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These patients are at significant bleeding risk postoperatively due to extensive aortic suture lines buy fluconazole online antifungal body lotion, and adequate blood products must be available for hemostasis early in the postoperative course order 50mg fluconazole overnight delivery antifungal prescription. The early phase is catecholamine-mediated and usually best treated with β-blocking agents such as esmolol or labetalol purchase fluconazole once a day fungus plural. The pulmonary arteries are sometimes patch augmented, and any intracardiac issues, that is, significant atrioventricular valve regurgitation, or atrial septectomy, addressed. Analgesics such as morphine, acetaminophen, ibuprofen, and ketorolac are effective; the adjustment of the cerebral circulation is accompanied by diminution of these symptoms, usually after 48 to 72 hours. The relatively large brain of the young infant carries a large percentage of the cardiac output. This in turn decreases superior vena cava flow, decreasing pulmonary artery flow, which will lead to decreased systemic arterial oxygenation. Fontan Completion Total cavopulmonary connection is the intended final stage of palliation for most single-ventricle patients. The surgical procedure itself has undergone many modifications since its original description by Fontan and Baudet in 1971 (Fig. A fenestration is sometimes created from the Fontan circuit to the left atrium, which is a 3- to 5-mm communication allowing a right to left shunt, which lowers Fontan circuit pressure and allows greater systemic cardiac output at the expense of some arterial desaturation (286). It should be noted that many Fontan completions, even without fenestration, are not fully saturated due to the position of the coronary sinus in the left atrium. Because the total cavopulmonary connection results in the absence of a pulmonary ventricle to actively pump blood into the lungs, flow in this circuit depends in large part on negative intrathoracic pressure creating a gradient from the extrathoracic systemic venous return to the cavae and heart. Therefore, positive pressure ventilation will decrease venous return and cardiac output. Many centers will extubate the Fontan patient in the operating room and limit the dose of opioids to promote venous return and maintain cardiac output. The Fontan patient is highly dependent on adequate filling volume in the total cavopulmonary connection in the early postoperative period to maintain cardiac output. In the ventilated patient, the effect of the positive pressure can be overcome with higher filling pressures, that is, 15 to 20 mm Hg. A key point in the early postoperative period is not to get behind in volume replacement; bleeding from repeat sternotomy and large pleural and mediastinal tube output from increased right-sided pressures often result in significant need for volume replacement. Inotropic support in the form of milrinone, and low-dose epinephrine or dopamine is often needed in the early postoperative course. Nonfenestrated patients especially are prone to high Fontan pressures and pleural effusions; conversely, they will benefit the most from early extubation. Differential diagnosis of abnormal post-Fontan hemodynamic states is presented in Table 24. Other Left-Sided Obstructive Lesions Aortic stenosis, mitral stenosis, and Shone complex patients present with a myriad of anatomic variations; the preoperative status of the patient, including ventricular dysfunction and/or hypertrophy, and any medications including β-blockade, must be reviewed. Details of the surgery, whether valve repair with subaortic resection, or valve replacement, must be communicated. With normal or hyperdynamic left ventricular function preoperatively, relief of mechanical obstruction will often result in a hypertensive patient and β-blockade with esmolol should be considered, along with other vasodilator therapy, that is, nitroprusside or calcium channel blocker such as nicardipine in older patients. It is important to control blood pressure, as there will frequently be a long aortic suture line and bleeding or dehiscence can accompany uncontrolled elevated blood pressure. Patients with poor ventricular function may require significant inotropic support. Other important considerations are the status of the coronary arteries, and whether there is proximal coronary artery atresia and right ventricular–dependent coronary sinusoids. Cardiac Transplantation The preoperative status of the transplantation patient must be thoroughly reviewed and understood, particularly in reference to pulmonary hypertension, which can complicate the postoperative course. Generally speaking, ischemic times less than 5 hours are desirable, with better myocardial function in the early postoperative period. It is also important to understand if the patient has elevated panel reactive antibodies, or a positive cross match with the donor heart. In such cases, intraoperative or postoperative plasmapheresis may be instituted to minimize risk of early rejection. The denervated heart after transplantation will not respond to parasympathetic inputs and therefore vagolytic agents such as atropine and pancuronium will not increase the heart rate. Uncomplicated transplants, that is, first time sternotomy, no pulmonary hypertension, good