HBF-II LEC 02 Gross Anatomy ANS Thorax Lecture Notes 2024 Walker PDF

Gross Anatomy: ANS Thorax 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 functional anatomy of the thoracic autonomics differentiating both...

Gross Anatomy: ANS Thorax 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 functional anatomy of the thoracic autonomics differentiating both sympathetic and parasympathetic systems. 2. Describe the transmission of thoracic visceral afferent information and relate to baroreceptor/chemoreceptor reflexes and the clinical phenomenon of referred pain. Session Outline I. Autonomic Innervation of the Thoracic Viscera A. Functional Summary of the Thoracic Autonomics B. Gross Location of the Thoracic Autonomics C. Schematic Organization of the Thoracic Autonomics 1. Autonomic Innervation of the Heart 2. Autonomic Innervation of the Lungs II. Visceral Afferents within the Cardiopulmonary Plexus A. Visceral Afferents for Reflexes B. Visceral Afferents for Pain Gross Anatomy: ANS Thorax Page 2 of 10 Dr. Paul Walker I. Autonomic Innervation of the Thoracic Viscera A. Functional Summary of the Thoracic Autonomics This lecture focuses on the autonomic innervation of the heart and lungs. ANS innervation of esophagus will be discussed with the ANS abdomen lecture given later in the HBF-II course. Function of Autonomic Visceral Motor Innervation to Heart & Lungs: Cardiac muscle exhibits spontaneous contractility with pacemaker control exerted by the SA and AV nodes. The autonomic nerves modify the rate and strength of each contraction of cardiac muscle, thus controlling cardiac output. SANS increases heart rate and strength of contraction. PANS decreases heart rate and strength of atrial contraction. Smooth muscle of the bronchial tree is innervated by autonomic nerves and this changes the lumen patency of the airways. SANS causes bronchodilation and PANS causes bronchoconstriction. Autonomic innervation of mucus glands and blood vessels also provides moisture and warmth to the airways. Visceral Sensory fibers carry information from stretch (volume) baroreceptors that sense changes in distension of the walls of the airways and major vessels (aorta arch, carotid arteries). Also carry information from chemoreceptors located in the carotid arteries that detect O2 and CO2 levels in the blood. There are also visceral sensory fibers that transmit pain signals from the heart and lungs representing tissue damage (or potential damage). All visceral sensory information travels to the CNS along the nerve plexuses that also carry autonomic motor fibers to the viscera. As such, an autonomic nerve plexus (e.g. cardiac plexus) contains both visceral motor and visceral sensory fibers. B. Gross Location of the Thoracic Autonomics Fig 1 (Netter 213) The thoracic autonomics are mainly located in the posterior mediastinum and can be visualized with the heart and lungs removed (Fig 1). The thoracic sympathetic trunk is located adjacent to the attachment of the ribs to the thoracic vertebral bodies. Note that each thoracic sympathetic ganglion is anchored to its intercostal spinal nerve by a white ramus communicans laterally, and a gray ramus communicans medially. Look for what is labeled cardiac plexus and pulmonary plexus. Look for the left vagus nerve (CN X) as related to the aortic arch on the left side of the cadaver. The right vagus nerve is more difficult to see. Gross Anatomy: ANS Thorax Page 3 of 10 Dr. Paul Walker The right and left vagal nerves form the esophageal plexus, and while it is mostly made of branches of the vagus nerve, there are also sympathetic nerve fibers in the plexus. There is also something called splanchnic nerves (greater, lesser, and least) that appear to originate from the 5th through 12th thoracic sympathetic ganglia. We will identify these in lab but understand their significance later when we study the gastrointestinal (GI) system. Now let’s review autonomic innervation to the heart and lungs more schematically so you can better understand upcoming physiology and pharmacology lectures on cardiac and pulmonary function. C. Schematic Organization of the Thoracic Autonomics Fig 2 below summarizes the pre-ganglionic