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Fifty years ago 1 mg propecia with visa hair loss in men 1 syndrome, three or four days of antibiotic prophylaxis was rec- ommended in advance of a dental or surgical procedure buy propecia 5mg amex hair loss regrowth shampoo, whereas 1 Sources: (1–5) effective propecia 1mg hair loss jacksonville. On the other hand, individuals with rheumatic valvular disease should be given prophylaxis for den- tal procedures and for surgery of infected or contaminated tissues. While this can be used as an adjunct just prior to dental procedures, it should never replace the use of antibiotics for appropriate indications for prevention. A list of dental and other procedures for which endocarditis prophy- laxis is, or is not, recommended is given in Tables 12. This 103 104 is because of the likely presence of penicillin-resistant microorgan- isms, particularly in the upper respiratory tract and oral cavity of patients receiving oral penicillin. However, some authorities believe that a change to a macrolide or clindamycin is more effective for endocarditis prophylaxis. Summary Infective endocarditis remains a signiﬁcant cause (many times unsus- pected) of cardiovascular morbidity and mortality. Although there are no data from controlled studies to support the use of antibiotic prophylaxis to prevent infective endocarditis, it remains the accepted medical/dental standard of care. Clearly, antibiotics have been shown to be able to prevent bacteraemia following dental extraction. Fur- thermore, proper laboratory facilities and clinical acumen are re- quired to reduce the occurrence of this complication of rheumatic heart disease. American Heart Association Committee on the Prevention of Rheumatic Fever, Endocarditis and Kawasaki Disease. Recommendations for prevention, diagnosis and treatment of infective endocarditis. New criteria for diagnosis of infective endocarditis: utilization of speciﬁc echocardiographic ﬁndings. Prospects for a streptococcal vaccine Early attempts at human immunization Attempts to prevent group A streptococcal infections by immuniza- tion date back to the early years of the twentieth century (1–4). Efforts to develop a vaccine against group A strep- tococci were placed on a ﬁrmer scientiﬁc footing with the recognition that the principal virulence factor of group A streptococci was M- protein, a streptococcal wall constituent (5), and that opsonic anti- bodies to M-protein protected animals from lethal challenge. Such antibodies persisted for many years in humans (6) and appeared to be the basis of acquired type-speciﬁc immunity (7). Nevertheless, at- tempts to develop a safe and effective M-protein vaccine encountered considerable difﬁculties because of the multiplicity of M-protein sero- types (and genotypes), the toxicity of early M-protein preparations, and the immunological cross-reactivity between M-protein and hu- man tissues, including heart tissue (8) and synovium (9). M-protein vaccines in the era of molecular biology Although our knowledge of the structure and function of M-protein has advanced considerably in recent years (11–15), M-protein pre- parations used in vaccines are still not free of epitopes that elicit immunological cross-reactivity with other human tissues. Antibodies against M-proteins, for example, cross-react with alpha-helical human proteins, such as tropomyosin, myosin and vimentin. Primary struc- ture data have revealed that M-proteins of rheumatogenic streptococ- cal serotypes, such as serotypes 5, 6, 18 and 19, share similar sequences within their B-repeats, and it is likely that such sequences are responsible for eliciting antibodies that cross-react with epitopes in the heart, brain and joints (16). Most of the cross-reactive M- protein epitopes appear to be located in the B-repeats, the A-B ﬂanking regions, or the B-C ﬂanking regions, all of which are some distance from the type-speciﬁc N-terminal epitopes (16–18). In contrast, antibodies raised against synthetic N-terminal peptides that correspond to the hypervariable portions of M-protein serotypes 5, 6 and 24 are opsonic, but do not cross-react with human tissue (17– 19). Further studies have shown that peptide fragments of M- 106 proteins, incorporated into multivalent constructs as hybrid proteins or as individual peptides linked in tandem to unrelated carrier pro- teins, elicited opsonic and mouse-protective antibodies against mul- tiple serotypes, but did not evoke heart-reactive antibodies (20, 21). These estimates were based on sero- type distribution data from economically developed western coun- tries, and such a vaccine might need to be reconstituted, based on prevalent local strains. Current studies are directed toward utilizing commensal gram-positive bacteria as vaccine vectors (22–23). One of these is C5a peptidase, an enzyme that cleaves the human chemotactic factor, C5a, and thus interferes with the inﬂux of polymorphonuclear neutrophils at the sites of inﬂammation (24). Intranasal immunization of mice with a defective form of the streptococcal C5a peptidase reduced the colo- nizing potential of several different streptococcal M-serotypes (25). A second potential vaccine target is streptococcal pyrogenic exotoxin B (SpeB), a cysteine protease that is present in virtually all group A streptococci. Mice passively or actively immunized with the cysteine protease lived longer than non-immunized animals after infection with group A