Innervation of Thoracic and Abdominal Viscera PDF

# INNERVATION OF THORACIC AND ABDOMINAL VISCERA ## OBJECTIVES: - Describe the sympathetic and parasympathetic input to and location of the major plexuses in the thorax and abdomen. - Understand the functional significance of the autonomic innervation of viscera in the thoracic and abdominal region...

# INNERVATION OF THORACIC AND ABDOMINAL VISCERA ## OBJECTIVES: - Describe the sympathetic and parasympathetic input to and location of the major plexuses in the thorax and abdomen. - Understand the functional significance of the autonomic innervation of viscera in the thoracic and abdominal regions. - Describe the distribution of the vagus nerve and pelvic splanchnic nerves. - Understand the pathway taken by visceral pain. ## INNERVATION OF THORACIC VISCERA A nerve plexus is a network of intermingled nerve fibers originating from various nerves. Innervation of thoracic viscera is from autonomic plexuses that receive input from parasympathetic and sympathetic sources. The parasympathetic input comes entirely from the vagus nerves and is preganglionic. Sympathetic fiber input comes from postganglionic neurons in sympathetic trunk ganglia of the upper five thoracic vertebrae and the superior, middle, and inferior cervical ganglia. ## Innervation of the Heart The heart receives autonomic innervation from the cardiac plexus, which has superficial and deep components. The superficial cardiac plexus lies under the concavity of the arch of the aorta; the deep cardiac plexus is located anterior to the bifurcation of the trachea within the fibrous pericardium. Communicating fibers join both plexuses. Parasympathetic motor (GVE) contributions to these plexuses are preganglionic fibers from cervical cardiac and thoracic cardiac branches from both vagus nerves. Most of these parasympathetic fibers synapse in ganglia in the walls of the atria and coronary arteries. Sympathetic motor contributions (also GVE) to the cardiac plexuses are postganglionic fibers from sympathetic trunk ganglia of T1-T5 spinal cord segments (thoracic cardiac branches) and from the superior, middle, and inferior cervical ganglia (cervical cardiac branches). Parasympathetic sensory (GVA) fibers originate from the inferior ganglion of the vagus, while sympathetic sensory (GVA) fibers originate from posterior root ganglia of T1-T5 nerves. Sympathetic and parasympathetic fibers follow cardiac vasculature to subsidiary plexuses and effector structures. Sympathetic stimulation tends to increase the heart rate and strength of contraction (atrial plexuses) as well as dilation of coronary arteries (coronary plexuses). Parasympathetic stimulation has the opposite effects. ## Innervation of the Lungs Each lung receives autonomic (GVE & GVA) innervation from pulmonary plexuses located anterior and posterior to the roots of the lungs. The anterior pulmonary plexus, a subsidiary plexus of the deep cardiac plexus, is located on the anterior aspect of tracheal bifurcation at the root of the lung. The posterior pulmonary plexus is a ganglionated plexus posterior to the root of the lung with connections to the anterior pulmonary plexus and cardiac plexus. Parasympathetic contributions (preganglionic) are from the cardiac plexus and directly from the vagus nerves as they pass posterior to the bronchi. Sympathetic contributions (postganglionic) are from the cardiac plexus and direct branches from sympathetic trunk ganglia of T1-T6 spinal cord segments. Parasympathetic stimulation tends to cause bronchoconstriction and increased mucus secretion, while sympathetic stimulation has opposite effects. ## Innervation of the Esophagus The esophageal plexus receives parasympathetic contributions (preganglionic) from the vagus nerves. Upon reaching the inferior two thirds of the esophagus, both left and right vagus nerves separate into fascicles to form most of this plexus. Sympathetic components (postganglionic) arise from thoracic ganglia (T1-T10 spinal cord segments) adjacent to the esophagus; there may be some contribution from the cardiac plexus or from branches ascending from abdominal plexuses. Sensory components (GVA) travel with both vagal and sympathetic nerves. Near the esophageal hiatus in the diaphragm, the nerve fascicles reunite. Due to the rotation of the stomach during development, the left vagus now lies anterior to the stomach and the right vagus lies posterior. Thus, they