origin of PANS compared to SANS. This figure was used in the Intro ANS lecture given during the HBF-I course. Fig 2 (Gray’s Anatomy for Students) PANS The PANS is commonly referred to as the ‘craniosacral’ part of the autonomic nervous system. Cell bodies of pre-ganglionic PANS neurons are located in cranial nerve nuclei of the brainstem (cranial part) and the sacral spinal cord (sacral part). For the HBF-II course, we will only be concerned with cranial nerve X for the PANS system. CN X- Vagus Nerve: originates from the caudal brainstem (medulla). SANS The origin of the SANS is commonly referred to as the ‘thoracolumbar’ part of the autonomic nervous system. Cell bodies of preganglionic SANS neurons are located in a long cell column that runs vertically in the gray matter of the spinal cord from thoracic T1 level to lumbar L2 level (T1-L2). For innervation of the thoracic viscera (heart, lungs, esophagus), preganglionic sympathetic neurons are located in the upper half of the thoracic sympathetic cell column (T1-T6). T1-T4 for heart, T1-T5 for lungs, T5-6 for esophagus Gross Anatomy: ANS Thorax Page 4 of 10 Dr. Paul Walker Below Fig 3 shows the preganglionic & postganglionic organization of the thoracic autonomics: Fig 3 Compare the location of pre-ganglionic cell bodies for SANS vs. PANS. Compare the location of post-ganglionic cell bodies for SANS vs. PANS. What neurotransmitters do pre-ganglionic SANS vs. PANS axons release? What neurotransmitters do post-ganglionic SANS vs. PANS axons release? What receptors respond to transmitter(s) released by post-ganglionic SANS vs. PANS axons? Gross Anatomy: ANS Thorax Page 5 of 10 Dr. Paul Walker 1. Autonomic Innervation of the Heart Fig 4 below summarizes the pre- and post-ganglionic sympathetic innervation of the heart. Preganglionic axons destined to regulate the cardiopulmonary system project out of the upper thoracic spinal cord (T1-T4) and enter the sympathetic trunk via white rami communicans at the same spinal level as their cell body origin. Once in the trunk, they: Synapse in the ganglia at the same level entered. Pass through the ganglia and ascend in the sympathetic trunk to synapse in the cervical sympathetic ganglia. Fig 4 (Gray’s Anatomy for Students) Postganglionic SANS fibers do not return to spinal nerves via gray rami communicans, but instead exit the ganglia anteriorly and join the cardiac plexus of nerves. These nerves can be found in both neck and thorax regions. (Neck surgeons need to be aware of these nerves to the heart.) The targets of postganglionic SANS fibers are located in the epicardium where the SA & AV nodes and conduction system are innervated. Axons also travel with the coronary arteries to reach the ventricular myocardium and smooth muscle of the coronary arteries. SANS causes increased heart rate and increased force of contraction via beta- adrenergic (1, 2) receptors. SANS also induces coronary artery vasodilation in a mechanism that is coupled to increased metabolic demand of heart muscle. Clinical Note: Beta blockers are antihypertensive medicines that work through suppression of beta-adrenergic (1, 2) receptor-mediated signaling. Gross Anatomy: ANS Thorax Page 6 of 10 Dr. Paul Walker The below Fig 5 adds the contribution of preganglionic PANS innervation to the cardiac plexus. The PANS fibers join the SANS fibers to form the cardiac plexus of autonomic nerves to the heart. Fig 5 (Gray’s Atlas of Anatomy) Preganglionic PANS axons originate from the caudal brainstem (medulla) and travel in CN X (vagus nerve). This nerve exits the skull superiorly and travels in the neck inferiorly toward the thorax. Within the neck and more inferiorly in the thorax, the vagus nerve gives off branches that join the cardiac plexus. These are still preganglionic PANS axons. Preganglionic PANS axons synapse in terminal PANS ganglia located deep to the epicardium of the heart. Postganglionic PANS fibers innervate specialized cardiac muscle in SA and AV nodes and thus affect atrial contractility. PANS causes decreased heart rate and decreased force of atrial contraction via muscarinic M2 receptors. PANS also reduces coronary artery blood flow due to decreased metabolic