynaptic neurons leave the spinal cord in the ventral roots of the thoracic, lumbar spinal nerves and enter the sympathetic trunk through white rami communicantes. Sympathetic visceral motor impulses may take several pathways at this point.

1.

The axons of presynaptic sympathetic neurons carrying impulses to peripheral blood vessels, skin glands, and smooth muscles synapse at the sympathetic chain ganglia. The axons of the corresponding postsynaptic neurons then join spinal nerves through gray rami communicantes to reach their targets.
2.

The axons of presynaptic sympathetic neurons carrying impulses to structures in the head and neck ascend within the sympathetic chain from upper thoracic spinal cord levels and synapse on cervical sympathetic ganglia. The axons of the corresponding postsynaptic neurons typically follow branches of the carotid arteries to their targets.
3.

The axons of presynaptic sympathetic neurons carrying impulses to thoracic viscera such as the heart, lungs, and esophagus enter the ventral rami of spinal nerves and typically synapse at adjacent sympathetic chain ganglia. The axons of the corresponding postsynaptic neurons typically travel by direct branches to the cardiac, pulmonary, and esophageal autonomic plexuses.
4.

The axons of most sympathetic presynaptic neurons to abdominal and pelvic viscera pass through the sympathetic trunk without synapsing, form distinct thoracic splanchnic or lumbar splanchnic nerves, and synapse at ganglia within one of the many autonomic nerve plexuses clustered around the major branches of the abdominal aorta. The axons of the corresponding postsynaptic neurons typically follow the appropriate visceral branches of the aorta to reach their targets.
5.

The axons of some lower lumbar presynaptic neurons carrying impulses to the distal portions of the urogenital organs and the perineal erectile tissue may descend in the sympathetic chain to synapse at the sacral sympathetic chain ganglia. The axons of the corresponding postsynaptic neurons, the “sacral splanchnic nerves,” travel anteriorly by direct, and possibly vascular, branches to reach their targets.

Parasympathetic Nervous System

The nerve cell bodies of presynaptic parasympathetic neurons are found in the brainstem visceral motor nuclei and in spinal cord segments S2 through S4. In general, parasympathetic nerves decrease heart rate, increase glandular secretion, and increase smooth muscle contraction.

The vagus nerve is one of the longest nerves in the body and contains parasympathetic fibers. It originates from the brain stem as the 10th cranial nerve. The right and left vagus nerves leave the cranial cavity through the jugular foramina and run inferiorly within the carotid sheath with the carotid arteries and internal jugular veins. Its visceral branches innervate glands as well as muscles of the cardiac, respiratory, and GI systems.

Autonomic Innervation of Thoracic Organs

The cardiac, pulmonary, and esophageal plexuses are mixed sympathetic and parasympathetic plexuses. The sympathetic contributions to these plexuses are typically from postsynaptic neurons with nerve cell bodies in the T1–T4 sympathetic trunk. The parasympathetic contributions are branches of the vagus nerve that synapse at ganglia within the plexuses or in the walls of thoracic organs.

The superficial portion of the cardiac plexus lies anterior to the right pulmonary artery and inferior to the aortic arch. The deep portion of the cardiac plexus lies posterior to the aortic arch and anterior to the tracheal bifurcation just superior to the carina. The plexuses then give rise to varying branches that reach the sinoatrial node to regulate heart rate.

The right and left pulmonary plexuses lie along the anterior and posterior surfaces of the mainstem bronchi and lung hila. Overall, the pulmonary plexus innervates the bronchial tree and visceral pleura, although the posterior pulmonary plexus innervates more than 70% of the lungs.

An esophageal plexus is a group of nerve fibers with parasympathetic and sympathetic innervation that enter the esophagus at various levels. As the vagus nerve moves through the thorax, the anterior and posterior trunks become the esophageal plexus. Like most parasympathetic nerves, the preganglionic nerve fibers enter the wall of the organ and eventually synapse with postganglionic neurons. The vagus nerve components of the esophageal plexus contain nerve fibers that are both excitatory and inhibitory depending on the neurotransmitters released and their origin in the dorsal motor nucleus.

Autonomic Innervation of Abdominal and Pelvic Organs

Sympathetic Innervation of Abdominal and Pelvic Organs

The greater (superior thoracic) splanchnic nerve typically consists of presynaptic fibers originating from the T5 to T9 spinal cord segments. However, nerve fibers can arise from levels as high as T1 and as low as T11 ( Fig. 2.3 ). The greater splanchnic nerve is usually located anterolateral to the T12 vertebral body. The greater splanchnic nerve leaves the thorax, perforates the crura of the diaphragm, and enters the retroperitoneum where the presynaptic axons synapse on postsynaptic neurons in the celiac plexus. Postsynaptic sympathetic fibers from the celiac plexus typically follow arterial branches of the celiac trunk to the stomach, proximal duodenum, pancreas, spleen, and hepatobiliary organs. Some presynaptic sympathetic fibers in the greater splanchnic nerve synapse directly on chromaffin cells in the adrenal medulla.

The lesser splanchnic nerve typically consists of presynaptic fibers originating from the T9 to T11 spinal cord segments. Presynaptic fibers in the lesser splanchnic nerve typically synapse on postsynaptic neurons located in the celiac or superior mesenteric plexuses. Postsynaptic axons typically follow branches of the superior mesenteric artery to the distal duodenum, remaining small bowel, ascending colon, and transverse colon proximal to the splenic flexure.

The least splanchnic nerve typically consists of presynaptic fibers from T11 to T12 spinal cord segments. Presynaptic fibers in the least splanchnic nerve typically synapse on postsynaptic neurons located in the aortorenal ganglion. Postsynaptic axons follow the renal arteries to the kidneys.

The lumbar splanchnic nerves generally arise from L1 to L2 spinal cord segments. Presynaptic axons synapse on postsynaptic neurons in the inferior mesenteric plexus. Postsynaptic axons follow branches of the inferior mesenteric artery to the splenic flexure, descending colon, sigmoid colon, and proximal rectum. Postsynaptic axons also descend into the pelvis via direct branches, the hypogastric trunks, to reach the superior hypogastric plexus, inferior hypogastric plexus, and specific visceral plexuses on the lateral walls of the rectum and pelvic organs. Branches of the cervical (uterovaginal) or prostatic plexuses descend along the urethra to reach perineal erectile tissue.

Visceral afferent fibers from the GI tract typically return to the spinal cord along sympathetic visceral motor pathways.

Parasympathetic Innervation of Abdominal and Pelvic Organs

The left and right vagus nerves enter the abdomen on the esophagus, passing through the esophageal hiatus as the anterior and posterior branches of the esophageal plexus, respectively. The anterior branch innervates the intraabdominal esophagus and stomach. The posterior branch innervates the liver, biliary tree, gallbladder, and lesser omentum, and joins the celiac plexus. Vagal branches from the celiac plexus supply the small intestine and large intestine proximal to the splenic flexure. Postsynaptic parasympathetic neurons are situated in the wall of the gut in the myenteric (Auerbach) and submucosal (Meissner) plexuses ( Fig. 2.2 ).

Abdominal and Pelvic Autonomic Nerve Plexuses

A series of intercommunicating nerve plexuses and ganglia surround the major branches of the abdominal aorta ( Fig. 2.4 ). The axons of presynaptic sympathetic neurons reach the aortic plexuses via the thoracic and lumbar splanchnic nerves and synapse at sympathetic ganglia within the plexuses. The axons of sympathetic postsynaptic neurons follow blood vessels to the abdominal organs.