Introduction to Autonomic Nervous System Structure PDF

Dr. Sabrina Belle [email protected] Read about it! Gray’s Anatomy for Students 4th Edition Pages 29-48 (covers somatic and autonomic nervous systems and Table 8.4 covers the axon fiber types) 1 Objectives 1. 2. 3. 4. 5. 6. 7. 8. Describe the basic structure and pathway of the autonomic (visceral)...

Dr. Sabrina Belle [email protected] Read about it! Gray’s Anatomy for Students 4th Edition Pages 29-48 (covers somatic and autonomic nervous systems and Table 8.4 covers the axon fiber types) 1 Objectives 1. 2. 3. 4. 5. 6. 7. 8. Describe the basic structure and pathway of the autonomic (visceral) nervous system. o What are the different options an axon can travel in the basic pathway? Describe where the cell bodies are located and the pathway of pre and post ganglionic axons in the sympathetic nervous system. o How does the pathway differ if axons are innervating blood vessels or glands in the periphery vs. visceral organs in the thorax and abdomen? Differentiate between the pathways of axons innervating blood vessels or glands in the periphery versus visceral organs in the thorax and abdomen. Describe where the cell bodies are located and the pathway of pre and post ganglionic axons in the parasympathetic nervous system. Describe the splanchnic nerves, their origination in the spinal cord and what system they belong to (sympathetic or parasympathetic). Describe why the Vagus is important and what system the nerve belongs to. Describe the enteric nervous system and its role in the autonomic nervous system. Recall the somatic (voluntary) nervous system – what neurons and axons are involved and what it ultimately achieves. 2 Image 1 Nervous System Components Somatic Nervous System Skin (sensation) / Bones / Skeletal muscle Image 2 Autonomic Nervous System Organs / Glands / Smooth muscle Glands 3 Nervous System Components Somatic Nervous System Autonomic Nervous System Skin (sensation) Bones Skeletal muscles Organs Glands Smooth muscle Voluntary Motor Somatic Sensory Visceral Motor Visceral Sensory (efferent) (afferent) (efferent) (afferent -axons ) are called -axons are called General Somatic Efferent (GSE) -axons are called General Somatic Afferent (GSA) -axons are called General Visceral Efferent (GVE) Sympathetic General Visceral Afferent (GVA) Enteric Parasympathetic 4 Autonomic (Visceral) Motor Pathway Image 3 *Autonomic (visceral) motor pathways have a 2-neuron chain from the spinal cord, starting in the lateral horn of the spinal cord. *From the cell body in the lateral horn, a preganglionic axon will travel to a ganglia (group of cell bodies in the PNS) where a synapse will occur. *From the post-ganglionic cell bodies, a post-ganglionic axon will then travel to the target organ – a visceral / autonomic structure! *The general "rule" for the autonomic nervous system, be it sympathetic or parasympathetic, is that pre-ganglionic axons are myelinated and post-ganglionic axons are unmyelinated. Image 4 Pre-ganglionic axon Pre-ganglionic Cell Body (in lateral horn of spinal cord - grey matter) Post-ganglionic axon Post-ganglionic cell body (star) (in ganglia of the peripheral ner vous system) (PNS) =Synapse 5 Autonomic Nervous System “Rule of 2’s” Image 5 *There are 2 divisions: 1-Sympathetic 2-Parasympathetic *Each division arises from 2 regions of the CNS: 1-Sympathetic = Thoracolumbar (Pre-ganglionic cell bodies located in the lateral horn of Thoracic & Lumbar regions (T1-L2) 2-Parasympathetic = Craniosacral Pre-ganglionic cell bodies associated with cranial nerves III, VII, IX, X and in cell bodies in S2-S4 in the spinal cord *Each system has a 2-neuron chain from either the spinal cord or cranial nerve nuclei 6 Sympathetic Division (ANS) Image 6 *Now as we know, the sympathetic nervous system has cell bodies from T1-L2 spinal cord regions (lateral horn grey matter) *The axons originating in these cell bodies are different than voluntary motor axons, because they are visceral and need to travel internally to synapse on ganglia and ultimately a visceral structure. We call these axons General Visceral Efferent (GVE) type axons. *And they have a different path then General Somatic Efferent (GSE) type of axons! 