demand of heart muscle. Gross Anatomy: ANS Thorax Page 7 of 10 Dr. Paul Walker 2. Autonomic Innervation of the Lungs Fig 6 (Netter 214) Fig 6 shows a schematic of the pulmonary plexus of autonomic nerves. Location of pre-ganglionic SANS are similar to heart (sympathetic cell column in upper thoracic spinal cord). Fig 6 shows T1-T5 contribution which is reported. As with the heart, the preganglionic SANS axons regulating the lung either synapse in the sympathetic ganglion at the same level or ascend in the sympathetic trunk until they reach the cervical sympathetic ganglia. As with the heart, the postganglionic SANS axons regulating the lung exit the anterior surface of the ganglia without reentering spinal nerves and join the nerves of the cardiac plexus to travel to lungs. PANS preganglionic axons from the vagus nerve destined to innervate the lungs distal to the primary bronchus diverge from the cardiac plexus and distribute along anterior and posterior surfaces of the primary bronchi. They track along the pulmonary tree distally to reach the terminal bronchioles. PANS preganglionic axons of the vagus that innervate proximal parts of the pulmonary system (larynx and trachea) do not travel with the other nerves of the pulmonary plexus, but instead travel with vagal branches known as the superior laryngeal nerve (to superior larynx) and the recurrent laryngeal nerve (inferior larynx and trachea). We will see the recurrent laryngeal nerve in gross anatomy lab during this course, but will study its function in more detail later. Postganglionic SANS fibers innervate bronchiolar and vascular smooth muscle of the pulmonary tree as well as mucus glands. The receptors used are: 2 adrenergic receptors enhance airflow by producing bronchodilation.  adrenergic receptors reduce mucus secretion (via vasoconstriction). Clinical Note: Inhalant bronchodilators that act at 2 adrenergic receptors are commonly used to treat asthma. These agents are mixed with corticosteroids, which are incorporated to blunt the inflammatory response. Postganglionic PANS fibers also innervate bronchiolar and vascular smooth muscle of the pulmonary tree as well as mucus glands. The lung receptors that bind acetylcholine are: M3 muscarinic receptors constrict bronchiole smooth muscle to produce bronchoconstriction and also increase mucus secretion. Gross Anatomy: ANS Thorax Page 8 of 10 Dr. Paul Walker II. Visceral Afferents within the Cardiopulmonary Plexus Fig 7 https://www.memorangapp.com/flashcards/157699/Cardiovascular%2BPhysiology%2B-%2BBaroreceptors%2Band%2Bchemoreceptors A. Visceral Afferents for Reflexes Visceral afferents carry 2 types of sensory information from the heart. 1. Reflex- Fibers from stretch mechanoreceptor (baroreceptors) and chemoreceptors located in the great vessels (aortic arch, carotid bifurcation) travel with the vagus nerve (CN X) and glossopharyngeal nerve (CN IX) to the brainstem where they will modify cardiovascular and respiratory reflexes. Cell bodies of sensory neurons located in ganglia located close to the brainstem (base of skull). Stretch receptors in the terminal bronchioles are part of the Hering-Breuer reflex activated during inspiration when the lungs expand to take in air. 2. Irritant- some sensory fibers detect irritation to the pulmonary tree (illustrated in previous Fig 6). These are associated with the vagus nerve and participate in the afferent limb of the cough reflex as well as other autonomic responses to modulate mucus secretion within the pulmonary system. Clinical Applications When a person moves from reclining to standing posture, the resultant pooling of blood in the lower half of the body is countered by vasoconstriction in certain organs to drive blood to appropriate areas (e.g. brain) while also increasing cardiac output. This baroreceptor reflex functions as a buffer to maintain blood pressure during sudden changes in posture. Failure of this reflex is known as orthostatic hypotension- a severe drop in blood pressure when the patient assumes the upright position. A patient with this problem becomes dizzy and may faint (reflex syncope) due to decreased blood flow to the brain. As shown above in Fig 