donor heart function, minimal bleeding, can usually be extubated 12 to 24 hours postoperatively. Transplants complicated by bleeding, pulmonary hypertension, complex reconstructions, or small infants may require prolonged postoperative ventilation and inotropic support. Institutional approaches and protocols for sedation and analgesia are highly desirable, to standardize care as much as possible and to decrease the incidence of undertreated pain and patient distress from anxiety. The ventilated postsurgical patient will require significant doses of analgesic and sedative drugs, most often in the form of opioids (morphine, fentanyl), which should be given as continuous infusions or on a scheduled basis after major surgery. Similarly, sedation for anxiolysis and to prevent awareness is often provided by benzodiazepines (midazolam, lorazepam). Muscle relaxation is rarely indicated, except where patient movement or coughing will interfere with care, that is, ventilation. A peripheral nerve stimulator placed over the ulnar nerve should be used and state of neuromuscular blockade assessed and titrated frequently. Administration of unnecessarily large doses of sedatives, analgesics, and muscle relaxants can lead to a polyneuropathy syndrome characterized by prolonged muscular weakness that may significantly complicate weaning from support (295,296). The minimum effective doses necessary to achieve desired sedation and analgesia should be used. Neuraxial anesthesia, in the form of thoracic epidural or paravertebral catheters, with continuous local anesthetic infusions with or without opioids, is often very effective for thoracotomy pain. Continuous infusions of sedative agents may be necessary for difficult to sedate patients. Propofol should never be used for this purpose in children, because of its clear association with propofol infusion syndrome in children receiving large doses for significant periods of time in intensive care units. This syndrome is characterized by mitochondrial failure, severe myocardial dysfunction, acidosis, and cardiovascular collapse leading to death (298,299). This agent should be used with caution in young infants due to delayed hepatic clearance, and in patients with bradycardia, or who are hemodynamically unstable. Despite its potential respiratory benefits, dexmedetomidine was not shown to facilitate early extubation in a recent retrospective review (304). Additional agents that may be used include ketamine, especially for painful procedures; barbiturates, and chloral hydrate. Risk for this syndrome is generally higher with continuous infusions of large doses of potent synthetic opioids, that is, fentanyl and infusions of short acting benzodiazepines, that is, midazolam. Low-dose naloxone infusion may be effective at preventing tolerance syndromes (307). Also, intravenous acetaminophen, ibuprofen, or ketorolac are now available, and should be considered for their opioid sparing effects when no contraindications exist. Patients should be assessed for withdrawal syndromes using the various semi-objective grading scales, and if established, a plan to gradually wean the sedative and analgesic drugs, that is, 5% to 10% per day, and substitute long acting drugs (methadone and lorazepam) or drugs from other classes (barbiturates, transdermal clonidine patch), for withdrawal symptoms. Consultation from an acute pain service expert is recommended for difficult cases.
Evaluation of the lymphatic circulation after Fontan operation reveals marked abnormalities with tortuous dilated lymphatic channels coursing through the abdomen and chest as they drain lymph away from tissues (94) effective fluconazole 50 mg fungus vs eczema. Thoracic duct diameters are markedly increased and obstruction due to channel distortion and scarring is common generic 50mg fluconazole overnight delivery fungal wart. Novel catheter–based techniques allow access to the lymphatic system through the use of very thin needles 200mg fluconazole sale fungus covered chest nagrand. Lymphatic channels draining lymph into the airway can be embolized, thereby providing a new treatment strategy for plastic bronchitis (96). While potentially life- saving, such lymphatic embolization or thoracic duct reimplantation techniques do not alter the primary deficiency of the Fontan circulation, that of chronic venous congestion and venous hypertension, the stimulus for lymphatic abnormality in the first place. Treatment strategies involving pharmacologic manipulation of the impedance to systemic venous flow through pulmonary vasodilation, or perhaps design of new ways to actively propel blood flow into the pulmonary circulation that would result in lower venous pressure and improved ventricular output are necessary in order to optimally manage the complications in patients born with one effective ventricle. Prevalence of congenital anomalies in newborns with congenital heart disease diagnosis. Critical congenital heart disease–utility of routine screening for chromosomal and other extracardiac malformations. Associated noncardiac congenital anomalies among cases with congenital heart defects. Congenital heart defects and major structural noncardiac anomalies in Alberta, Canada, 1995–2002. High prevalence of respiratory ciliary dysfunction in congenital heart disease patients with heterotaxy. Heterotaxy and complex