streptococci (26). Epidemiological considerations Once a safe and effective streptococcal vaccine is available many practical issues would need to be addressed. Other issues, such as cost, route of administration, number and frequency of required doses, potential side-effects, stability of the material under ﬁeld conditions, and dura- bility of immunity, would all inﬂuence the usefulness of any vaccine. The most promising approaches are M-protein-based, including those using multivalent type-speciﬁc vaccines, and those directed at non-type-speciﬁc, highly conserved portions of the molecule. Success in developing vaccines may be achieved in the next 5–10 years, but this success would have to contend with important questions about the safest, most economical and most efﬁcacious way in which to employ them, as well as their cost-effectiveness in a variety of epidemilogic and socio-economic conditions. A review of past attempts and present concepts of producing streptococcal immunity in humans. Intravenous vaccination with hemolytic streptococci: its inﬂuence on the incidence of rheumatic fever in children. Persistence of type-speciﬁc antibodies in man following infection with group A streptococci. Epitopes of group A streptococcal M protein shared with antigens of articular cartilage and synovium. Rheumatic fever: a model for the pathological consequences of microbial-host mimicry. Streptococcal M protein: alpha-helical coiled-coil structure and arrangement on the cell surface. Alternate complement pathway activation by group A streptococci: role of M-protein. Inhibition of alternative complement pathway opsonization by group A streptococcal M protein. Streptococcal infections: clinical aspects, microbiology, and molecular pathogenesis. Type-speciﬁc immunogenicity of a chemically synthesized peptide fragment of type 5 streptococcal M protein. Multivalent group A streptococcal vaccine designed to optimize the immunogenicity of six tandem M protein fragments. Protection against streptococcal pharyngeal colonization with a vaccinia:M protein recombinant. Intranasal immunization with C5a peptidase prevents nasopharyngeal colonization of mice by the group A Streptococcus. Vaccination with streptococcal extracellular cysteine protease (interleukin-1 beta convertase) protects mice against challenge with heterologous group A streptococci. Acute rheumatic fever in Auckland, New Zealand: spectrum of associated group A streptococci different from expected. Adding to the burden on health systems of developing countries are the costs of outside referrals that are often required during the course of treatment. The socioeconomic costs were also borne by the parents of the patients, with 22% exhibiting absenteeism from work, and about 5% losing their jobs.
Testes produce testosterone generic 5mg propecia mastercard hair loss cream; it regulates the production of sperm and stimulates the development and maintenance of male sexual characteristics propecia 1mg hair loss in men 4 christ. Erythropoietin is a hormone release by the kidneys and liver that can stimulate red blood cell production propecia 1mg without prescription hair loss cure bald truth. If stress is extreme, unusual, or long-lasting, the normal homeostatic mechanism may not be sufficient. They may be any disturbance such as heat, cold, environmental poisons, toxins, raging infection, heavy bleeding or strong emotional reaction. K is partly responsible for controlling the water concentration of the + cytosol and as cells lose more K , they function less effectively. Stress-related disorders include: gastritis, ulcerative colitis, irritable bowel syndrome, peptic ulcers, hypertension, asthma, rheumatoid arthritis, migraine headaches, anxiety, and depression. Stress may increase susceptibility to infection by temporarily inhibiting certain components of the immune system. System works by a negative feedback mechanism and remember that immunosuppressant drugs are effective with mechanism. First published 2002 by Blackwell Science Ltd Reprinted 2002 Library of Congress Cataloging-in-Publication Data Faiz, Omar. The anatomical drawings are the work of Jane Fallows, with help No longer do medical students have to spend long hours in the dissect- from Roger Hulley, who has transformed our rough sketches into the ing room searching fruitlessly for the otic ganglion or tracing the small finished pages of illustrations that form such an important part of the arteries that form the anastomosis round the elbow joint. They now book and we should like to thank her for her patience and skill in carry- need to know only the basic essentials of anatomy with particular ing out this onerous task. Some of the drawings have been borrowed or emphasis on their clinical relevance and this is a change that is long adapted from Professor Harold Ellis’s superb book Clinical Anatomy overdue. However, students still have examinations to pass and in this (9th edn) and we are most grateful to him for his permission to do this. Finally, it is a pleasure to thank the standard format of the at a Glance series and is arranged in short, all the staff at Blackwell Science who have had a hand in the prepara- easily digested chapters, written largely in note form, with the appro- tion of this book, particularly Fiona Goodgame and Jonathan Rowley. Where necessary, clinical appli- cations are included in italics and there are a number of clinical Omar Faiz illustrations. We thus hope that this