are now called anterior and posterior vagal trunks. ## Thoracic Plexus | Thoracic Plexus | Sympathetic Contribution | Parasympathetic Contribution | |---|---|---| | Cardiac | Postganglionic fibers from cervical and thoracic (T1-T5) ganglia | Preganglionic vagal fibers from cervical and thoracic branches | | Pulmonary | Postganglionic fibers from cardiac plexus and thoracic (T1-T6) ganglia | Preganglionic vagal fibers from cardiac plexus and thoracic branches | | Esophageal | Postganglionic fibers from thoracic (T1-T10) ganglia | Preganglionic vagal fibers | ## INNERVATION OF ABDOMINAL VISCERA Innervation of abdominal (and pelvic) viscera is from a great continuous mass of autonomic fibers that lie principally on the abdominal aorta and its major branches (see Autonomic Nerve Plexuses of Abdomen Learning Tool on Elentra). Sympathetic motor (GVE) contributions are from preganglionic thoracic splanchnic nerves (greater, lesser, and least) and lumbar splanchnic nerves. The thoracic splanchnic nerves originate from neurons in the intermediolateral cell columns at T5-T12 spinal cord segments, while the lumbar splanchnic nerves originate from neurons at L1-2. These sympathetic fibers usually synapse in ganglia within the plexuses; the postganglionic neurons then follow branches of the aorta for distribution. Parasympathetic contributions are preganglionic fibers from the vagus nerves and pelvic splanchnic (S2-S4) nerves. Parasympathetic fibers usually synapse in ganglia within the walls of the viscera. Sensory fibers generally travel with both sympathetic and parasympathetic components. The aortic plexus is a ganglionated plexus that is the largest of the prevertebral (preaortic) plexuses and is found at the T12-L3 vertebral levels. Ganglia and subsidiary plexuses associated with the aortic plexus are named for arterial branches with which they are anatomically associated (celiac, renal, superior mesenteric, inferior mesenteric, etc.). Largest of the named ganglia is the celiac ganglion (adjacent to the celiac trunk) that receives sympathetic fibers from the greater splanchnic nerves (T5-T9). Some fibers from the greater splanchnic nerve do NOT synapse in the celiac ganglion but continue as preganglionic fibers to synapse in more lateral or inferior ganglia (e.g., superior mesenteric or aorticorenal), or synapse directly in the medulla of the suprarenal gland (where they synapse within chromaffin cells of the suprarenal gland). Parasympathetic fibers from the anterior vagal trunk generally bypass the celiac plexus and distribute directly to the stomach, duodenum, etc., while fibers from the posterior vagal trunk pass through the plexus – but do not synapse there – to reach the same effector organs. Continuing inferiorly, the aorticorenal plexus (adjacent to the renal arteries) usually receives preganglionic sympathetic fibers from the lesser splanchnic nerves (T10 & T11). This plexus (or the adjacent and more lateral renal plexus) also receives preganglionic fibers from the least splanchnic nerves (T12). Like the greater splanchnic nerves, most of the preganglionic fibers in the lesser and least splanchnic nerves synapse in these plexuses, but some may continue to synapse in other plexuses. Parasympathetic components are continuations of preganglionic vagal fibers from the celiac plexus, passing through without synapse. Fibers of the aortic plexus between the superior and inferior mesenteric plexuses (and their associated ganglia) are called the intermesenteric plexus. This plexus receives additional preganglionic sympathetic fibers (L1 and L2) from the upper two lumbar splanchnic nerves. The preganglionic parasympathetic fibers in this plexus represent the distal extent of vagal innervation and supply the large intestine to the left colic (splenic) flexure. The last named subsidiary plexus of the aortic plexus is the inferior mesenteric plexus, which receives preganglionic parasympathetic components from pelvic splanchnic nerves via the superior hypogastric plexus and hypogastric nerves (see "Vasculature, Innervation, and Functional Anatomy of Pelvis and Perineum"). The lower two lumbar splanchnic nerves feed into the superior hypogastric plexus (not considered as part of the aortic plexus). These lower lumbar splanchnic nerves are also formed by preganglionic L₁ and L2 fibers leaving the spinal cord (inferior extent of sympathetic outflow) but then descend the sympathetic trunks