structural heart defects in a mutant mouse model of primary ciliary dyskinesia. Increased postoperative respiratory complications in heterotaxy congenital heart disease patients with respiratory ciliary dysfunction. Epigenetic programming of hypoxic-ischemic encephalopathy in response to fetal hypoxia. Cardiovascular morbidity and mortality in Finnish men and women separated temporarily from their parents in childhood–a life course study. Impact of prenatal diagnosis in survivors of initial palliation of single ventricle heart disease: analysis of the national pediatric cardiology quality improvement collaborative database. Renal complications associated with the treatment of patients with congenital cardiac disease: consensus definitions from the Multi-Societal Database Committee for Pediatric and Congenital Heart Disease. Factors associated with acute kidney injury or failure in children undergoing cardiopulmonary bypass: a case-controlled study. Low renal oximetry correlates with acute kidney injury after infant cardiac surgery. Tabbutt S, Ghanayem N, Ravishankar C, et al; Pediatric Heart Network Investigators. Risk factors for hospital morbidity and mortality after the Norwood procedure: a report from the Pediatric Heart Network Single Ventricle Reconstruction trial. Renal function after cardiopulmonary bypass surgery in cyanotic congenital heart disease. Prevalence, predictors, and prognostic value of renal dysfunction in adults with congenital heart disease. Renal dysfunction is common among adults after palliation for previous tetralogy of Fallot. Gastrointestinal complications associated with the treatment of patients with congenital cardiac disease: consensus definitions from the Multi-Societal Database Committee for Pediatric and Congenital Heart Disease. Neonatal necrotizing enterocolitis: pathogenesis, classification, and spectrum of illness. Reducing the incidence of necrotizing enterocolitis in neonates with hypoplastic left heart syndrome with the introduction of an enteral feed protocol. Necrotizing enterocolitis in infants with ductal-dependent congenital heart disease. Necrotizing enterocolitis in neonates with congenital heart disease: risk factors and outcomes. Gastrointestinal morbidity after Norwood palliation for hypoplastic left heart syndrome. Diastolic flow parameters are not sensitive in predicting necrotizing enterocolitis in patients undergoing hybrid procedure. Abnormal abdominal aorta hemodynamics are associated with necrotizing enterocolitis in infants with hypoplastic left heart syndrome. Long-term survival, modes of death, and predictors of mortality in patients with Fontan surgery. End-organ consequences of the Fontan operation: liver fibrosis, protein-losing enteropathy and plastic bronchitis. Abnormal patterns of intraventricular flow and diastolic filling after the Fontan operation: evidence for incoordinate ventricular wall motion. Late ventricular geometry and performance changes of functional single ventricle throughout staged Fontan reconstruction assessed by magnetic resonance imaging. Hemodynamic performance of the Fontan circulation compared with a normal biventricular circulation: a computational model study. A cross-sectional study of exercise performance during the first 2 decades of life after the Fontan operation. Hepatic pathology may develop before the Fontan operation in children with functional single ventricle: an autopsy study. Prospective longitudinal study of coagulation profiles in children with hypoplastic left heart syndrome from stage I through Fontan completion. Coagulation factor abnormalities as possible thrombotic risk factors after Fontan operations. Protein-losing enteropathy after fontan operation: investigations into possible pathophysiologic mechanisms. Hepatic fibrosis and cirrhosis in the Fontan circulation: a detailed morphological study. Progression of liver pathology in patients undergoing the Fontan procedure: chronic passive congestion, cardiac cirrhosis, hepatic adenoma, and hepatocellular carcinoma. Computer-assisted image analysis of liver collagen: relationship to Ishak scoring and hepatic venous pressure gradient. Congestive hepatopathy after Fontan operation and related factors assessed by transient elastography. The precarious state of the liver after a Fontan operation: summary of a multidisciplinary symposium. Liver cirrhosis in Fontan patients does not affect 1-year post- heart transplant mortality or markers of liver function.
This again tients with long-term visual loss may not respond as well as improved her vision to 6/18 without further deterioration patients who present with more rapid visual loss due to optic (Fig discount fluconazole master card antifungal coconut oil. The second patient presented with an 8-month progres- Two patients with compressive lesions (lateral sphe- sive visual loss and at presentation could only see hand noid wing meningioma and an encircling fibrous dys- Fig fluconazole 150 mg for sale antifungal at home. The compressive lesion on the orbital apex and optic canal is indicated by the white arrows purchase fluconazole 50mg line antifungal bathroom cleaner. Injudicious use of inappropriate instruments has the potential to worsen the vision and this should be kept in mind during the procedure. Transethmoidal optic nerve contemplated unless there is an obvious bony fragment