HBF-II LEC 37 Gross Anatomy ANS Abdomen Notes 2024 Walker PDF

Gross Anatomy: ANS Abdomen Page 1 of 10 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately: 1. Describe the autonomic innervation of the abdominal viscera. 2. Descri...

Gross Anatomy: ANS Abdomen Page 1 of 10 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately: 1. Describe the autonomic innervation of the abdominal viscera. 2. Describe abdominal visceral afferents and relate to clinical signs of referred pain. 3. Discuss the anatomy of the enteric nervous system and its functional relevance. Session Outline I. Visceral Motor Innervation of the Abdominal Viscera A. Overview of the Abdominal Autonomics B. Anatomy of the Abdominal Autonomics II. Visceral Sensory Innervation of the Abdominal Viscera A. Visceral Afferents B. Referred Pain III. Enteric Nervous System A. Hirschsprung’s Disease (Congenital Megacolon) Gross Anatomy: ANS Abdomen Page 2 of 10 Dr. Paul Walker I. Innervation of the Abdominal Viscera A. Overview of the Abdominal Autonomics This lecture focuses on the innervation of the gastrointestinal tract and accessory organs. Visceral Motor Nerves Smooth muscle and blood vessels of the GI tract are innervated by ANS fibers, as well as accessory GI glands (pancreas, liver, gallbladder). ANS fibers regulate GI motility (peristalsis), secretions (enzymatic and mucus), and blood flow. PANS fibers increase GI peristalsis & gland secretion while SANS fibers decrease GI peristalsis & gland secretion, as well as blood flow to the viscera. The GI tract also has its own intrinsic nervous system called the ‘enteric plexus’ or ‘enteric nervous system’. This specialized nervous system is contained within the GI walls and the ANS serves to modify (increase or decrease) its activities. Visceral Sensory Nerves VA fibers carry GI reflex information to the medulla via the vagus nerve (CN X). VA fibers carry pain information to the spinal cord via sympathetic nerves. Visceral pain from the abdomen is usually referred to the body (soma) wall. Knowledge of referred pain sites is useful to localize pathology of the abdominal viscera. Fig 1 (Grays Anatomy for Students) PANS Preganglionic PANS fibers originate from the brainstem (medulla) and travel in the Vagus Nerve (CN X) to innervate the GI tract (esophagus, stomach, small intestine, ascending colon, transverse colon) and associated viscera (liver, gallbladder, pancreas) as far distally as the splenic flexure of the large intestine. For the GI tract distal to the splenic flexure, preganglionic PANS fibers originate from S2- S4 spinal levels and travel in Pelvic Splanchnic Nerves to innervate the descending colon, sigmoid colon, and rectum. SANS Preganglionic SANS fibers originate from T5- L2 spinal levels to innervate the entire GI tract and associated viscera (liver, gallbladder, pancreas) via thoracic splanchnic (greater, lesser, least) and lumbar splanchnic nerves. Gross Anatomy: ANS Abdomen Page 3 of 10 Dr. Paul Walker Fig 2 below provides a schematic plan for the organization of the thoracic autonomics: SANS Ach NE a1 Spinal Cord SANS Preganglionic SANS Postganglionic a2 T5-L2 Levels b1 VE Prevertebral b2 Sympathetic Ganglion CNS PNS Located Along the Aorta PANS M1 Ach Ach M3 PANS Preganglionic Brainstem PANS Postganglionic Vagus Nerve CN X Terminal Parasympathetic Ganglion Located in the Organ Spinal Cord S2-S4 Levels VE Fig 2 Compare the location of pre-ganglionic cell bodies. Compare the location of post-ganglionic cell bodies. What neurotransmitters do pre-ganglionic axons release? What neurotransmitters do post-ganglionic axons release? What receptors respond to neurotransmitters released from post-ganglionic axons? Gross Anatomy: ANS Abdomen Page 4 of 10 Dr. Paul Walker B. Anatomy of the Abdominal Autonomics Fig 3 (Grays Anatomy for Students) Preganglionic SANS Pre-ganglionic SANS axons arise from the sympathetic cell column located in the spinal cord at T5-L2 levels. Preganglionic axons enter the sympathetic trunk via white rami communicans at the same spinal level as their cell body origin (T5- L2) and travel through the respective sympathetic ganglion without T5- L2 synapsing. Preganglionic SANS