book will be helpful in revising and David Moffat consolidating the knowledge that has been gained from the dissecting room and from more detailed and explanatory textbooks. Preface 5 1 The thoracic wall I Thoracic outlet (inlet) First rib Clavicle Suprasternal notch Manubrium 5 Third rib 1 2 Body of sternum Intercostal space 4 Xiphisternum Scalenus anterior Brachial Cervical Costal cartilage plexus rib Costal margin 3 Subclavian 1 Costochondral joint Floating ribs artery 2 Sternocostal joint Fig. The outlet (inlet) of the thorax is outlined Transverse process with facet for rib tubercle Demifacet for head of rib Head Neck Costovertebral T5 joint T6 Facet for Tubercle Costotransverse vertebral body joint Sternocostal joint Shaft 6th Angle rib Costochondral Subcostal groove joint Fig. The thoracic cage is formed by the sternum and costal cartilages in • The 10th rib has only one articular facet on the head. They articulate posteriorly with the vertebrae by way of a single facet It is separated from the abdominal cavity by the diaphragm and com- on the head. It articulates teriorly and with the sternum anteriorly by way of the costal cartilages inferiorly with the body of the sternum at the manubriosternal joint. All have a single synovial joint except Atypical ribs (1st, 2nd, 10th, 11th, 12th) for the 2nd which is double. The head • The costochondral joints (between ribs and costal cartilages) are prim- bears a single facet for articulation. A neurological deﬁcit as well as vascu- 1st and 10th–12th ribs have a single synovial joint with their corres- lar insufﬁciency arise as a result of pressure from the rib on the lowest ponding vertebral bodies. These include the: • Sensory branches from the pleura (upper nerves) and peritoneum • External intercostal: this muscle ﬁlls the intercostal space from the (lower nerves). The ﬁbres run down- • The 1st intercostal nerve is joined to the brachial plexus and has no wards and forwards from rib above to rib below. The 2nd intercostal the posterior intercostal membrane which reaches as far back as the nerve consequently supplies the skin of the armpit and medial side of vertebral bodies. The ﬁbres of these muscles span more than one inter- The diaphragm separates the thoracic and abdominal cavities. It lies between the The muscular part has three component origins: internal intercostal and innermost intercostal muscle layers. The intercostal structures course under cover of the subcostal The right crus arises from the front of the L1–3 vertebral bodies and groove. Vascular supply and venous drainage of the chest wall The medial arcuate ligament is made up of thickened fascia which The intercostal spaces receive their arterial supply from the anterior overlies psoas major and is attached medially to the body of L1 and lat- and posterior intercostal arteries. The lateral arcuate ligament is • The anterior intercostal arteries are branches of the internal thoracic made up of fascia which overlies quadratus lumborum from the trans- artery and its terminal branch the musculophrenic artery. The median arcuate ligament is a ﬁbrous arch which connects left • The ﬁrst 2–3 posterior intercostal arteries arise from the superior and right crura. The lower nine posterior inter- •Asternal part: consists of two small slips arising from the deep sur- costal arteries are branches of the thoracic aorta. Openings in the diaphragm The anterior intercostal veins drain anteriorly into the internal thor- Structures traverse the diaphragm at different levels to pass from acic and musculophrenic veins. Lymph drainage from the: • T10, the oesophageal opening: transmits the oesophagus, vagi and • Anterior chest wall: is to the anterior axillary nodes. Only the upper six intercostal nerves run in their inter- • Motor supply: the entire motor supply arises from the phrenic nerves costal spaces, the remainder gaining access to the anterior abdominal (C3,4,5). Middle mediastinum Heart and roots of great vessels Anterior mediastinum Pericardium Thymus Posterior mediastinum Oesophagus Descending thoracic aorta Thoracic duct Fig. The superior mediastinum communicates with the root of the neck The dual drainage of the lower third forms a site of portal-systemic through the ‘thoracic inlet’. In advanced liver cirrhosis, portal pressure rises result- the manubrium, posteriorly by T1 vertebra and laterally by the 1st rib. They are predisposed to rupture, causing potentially life-threatening • Middle mediastinum: consists of the pericardium and heart. The incidence of adenocarcinoma of agus passes initially through the superior and then the posterior medi- the lower third of the oesophagus is currently increasing for unknown astina. From here, it insertion of stents and use of lasers to pass through tumour obstruction passes downwards and forwards to reach the oesophageal opening in have become the principal methods of palliation. It is situated between the abdom- • A double muscular layeralongitudinal outer layer and circular inal aorta and the right crus of the diaphragm. The muscle is striated in the upper two-thirds and • The thoracic duct carries lymph from the cisterna chyli through the smooth in the lower third. The internal thoracic artery divides behind the 6th The ascending aorta arises from the aortic vestibule behind the costal cartilage into superior epigastric and musculophrenic branches. The arch lies posterior to the lower half of • 2nd part: the part of the artery that lies behind scalenus anterior. It the manubrium and arches from front to back over the left main gives rise to the costocervical trunk (see Fig.