decompression. Endo- series presented in this chapter and from larger studies in scopic optic nerve decompression for traumatic blindness. Otolaryngol 1–4,10 Head Neck Surg 2000;123(1 Pt 1):34–37 the literature suggest that patients should be operated 3. Powered instrumentation in orbital and optic nerve mol 1994;38(6):487–518 decompression. Vasculature and morphometry of the the comatose and conscious patients after trauma. In last 10 years, new techniques have been introduced to aid addition, this route of access does not give access to the with the resection of tumors in regions which traditionally anterior and medial compartments of the maxillary sinus. As there is only a single fulcrum mors but, as techniques and adjuvant therapy develop, these around which the blade rotates, good access is provided to techniques will be increasingly applied to the resection of the medial and lateral walls, and the foor of the maxillary malignant tumors. Endoscopic resection of the medial maxilla is useful to access the anterior, posterior, and Endoscopic Medial Maxillectomy for Access to lateral walls of the maxillary sinus. The lateral Surgical Techniques for Access to the nasal wall and septum are infltrated with 2% lidocaine and Maxillary Sinus, Pterygopalatine Fossa, 1:80 000 adrenaline. A pterygopalatine fossa block is placed and Infratemporal Fossa via the greater palatine canal using 2 mL of lidocaine and adrenaline (see Chapter 2). This helps to reduce bleeding Canine Fossa Trephination for Access to the during the dissection of the medial wall of the maxilla and Maxillary Sinus (Videos 8 and 9) pterygopalatine fossa. The frst step in endoscopic medial maxillectomy is to Tumors that involve the medial wall, anterior foor, or an- remove the uncinate process and perform a large middle terior or anterolateral wall of the maxillary sinus cannot meatal antrostomy (Fig. The maxillary antrum is en- be accessed through a maxillary antrostomy irrespective of larged posteriorly up to the posterior wall of the maxillary how large this is made. Although this can orbital wall and allows removal of the residual medial max- be achieved through an inferior meatal puncture, place- illa without endangering the orbit. Most large tumors of the ment of a 4–mm microdebrider blade through the inferior maxillary sinus and/or pterygopalatine fossa will involve the meatal antrostomy tends to destabilize the inferior turbi- posterior ethmoids and sphenoid. This lary fap is performed and the frontal recess dissected with is because the nasal vestibule provides a fulcrum around exposure of the frontal ostium. The bulla ethmoidalis is re- which the blade is rotated, causing signifcant disruption moved and a posterior ethmoidectomy and sphenoidotomy 200 16 Endoscopic Resection of Tumors 201 A, B C Fig. The white arrow indicates the trephination port in the anterior face of the maxillary sinus. Any tumor large intranasal component of a soft nonvascular tumor, the extension into the anterior and posterior ethmoids can be tumor is debulked (Fig. If the tumor is very vascular or assessed and, if necessary, biopsies or frozen sections of the frm then it can be pushed superiorly or partially debulked. This Because of the posterior location of angiofbroma, debulking helps ensure complete tumor clearance. To perform the medial maxillectomy, the inferior tur- Turbinectomy scissors are used to cut along the crushed binate is medialized. A Tilley’s packing forceps is used to region of the inferior turbinate up to the point where the crush the turbinate just distal to the junction of the anterior turbinate inserts into the lateral nasal wall (Fig. Here the mucosal incision is turned ver- tically toward the posterior region of the maxillary sinus antrostomy. A sharp chisel is used to cut the bone under the mucosal incisions following the mucosal incision (Fig. The posterior vertical cut needs to enter the maxillary sinus adjacent to the posterior wall of the maxillary sinus and into the large antrostomy5 (Fig. Once the bone forming the medial maxillary wall is mobilized, the nasolacrimal duct will tether the bone an- teriorly and the duct will be visualized (Fig. Note the microdebrider blade that has been placed through the canine fossa trephine. Tumor can now be sinus, further resection of the anteromedial wall and frontal removed from the maxillary sinus under direct visualization. In such If additional access is required and the tumor does not attach cases a canine fossa trephine is not thought to be suitable due to the anterior wall of the maxillary sinus, a canine fossa to the small risk of seeding the tumor into the soft tissues of puncture can be performed. Although seeding is unlikely to occur, this risk is endoscope to be introduced through the anterior wall of the thought to be greater if the entry point into the maxillary sinus maxillary sinus which can be useful to access areas within the is through tumor rather than through normal mucosa. This access is achieved the required angle for dissection in difcult areas such as the by performing a hemitransfxion incision in the opposite nasal anterior wall or anterolateral region of the maxillary sinus. The instrument can then that attaches extensively to the anterior face of the maxillary be passed through the hemitransfxion incision, through the Fig. This allows the passage