axons exit the anterior surface of the sympathetic ganglion as splanchnic nerves that travel to prevertebral ganglia located along the abdominal aorta and its major branches. Thoracic Spanchnics Greater splanchnic n. T5-T9 Lesser splanchnic n. T10-11 Least splanchnic n. T12 Lumbar & Sacral Splanchnics L1-L2 Target sympathetic ganglia for the splanchnic nerves are located in the abdominal cavity and are called prevertebral ganglia because of their positions along the abdominal aorta anterior to the vertebral column. Preganglionic SANS splanchnic axons synapse on postganglionic SANS neurons in prevertebral ganglia. Postganglionic SANS axons from the prevertebral ganglia exit and use arterial branches to the intestines to get to the gut wall. Here they synapse on neurons of the enteric plexus. Fig 4 (right) shows the greater splanchnic nerve emerging from the thoracic sympathetic trunk. Can you identify the greater splanchnic nerve, thoracic sympathetic trunk and ganglia, white rami communicantes, and intercostal nerves? Fig 4 (Prosection by Ben James, CLS 2020) Gross Anatomy: ANS Abdomen Page 5 of 10 Dr. Paul Walker Postganglionic SANS Fig 5 (Netter) The main prevertebral SANS ganglia affecting the abdominal viscera are illustrated in Fig 5. Prevertebral ganglia: Celiac Aorticorenal Superior Mesenteric Inferior Mesenteric Three of the above ganglia are located close together superiorly around the celiac trunk and SMA. This superior set includes celiac, aorticorenal, and superior mesenteric ganglia. They provide SANS regulation of the proximal 2/3 of the GI tract (up to the splenic flexure of the large intestine). This part of the GI tract is referred to as foregut and midgut. The superior set of ganglia receive preganglionic SANS axons from thoracic splanchnic nerves representing T5-T12 spinal levels. The Inferior Mesenteric ganglion is located more inferiorly near the IMA and regulates the distal 1/3 of the GI tract known as the hindgut. This part of the GI tract is distal to the splenic flexure of the large intestine and includes the descending colon, sigmoid colon, and rectum. The descending colon and sigmoid colon are located in the abdominal, and the rectum descends into the pelvic cavity. The preganglionic SANS innervation of the inferior mesenteric ganglion is different as compared to the superior set of ganglia. The IM ganglion receives preganglionic lumbar splanchnic nerves from L1-2 spinal levels, as well as preganglionic sacral splanchnic nerves. Similar to lumbar splanchnics, the sacral splanchnic nerves contain preganglionic SANS fibers that originate from L1-L2 spinal levels. These fibers descend within the sympathetic trunk into the pelvis and emerge as sacral splanchnic nerves from the anterior surfaces of the sacral sympathetic ganglia. Some of the sacral splanchnic nerves from the pelvic cavity use the right and left hypogastric nerves to gain access to the abdominal autonomic plexus via the superior hypogastric plexus (Fig 5). Gross Anatomy: ANS Abdomen Page 6 of 10 Dr. Paul Walker Preganglionic PANS Proximal 2/3 of the GI tract (foregut, midgut)- Vagus nerve Distal 1/3 of the GI tract (hindgut)- Pelvic splanchnic nerves that have preganglionic origin from S2-S4 spinal cord (region in the conus medullaris). Fig 6 (Gray’s Atlas of Anatomy) Summary of Preganglionic Origins of Abdominal Autonomics Foregut & Midgut Hindgut PANS- Vagus nerve (brainstem) PANS- Pelvic splanchnic nerves (S2-S4) SANS- Thoracic splanchnics (T5-T12) SANS- Lumbar & sacral splanchnics (L1-L2) Gross Anatomy: ANS Abdomen Page 7 of 10 Dr. Paul Walker II. Visceral Sensory Innervation of the Abdominal Viscera A. Visceral Afferents Fig 7 (Grays Atlas of Anatomy) Mechanoreceptors located in the GI tract detect distention of the gut wall and this information is carried to the medulla by the vagus nerve. The cell bodies of these vagal sensory neurons are located in ganglia near the brainstem. This sensory information is important for GI reflexes discussed in the upcoming Gastrointestinal Motility lecture. Mechanoreceptors located in the rectum carry stretch information to the spinal cord via the pelvic splanchnic nerves. Pain receptors located