to exit at vertebral levels L3 and L4. ## Abdominal Plexus | Abdominal Plexus | Sympathetic Contribution | Parasympathetic Contribution | |---|---|---| | Celiac | Greater splanchnic nerves (T5-T9) | Vagus nerves | | Aorticorenal | Lesser splanchnic nerves (T10 & T11) | Vagus nerves (from celiac plexus) | | Aorticorenal / Renal | Least splanchnic nerves (T12) | Vagus nerves (from celiac plexus) | | Superior Mesenteric | Continuation of fibers from celiac plexus | Vagus nerves (from celiac plexus) | | Intermesenteric | Upper two lumbar splanchnic nerves (L1 & L2) | Vagus nerves (from superior mesenteric plexus) | | Inferior mesenteric | Upper two lumbar splanchnic nerves (L1 & L2) through intermesenteric plexus | Pelvic splanchnic nerves (S2-4) (from superior hypogastric plexus and hypogastric nerves) | | Superior hypogastric | Lower two lumbar splanchnic nerves (L₁ & L2) | Pelvic splanchnic nerves (S2-4) (from hypogastric nerves) | ## SUMMARY Nerves to thoracic and abdominal viscera travel along vessels supplying various organs. Preganglionic sympathetic (splanchnic) contributions to the abdominal plexuses usually synapse on ganglion cells within these plexuses. Postganglionic fibers subsequently distribute along the vessels. Preganglionic parasympathetic fibers generally do not synapse on postganglionic cells of the plexus but rather distribute along vessels to synapse on postganglionic nerve cell bodies in the walls of organ to be innervated. ## CLINICAL ANATOMY In general, visceral pain from all organs within the thoracic and abdominal cavity travels back to the spinal cord along the precise reverse of the pathway that brought sympathetic supply to these organs. Thus, pain returns to the same spinal cord segment supplying the preganglionic sympathetic neurons for the organ. This anatomical arrangement is believed to underlie the concept of referred pain, where pain from viscera is perceived as coming from the somatic area of the body supplied by those same spinal cord segments. This referred pain pattern can often map to dermatomes that represent the same spinal cord levels. Some specific examples are: - Sensory fibers mediating pain from the heart accompany cervical and thoracic cardiac sympathetic nerves from T1-5 spinal cord levels. This heart pain is often referred to the 1st-5th intercostal spaces and medial aspect of the upper limb. - Pain fibers from the gall bladder enter the spinal cord at T7-9 spinal cord levels. This pain is often referred to the 7th-9th intercostal spaces. - Pain fibers from the appendix enter the spinal cord at T10-11 levels. This pain is often referred to the umbilical region. Peristalsis of the gastrointestinal tract is controlled by parasympathetic fibers. Vagal parasympathetic fibers stimulate acid secretion by the parietal cells of the stomach. Transection of the vagal trunks is often done to reduce excess acid formation associated with peptic ulcer. ## SAMPLE QUESTIONS 1. The major fiber component of the greater splanchnic nerve is: a. Preganglionic sympathetic. b. Postganglionic sympathetic. c. Preganglionic parasympathetic. d. Postganglionic parasympathetic. e. Branchiomotor. 2. Transection of the vagus nerve in the upper thorax would least affect which of the following organs? a. Esophagus. b. Heart. c. Stomach. d. Gall bladder. e. Kidney. 3. Sympathetic preganglionic efferent fibers to the appendix originate from spinal cord levels T10-11. What is the most likely location of referred pain? a. Right shoulder. b. Umbilical region. c. Inferior border of the scapula. d. Anterior neck. e. Inguinal region. ## ANSWERS 1. The answer is **A**. Sensory fibers also travel in the greater splanchnic nerve. 2. The answer is **B**. The parasympathetic cardiac fibers from the vagus arise in the cervical region and travel toward the cardiac plexus independent from the vagus proper. 3. The answer is **B**. # ABDOMINAL INNERVATION A diagram of the abdominal innervation is included. It depicts the following: - Anterior vagal trunk - Posterior vagal trunk - Celiac ganglion - Greater splanchnic nerves - Lesser splanchnic nerves - Least splanchnic nerves - Superior mesenteric ganglion - Aorticorenal ganglion - Right kidney - Left kidney - Renal plexus - Upper lumbar splanchnic nerve - Sympathetic ganglion and trunk - Lower lumbar splanchnic nerve - Intermesenteric plexus - Inferior mesenteric ganglion - Superior hypogastric plexus - Hypogastric nerves