of a 70-degree dia- fxion incision anteriorly in the left nostril (contralateral side to the mond tipped drill (D). C, carti- right nasal cavity demonstrating the working tip of the drill passing into lage; F, fap. The mucosa giving greater access to the anterior wall of the maxillary sinus from the posterior wall of the maxillary sinus is elevated (Fig. This exposes the bone and removal of this of approach and usually allows complete access to the entire bone is necessary to expose the pterygopalatine fossa. The punch Access to the Pterygopalatine Fossa (Videos 41 and 42) is introduced into the sphenopalatine foramen and the bone anterior to the foramen removed until the posterior Access to the pterygopalatine fossa is achieved by remov- wall of the maxillary sinus is reached (Fig. In most cases removal of this bone can be done either with the punch a medial maxillectomy is unnecessary as most of the ptery- or with a 45-degree through-biting Blakesley. Bone is re- gopalatine fossa can be accessed through a large middle moved until the contents of the pterygopalatine fossa are meatal antrostomy. In addition the vidian nerve enters the posterior aspect of the fossa before moving laterally to end in the pterygopalatine ganglion which is suspended from the maxillary nerve (Fig. The pterygopala- tine fossa narrows gradually as it opens laterally into the region of the infraorbital fssure and pterygomaxil- lary fssure before widening into the infratemporal fossa (Figs. The roof of the pterygopalatine fossa is formed by the greater wing of the sphenoid bone and the infraorbital fssure, foramen rotundum, and the maxillary nerve coursing from the foramen rotundum from medial to lateral across the roof of the fossa just below the orbital apex (Figs. The frst fact to be appreciated is that the pterygopalatine fossa forms a relatively small part of the total area behind the posterior wall of the maxillary sinus Access to the Infratemporal Fossa (Videos 41 and 42) (Fig. Second the frst structures to be encountered when entering the fossa are the blood vessels (Fig. To access the infratemporal fossa, all of the bone of the pos- The neural structures all lie deep to this plexus of arteries terior and lateral wall of the maxillary sinus needs to be (Fig.
Due to the lack of action by city ofcials (your employees) order fluconazole 200mg fast delivery fungus gnats flowering, residents have organized into a coalition to alert the media and government ofcials to the increasing num- ber of illnesses and birth defects that have occurred in the subdivision (Beck buy fluconazole 200mg antifungal oral rinse, 1979) generic 50mg fluconazole mastercard fungus gnat larvae uk. City employees that have been reluctant to take action on this issue need to be reprimanded, reassigned, or terminated from their positions. The company that dumped the chemicals should be forced, through legal or legislative means, to clean up the chemical dumping site. The city manager should contact state and federal ofcials in a coordinated efort to resolve this issue. The public should be kept reassured that steps are being taken to resolve the issue and that the city will do everything in its power to correct a serious environmental oversight. Stage 4 of the Disaster The school that was built on top of the landfll has been demolished, but the school board and the corporation are refusing to admit any liability (University of Bufalo Libraries, 2007). The president of the United States has just declared the neighborhood a federal emergency and has relocated all residents out of Case Studies: Nuclear, Biotoxins, or Chemical ◾ 193 the subdivision (University of Bufalo Libraries, 2007). The city manager should be active in coordination of all activities in relocating residents to temporary housing that is satisfac- tory, as well as working closely with federal ofcials in resolving the crisis. Money will need to be allocated to a legal fund that will be used to not only hold the school district and com- pany liable for negligence, but also protect the city from lawsuits. The city manager needs to fgure out if the land can be used for any useful purpose other than residential. If the land can be used for something other than residential, a budget needs to be formulated in an efort to revitalize an area that has been so devastated by the chemical dumping incident. The city manager should provide government investigators any information that is needed. In the future the city manager should ensure city inspectors have a very strict building code that must be signed of on by the building inspection department as well as having an environmental impact study prior to construction. Key Issues Raised from the Case Study Administrators have an ethical and professional duty to protect their citizenry from items that can potentially produce health-related problems. Administrators and local ofcials should never ignore or deny that a problem exists. Administrators should take their residents’ concerns seriously and investigate legitimate concerns that could negatively impact their community. If issues are found, administrators should act decisively to resolve any issues favorably. Not holding the industry accountable for disposal of waste was the initial failure that would later haunt the community. Additionally, the school board 194 ◾ Case Studies in