7 Spinal Cord & Associated Structures Image 7 Paravertebral Ganglion (collectively these form the Sympathetic Chain of ganglia) Dorsal Root Dorsal primar y rami Dorsal Root Ganglion Ventral primar y rami Ventral Root Sympathetic Chain Gray Rami Communicans Splanchnic nerve Prever tebral ganglion White Rami Communicans 8 Image 8 Paravertebral Ganglion (These form the sympathetic chain!) Prevertebral Ganglion 9 Information Flow (Visceral) Image 9 Dorsal Primary Rami Grey Matter (Cell bodies) Spinal Ner ve Ventral Primary Rami Grey Rami Communicans White Rami Communicans Paravertebral Ganglion Prever tebral Ganglion DRG = Dorsal Root Ganglion 10 Autonomic Pathway *Pre-ganglionic (GVE) axons leave the lateral horn, travel in the ventral root, but then exit the spinal nerve in the white rami communicans to reach a paravertebral ganglion (sympathetic chain). *From the paravertebral ganglion, pre-ganglionic (GVE) axons then travel on splanchnic nerves to reach another ganglia (prevertebral) where they synapse. Post-ganglionic axons then leave that prevertebral ganglion and travel to their target visceral organ. *If a sympathetic nerve’s “job” is to innervate blood vessels or sweat glands of the skin, those axons will synapse in the paravertebral ganglion and then travel on the gray rami communicans to leave the sympathetic chain and enter the spinal nerve. *One exception is the innervation of the adrenal gland – there is no synapse in the prevertebral ganglion – the preganglionic axons synapse directly in the adrenal medulla 11 Sympathetic Innervation of Visceral Organs in the Thorax, Abdomen, Pelvis (but NOT Heart and Lungs!) 12 Sympathetic Innervation of the Heart & Lungs (T1-T4) Cardiopulmonary Splanchnic Nerves The heart and lungs are mainly innervated by T1-T4 through cardiopulmonary splanchnic nerves and a synapse can occur in the paravertebral ganglion. However, if you want to get more detailed, efferent sympathetic innervation to the heart and lungs can also consist of some nerves originating in the superior, middle and inferior cervical ganglion as well as the T1-T4 segments. So, the nerves from T1-T4 have their own route and may be accompanied by nerves from the cervical ganglion. All of these ner ves then branch to for m smaller nerves that then wind themselves around the vessels and myocardium. 13 Innervation of Glands and Blood Vessels in the Skin *Note that ONLY the sympathetic nervous system does this! 14 Options in the Sympathetic Autonomic Pathway 1 Innervation of blood vessels and glands in the skin: Axons can enter the sympathetic chain, synapse in the paravertebral ganglion and exit on the spinal nerve at the same level 2 Innervation of blood vessels and glands in the skin: Axons can enter the sympathetic chain and either ascend or descend to synpase in a paravertebral ganglion and exit on the spinal nerve 3Innervation of visceral organs: Axons can enter the sympathetic chain, the signal can pass through the paravertebral ganglia, at the same level and then exit on a splanchnic nerve to synapse in a prevertebral ganglion 4 Innervation of visceral organs: Axons can enter the sympathetic chain and either ascend or descend to pass through a paravertebral ganglion at another level and exit on a splanchnic nerve, to then synapse in a prevertebral ganglion. Image 10 15 Sympathetic Division of the Autonomic Nervous System (ANS) *T1-T4 axons travel to sympathetically innervate the heart and lungs – after they synapse in the paravertebral ganglion, they form cardiopulmonary splanchnic nerves *For T5 and below, after passing through the paravertebral ganglion (no synapse), certain preganglionic axons will bundle together to form splanchnic nerves! *T5-T9 axons form the Greater Splanchnic Nerve *T10-T11 axons form the Lesser Splanchnic Nerve *T12 axons form the Least Splanchnic Nerve *L1-L2 axons form the Lumbar Splanchnic Nerves Image 11 *Sacral Splanchnic Nerves originate from the sympathetic chain and network with the Lumbar Splanchnic Nerves 16 Sympathetic Division (ANS) *Paravertebral ganglia extend as two chains from the base of the skull to the ganglion impar on the anterior aspect of the coccyx where the two chainsmerge. *There are cervical, thoracic, lumbar and sacral regions of ganglia. *The splanchnic nerves (from the paravertebral ganglia) then course to prevertebral ganglia (also called pre-aortic) surrounding the large unpaired vessels on the abdominal aorta. There the signal will synapse. *Greater Splanchnic ➔ Celiac Ganglia Lesser Splanchnic ➔ Superior Mesenteric Ganglia *Least Splanchnic & Lumbar Splanchnics ➔ Inferior Mesenteric Ganglia *Sacral Splanchnics (originate in the sacral part of the sympathetic chain) ➔ Network with lumbar splanchnic nerves *From the ganglion surrounding the blood vessel, the axons are able to “wind” themselves around the vessels and travel with them to the target visceral organ! 17 Sympathetic Division (ANS) =synapse *The sympathetic nervous system is said to have a “Short preganglionic component” and a “Long postganglionic component”. *The picture illustrates why this is true – the length of the axon from the lateral horn in the spinal cord to the prevertebral ganglia synapse is “short” whereas the post- ganglionic axons, from the prevertebral ganglia, traveling to reach the visceral organs are “long” because they have to travel a “long” way to reach their destination. 