7, VA fibers carry information proximally to the brainstem from baroceptors located in the aortic sinus and the carotid sinus. These fibers are part of CN X (vagus) for aortic sinus and CN IX (glossopharyngeal) for the carotid sinus. Brainstem neurons involved in the baroreceptor reflex send signals to regulate both PANS and SANS pre- ganglionic neurons. The net effect is to increase cardiac output and also increase the resistance in the vascular beds of skeletal muscle and abdominal visceral organs, but not in the vessels of the heart and brain. Certain diseases produce pathophysiology of the baroreceptor reflex: Familial Dysautonomia- loss of visceral afferent fibers traveling in the vagus nerve. Multiple System Atrophy- loss of preganglionic SANS in the spinal cord. Gross Anatomy: ANS Thorax Page 9 of 10 Dr. Paul Walker The chemoreceptor reflex, also shown above in Fig 7, maintains homeostasis of blood gases by regulating respiration, cardiac output and blood flow through peripheral vessels. VA fibers return information from the chemoreceptors located in the carotid bodies and aortic bodies via CN IX & CN X. This detects changes in partial oxygen pressure (PO 2) and partial carbon dioxide pressure (PCO2). A brainstem chemoreceptor reflex pathway activates SANS when PO2 drops and PCO2 rises resulting in an increase in cardiac output and increased peripheral vascular tone. These changes also decrease blood flow to skeletal muscle and viscera, but blood flow to brain remains unchanged preserving vital functioning of the CNS. The cardiovascular component of the chemoreceptor reflex is closely matched with respiration, which is a somatic motor function mediated by the phrenic nerve. The somatic motor cell bodies of the phrenic nerve are located at spinal levels C3-5 and are controlled by a descending respiratory pathway from the brainstem. Under different circumstances, this system regulates the cardiovascular system in ways that you wouldn’t predict. For example, holding the breath (e.g. while under water) does not increase heart rate when PCO 2 levels rise, but rather causes the heart rate to decrease. Fig 8 (Gray’s Anatomy for Students) B. Visceral Afferents for Pain: 1. Pain- Fibers carrying pain information from the thoracic viscera are in the same autonomic plexuses that contain SANS and PANS fibers (Fig 8). Cell bodies of these VA neurons are found in the in dorsal root ganglion (DRG) at upper thoracic spinal levels. There are also somatic afferent (SA) sensory neuronal cell bodies in these DRGs that transmit cutaneous information from skin dermatomes. Therefore, the spinal cord receives both visceral afferent and somatic afferent pain signals. Gross Anatomy: ANS Thorax Page 10 of 10 Dr. Paul Walker Fig 9 (Gray’s Anatomy for Students) Clinical Applications Damage to the thoracic visceral causes VA fibers to send signals to the spinal cord as shown in Figs 8-9. The information enters the cord and synapses on spinal sensory neurons that also receive somatic afferent (SA) information from skin dermatomes. As such, a patient with a damaged organ such as the heart (depicted in Fig 9) feels pain coming from the body surface in upper thoracic dermatome regions. This is called ‘referred pain’ and is clinically valuable as a diagnostic tool for various diseases. The example shown in Fig 9 depicts how VA signals from damaged heart tissue cause pain feelings ‘referred’ to dermatomes of the left arm, shoulder, and chest. It should be noted that there are gender variations in the perception of referred pain that you will learn as part of your clinical training. Others types include: Referred pain from the lungs is typically reported to the shoulder (scapular) region. Pain from the diaphragm is referred more superiorly to the neck region because the afferent pain fibers from the diaphragm travel with motor fibers of the phrenic nerve (C3-C5). The feeling of ‘heart burn’ related to acid reflux of esophagus is likely related to referred pain to medial T5-T6 dermatome regions. Later, we’ll discuss other types of referred pain coming from abdominal organs (e.g. gall bladder, appendicitis, kidney stone, etc) as related to GI and urinary systems.