in the GI tract and accessory organs detect excessive stretch or other types of damage and carry this information to the spinal cord following nerve plexuses that also convey the sympathetic motor axons. The cell bodies are located in DRG located at spinal levels T5-L2. This sensory information is important clinically in terms of referred pain. Gross Anatomy: ANS Abdomen Page 8 of 10 Dr. Paul Walker B. Referred Pain Visceral afferent (VA) nerves bring pain signals from the internal abdominal organs to the spinal cord at the same levels that also receive somatic afferent (SA) signals from the body surface. Therefore, damage from the internal abdominal organs is often perceived as somatic pain in particular dermatome regions. Look at Fig 8 to review dermatome levels as well as the 4- or 9-quadrant organization abdominal viscera that you’ll learn in gross anatomy lab. Fig 9 below illustrates some common areas of referred pain from the abdominal viscera and Table 1 (next page) relates them to spinal level and regional organization. Fig 8 (Netter) Fig 9 (Netter) Gross Anatomy: ANS Abdomen Page 9 of 10 Dr. Paul Walker Table 1 below (Modified from Netter’s Clinical Anatomy) SPINAL CORD ANTERIOR ABDOMINAL REGION ORGAN LEVEL OR QUADRANT Stomach T5-T9 Epigastric or left hypochondrium Spleen T6-T8 Left hypochondrium Duodenum T5-T8 Epigastric or right hypochondrium Pancreas T7-T9 Inferior part of epigastric Liver or gallbladder T6-T9 Epigastric or right hypochondrium Jejunum T6-T10 Umbilical Ileum T7-T10 Umbilical Umbilical or right lumbar or right Cecum T10-T11 lower quadrant Umbilical or right inguinal or right Appendix T10-T11 lower quadrant Ascending colon T10-T12 Umbilical or right lumbar Sigmoid colon L1-L2 Left lumbar or left lower quadrant Kidney T10-L2 Lower hypochondrium or lumbar Ureter T11-L2 Lumbar to inguinal (loin to groin) No need to memorize Table 1. Just use it to understand some general principles of referred pain from the abdominal viscera. Why is liver or gallbladder pain sometimes referred to the shoulder? (Hint- return to Fig 7) III. Enteric Nervous System Fig 10 (Netter) Postganglionic SANS fibers exit their respective prevertebral ganglia and travel to the gut wall with branches of the intestinal arteries (Fig 10). Preganglionic PANS branches of the vagus (Fig 10) and pelvic splanchnic nerves (not shown) also travel with the arteries to the gut wall along with the sympathetic fibers. The target of SANS and PANS fibers is the enteric nervous system (also called the enteric plexus): a network of neurons located in the gut wall that controls peristalsis and gland secretion. The enteric nervous system is modified by both SANS and PANS systems. Gross Anatomy: ANS Abdomen Page 10 of 10 Dr. Paul Walker Fig 11 (Grays Anatomy for Students) Enteric Plexus (cont’d) Made of 2 parts (Fig 11): Myenteric Plexus (Auerbach’s) is a network of neurons located between longitudinal and circular muscle layers of the GI tract (from esophagus to rectum). Submucosal Plexus (Meissner’s) is a network of neurons located in submucosa of the GI tract (from esophagus to rectum). Both plexuses are connected and they are modulated by ANS input. Visceral afferents including reflex (vagal) afferents also located in plexus. Local sensory neurons within the network are critical to sensing information from the gut wall and organizing how other plexus neurons control peristalsis and secretions. You’ll receive more functional information on the enteric plexus in a physiology lecture on Gastrointestinal Motility. A. Hirschsprung’s Disease (Congenital Megacolon) HD is caused by a genetic defect that results in the failed migration of neural crest cells which form the enteric plexus in the distal regions of the hindgut. Affected areas lack the enteric plexus and are called ‘aganglionic’ regions of the bowel. Both Auerbach’s and Meissner’s plexuses are missing from these regions and peristalsis does not occur. The problem causes bowel constriction of the aganglionic regions and distension (Fig 12) and hypertrophy of proximal regions that contain a functional enteric plexus. Surgery is required to correct the problem. Fig 12 (Netter’s Clinical Anatomy)