The latter function may be speciﬁ- in body cavities cheap 1mg propecia mastercard hair loss 7 months postpartum, be spilt on the ﬂoor and absorbed cally used as a ‘disconnection alarm’ generic 1mg propecia overnight delivery hair loss 9 reasons. In paediatric practice buy propecia 5mg cheap hair loss 6 months after surgery, where small volumes of blood loss are relatively more Many other physiological parameters can be, and important, all absorbent materials are washed to are, monitored during anaesthesia when appropri- remove the blood and the resultant solvent as- ate. Cephalic vein The anaesthetic record On every occasion an anaesthetic is administered, a comprehensive and legible record must be made. The details and method of recording will vary with each case, the type of chart used and the equip- Dorsal metacarpal veins ment available. Laterally these are joined by • ﬂuids administered and lost: type and volume; veins from the thumb and continue up the radial • use of local or regional anaesthetic techniques; border of the forearm as the cephalic vein (Fig. These have the advantage of allowing the large vein in the middle of the ventral (anterior) anaesthetist to concentrate on caring for the pa- aspect of the forearm—the median vein of the tient, particularly during an emergency, rather forearm (Fig. The antecubital fossa The cephalic vein passes through the antecubital Intravenous cannulation and ﬂuid fossa on the lateral side and the basilic vein enters administration the antecubital fossa very medially, just in front of Intravenous cannulation is used to allow: the medial epicondyle of the elbow. These veins are •drugs to be given to induce and maintain joined by the median cubital or antecubital vein (see anaesthesia; Fig. Veins in this region tend to be used • ﬂuids to be given to maintain or restore the either in an emergency situation or when attempts patient’s circulating volume; to cannulate more peripheral veins have failed. Some de- proximity and easily damaged by needles or vices have ﬂanges or ‘wings’ to facilitate attach- extravasated drugs. All cannulae have a standard Luer-lock ﬁtting for attaching a giving set and some have a valved injection port through Equipment which drugs can be given (Fig. Devices of different lengths and diameters are • Seldinger type This is used predominantly to used; the term ‘cannula’ is used for those 7cm or achieve cannulation of the central veins (see less in length, and ‘catheter’ for those longer than below), but peripheral devices are available, de- 7cm. The main types of cannulae used are: Technique for cannulation of a • Cannula over needle The most popular device, peripheral vein available in a variety of sizes, most commonly 14 gauge (2. The other end of the relatively mobile and capable of considerable vari- needle is attached to a transparent ‘ﬂashback ation in their diameters. This reduces pain, • The junction of two veins is often a good site and makes the patient less likely to move and less as the ‘target’ is relatively larger. Often a ture site to prevent bleeding, and made worse by slight loss of resistance is felt as the vein is entered forgetting to remove the tourniquet! This indicates that the tip of the The degree of damage to the overlying tissues needle is within the vein. This ensures that the ﬁrst part of technique and lack of knowledge of the local the plastic cannula lies within the vein. However, a cannula may pre- •W ithdraw the needle 5–10mm into the cannula vent this and allow air to enter the circulation. Most likely following cannulation of a central vein As this is done, blood may be seen to ﬂow between (see below). The safest action is to withdraw the whole • The cannula and needle should now be ad- cannula and re-attempt at another site. The needle is re- • Thrombophlebitis Related to the length of time tained within the cannula to provide support and the vein is in use and irritation caused by the sub- prevent kinking at the point of skin puncture (Fig. There are many different types of equipment and ap- Complications proaches to the central veins, and the following is Most are relatively minor but this must not be used intended as an outline. A ﬂexible guidewire is then passed down Access to the central veins the needle into the vein and the needle carefully withdrawn, leaving the wire behind. The catheter The antecubital fossa is now passed over the wire into the vein, some- This route has a relatively low success rate, but times preceded by a dilator. The advantage of this fewer complications, the most important of which method is that the initial use of a small needle in- is thrombophlebitis after prolonged use (>48h). This approach is associated with the highest inci- dence of success (95%), and a low rate of complica- Fluid ﬂow through a cannula tions (Table 2. The right internal jugular offers certain advantages: there is a ‘straight line’ to the This determined by four factors: heart, the apical pleura does not rise as high on this • Internal diameter Theoretically, ﬂow is propor- side, and the main thoracic duct is on the left. This is rarely achieved in practice, but This can be approached by both the supra- and in- an increase of four- to ﬁvefold will be seen. Both are technically more dif- • Length Flow is inversely proportional to the ﬁcult than the