18 Sympathetic Division (ANS ) Image 12 *Visceral structures in the head do require sympathetic innervation – but there are no sympathetic cell bodies in the brainstem! *Therefore, cell bodies in thoracic levels will send their axons to the cervical ganglion and then post- ganglionic axons travel as the carotid plexus, a collection of nerve fibers around the Internal Carotid Artery. As the artery gives off branches, sympathetic fibers piggyback on the smaller vessels to get to 19 Parasympathetic Division (ANS) =synapse *Parasympathetic = Craniosacral *Pre-ganglionic cell bodies are located in brainstem nuclei of cranial nerves III, VII, IX, X, and in cell bodies of S2-S4 in the spinal cord Image 13 *The parasympathetic nervous system is said to have a “long pre-ganglionic component” and a “short post-ganglionic component”. *The length of the axon from the cranial nerve nucleus or sacral spinal cord level is “long” whereas the post-ganglionic axons, from the associated ganglia, traveling to reach the visceral organs are “short” *The “Splanchnic nerves” carrying parasympathetic axons from the sacral spinal cord levels are called “Pelvic Splanchnic Nerves” 20 22 Parasympathetic Division *Each cranial nerve that carries parasympathetic axons has an associated ganglion as follows: • CN III = Ciliary ganglion • CN VII = Pterygopalatine ganglion & o Submandibular ganglion CN IX = Otic ganglion • CN X = ganglia near the innervated organ *Vagus is called the “wanderer” for it supplies a myriad of visceral organs in the head and then “wanders” and travels as far caudally as the left colic flexure of the large intestine, (see red arrow). *The Vagus carries preganglionic parasympathetic axons to ganglia close to the visceral organs to be innervated. Image 14 *Pre-ganglionic parasympathetic axons that will form Pelvic Splanchnic nerves leave the ventral root before the spinal nerve is formed, hence there are NO parasympathetic fibers in any spinal nerve. The GVE axons then travel to 21 Vagus Nerve - Parasympathetic Division Image 15 22 On slide 21 it states, “Pre-ganglionic parasympathetic axons that will form Pelvic Splanchnic nerves leave the ventral root before the spinal nerve is formed, hence there are NO parasympathetic fibers in any spinal nerve.” What does this mean? *If we look at the image to the left and say that it represents a parasympathetic signal originating from S2S4. What is the target? Organs / viscera distal to the left colic flexure. *So, do these signals need to travel in a spinal nerve? Nope…they travel as we see here. *Spinal nerves would only have GVE sympathetic fibers as those fibers innervate blood vessels and sweat glands of the skin. There is no “job” for a parasympathetic axon to travel in any part of a spinal23 nerve. Enteric Nervous System *The enteric nervous system is the intrinsic nervous system of the Gastrointestinal Tract. *Contains two neural plexuses: 1Submucosal (Meiss ner’s) Plexus – the deeper plexus between the circular muscle and luminal mucosa. Regulates GI blood flow and epithelial function 2Myenteric (Auerbac h’s) Plexus – the more superficial plexus between the longitudinal and circular layers of the muscularis externa. Aids in motility, contraction and relaxation of smooth muscle. 24 Enteric Nervous System – “The Second Brain” *The enteric nervous system is a "mini-brain" as it contains all the elements of a nervous system including: o Sensory neurons: Respond to mechanical, thermal, osmotic, and chemical stimuli o Interneurons: Integrate information from sensory neurons and feedback to the enteric motor neurons o Motor neurons: Control motility, secretion, and absorption by acting on smooth muscle and secretory cells *The enteric nervous system communicates extensively with the autonomic nervous system, but is capable and does operate on its own, independent from the central nervous system. Image 16 Image 17 25 General Functions of the Autonomic Nervous System Sympathetic Parasympathetic *“Fight or Flight” responses *“Rest and Digest" responses *Increases heart rate and the contractility of cardiac cells, increasing the blood flow to skeletal muscles / vasodilator for the coronary vessels. *Can decrease heart rate and blood pressure *Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange. *Dilates pupils and relaxes the ciliary muscle to the lens *Inhibits digestion by diverting blood flow away from the gastro-intestinal tract / inhibits peristalsis. *Constricts intestinal sphincters and the urinary sphincter. *Promotes ejaculation / orgasm *Can constrict the bronchiolar diameter when the need for oxygen decreases. *Constricts the pupil and contracts the ciliary muscle of the lens, allowing for accommodation. *Increases digestion by dilating blood vessels of the GI tract, increasing blood flow / enhances peristalsis and stimulates salivary gland secretion *Promotes erection of genitals and sexual arousal. 26 End! 27