internal jugular route and there is a length of the cannula—doubling the length will signiﬁcant incidence of causing a pneumothorax halve the ﬂow. The main advantage of this route is com- • Viscosity Flow is inversely proportional to the fort for the patient during long-term use. Colloids and blood ﬂow more slowly than a must not be made because of the risk of airway crystalloid, particularly when they are cold. If such ﬂuids are used to restore the circulating volume, three to four times the deﬁcit Always use a large-diameter, short cannula during will need to be given. If crystalloids containing a resuscitation as the rate of ﬂow is determined primarily lower concentration of sodium than plasma (e. Intravenous ﬂuids extracellular and intracellular volumes), and as During anaesthesia ﬂuids are given intravenously little as 10% will remain intravascular. Crystalloids to replace losses due to surgery and provide the are used primarily either as an emergency resusci- patient’s normal daily requirements. Three types tation ﬂuid or to provide a patient’s daily require- are used: crystalloids, colloids, and blood and its ments of water and sodium. Colloids Crystalloids These are suspensions of high molecular weight These are solutions of crystalline solids in water. However, they have a ﬁnite life in the plasma and will eventually be either metabolized or ex- Risks of intravenous blood and creted and therefore need replacing. In the intraoperative period the between exposure and the development of anti- most commonly used are red cell products, platelet bodies. In order to try and eliminate these risks, Each unit contains approximately 510mL with a techniques now exist for using the patient’s own haematocrit of 35–45%. Each unit prior to surgery, the patient builds up a bank contains 250mL with a haematocrit of 60–75%, of two to four units of blood for retransfusion and is hence very viscous with a poor ﬂow rate. Each unit contains 300mL • Cell savers These devices collect blood lost dur- with a haematocrit of 50–70%. Given The type and volume of ﬂuid administered during via a standard giving set without the use of a surgery varies for each and every patient, but must microaggregate ﬁlter, as this will result in the loss take into account: of signiﬁcant numbers of platelets. Any 60 Anaesthesia Chapter 2 deﬁcit due to fasting is predominantly water from Fluid losses from the ﬁrst two causes are difﬁcult to the total body water volume. If evaporative is calculated at the normal daily maintenance rate losses are considered excessive, then 4% glucose of 1. Although this deﬁcit can be replaced with should be replaced with a solution similar in com- a ﬂuid such as 4% glucose plus 0. The other main cause of a preoperative deﬁcit is volume required is proportional to surgical trauma losses either from or into the gastrointestinal tract. Blood pres- This ﬂuid usually contains electrolytes and effec- sure, pulse, peripheral perfusion and urine output tively depletes the extracellular volume. It is best will give an indication as to the adequacy of re- replaced with a crystalloid of similar composition, placement, but in complex cases where there are particularly in respect of the sodium concentra- other causes of ﬂuid loss, particularly bleeding, the tion; for example 0. Blood loss is slightly more obvious and easier to Acute blood loss preoperatively can be replaced measure.
A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices order propecia 1 mg fast delivery hair loss legs men. Axons from these ganglionic neurons (postganglionic nerve fibers - dotted lines) then project to target effectors throughout the body order propecia 5 mg amex hair loss in men 200. To continue with the analogy of the circuit diagram generic propecia 5 mg otc hair loss in men will trichomoniasis, there are three different types of “junctions” that operate within the This OpenStax book is available for free at http://cnx. The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (the organ, tissue, or gland to be innervated). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called white rami communicantes (singular = ramus communicans); they are myelinated and therefore referred to as white (see Figure 15. The axon from the central neuron (the preganglionic fiber shown as a solid line) synapses with the ganglionic neuron (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea—via gray rami communicantes, which are unmyelinated axons. In some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (see Figure 15. The cervical ganglia are referred to as paravertebral ganglia, given their location adjacent to prevertebral ganglia in the sympathetic chain. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see Figure 15. Collateral ganglia, also called prevertebral ganglia, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons. They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion (see Figure 15. The word celiac is derived from the Latin word “coelom,” which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured). Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion. The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10–20 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs. Parasympathetic Division of the Autonomic Nervous System The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord. The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 15. The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.