Disaster Response and Emergency Management ignored warnings of the chemical company, building a school on top of a chemical dumping ground, which set the stage for the tragic illnesses that residents blamed upon the chemicals that had been buried in that location. The high number of birth defects and illnesses that appear to be linked to the chemical dumping at Love Canal have had long-term health efects upon the residents. In short, the public was not safeguarded by the local government as it should have been from the chemical company’s disposal process. Items of Note The residential houses have almost all been demolished in the Love Canal area and new development has been occurring since the 1990s. One power plant in particular is located close to where over 25,000 people reside (Washington Post, 1999). You have received a report that a main feed water pump critical for cooling the reactor has failed at this nuclear power plant (Cantelon and Williams, 1982). An emergency response team should be sent to the reactor to determine how to get the water pump back online quickly. However, the director should also mandate an evacuation of residents that could be impacted by a potential radioactive leak if the power plant had a meltdown and radioactive isotopes escape into the atmosphere. The director should inform federal, state, and local ofcials of the problem and keep those individuals informed as events transpire. The public should be informed on the status of the power plant and the potential dangers that could impact residents nearby. A team of engineers should be called in quickly to either repair the water pumps or fnd an alternative way of cooling the reactor. Clinical tial distribution of ions and selective membrane Connection permeability with four major cations and anions contributing to the resting membrane Retrograde axonal transport is of + − potential. Na and Cl ions are concentrated clinical importance because it is + extracellularly, and K and organic anions the route by which toxins such as tetanus and (proteins and amino acids) are concentrated viruses such as herpes simplex, rabies, and intracellularly. Synapses Proteins and amino acids do not move through the membrane as part of the resting membrane Axonal endings or terminals occur in relation to potential. An important anatomic characteristic thereby maintaining the membrane potential of the synapse is that the axonal ending is sepa- at about −60 mV. An important physiologic characteristic of a synapse Electrotonic Conductance in the is polarization; that is, the impulse always travels Soma-dendritic Membrane from the axon to the next neuron in the circuit or Electrotonic transients in the resting membrane to the muscle or gland cells supplied by the axon. Neurotransmitters, manu- potential shifts are electrotonically summated, factured and released by the neurons, cross the temporally and spatially, as they are conducted synaptic cleft to affect the postsynaptic neuron, passively from the soma and dendrites to the muscle, or gland cell. However, the neurotransmit- Action Potential Initiation and ters at synapses between neurons may be excit- Conductance atory, enhancing the production of an impulse in Depolarization of the axon hillock-initial segment the postsynaptic neuron, or inhibitory, hindering region to about −45 mV results in the generation impulse production in the postsynaptic neuron. The subsequent falling phase of the action poten- Physiologic Properties tial is slightly more prolonged and occurs by the effux of K+. Starting at the initial axon segment Resting Membrane Potential and continuing through to its terminal branches, Under steady-state conditions, neurons are the propagation of the action potential occurs as a electrically polarized to about –60 mV by the nondecremental voltage change. The velocity of separation of extracellular cationic charges propagation of an action potential is dependent from intracellular anionic charges. Saltatory Conduction changes occur discontinuously along the axonal membrane at small gaps (1 μm) between the edges In unmyelinated, generally small-diameter (0. Conversely, tance and concentrated Na+ channels at the nodes in large-diameter (13–20 μm) myelinated axons allow the action potential to jump (saltatory (type I or Aα), impulse propagation is much faster conduction) between nodes, increasing the speed (80–120 m/s) because Na+ and K+ conductance of conduction in myelinated axons (Fig. Chapter 1 Introduction, Organization, and Cellular Components 13 Normal Action Potential Propagation Saltatory Conduction in Myelinated Axon A. Node Myelin Na+ K+ K+ Na+ K+ K+ Na+ K+ K+ Na+ K+ K+ Na+ Current Flow Axon Na+ Na+ Na+ Na+ K+ K+ Na+ Myelin Node Nonsaltatory Conduction in Unmyelinated Axon B. Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ Axon Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ Action Potential Propagation Block C. Impulse blockade + K+ K+ Na+ K+ K+ + K+ K+ + K+ K+ + Na Na Na Na Axon Na+ Na+ K+ K+ Na+ K+ K+ Na+ K+ K+ Na+ Figure 1-10 Normal and abnormal action potential propagation. In myelinated axons, action poten- tial propagation is rapid because of saltatory current fow through the nodes of Ranvier where Na+ chan- nels are concentrated. In unmyelinated axons, action potential propagation is slower because Na+ channels are uniformly distributed in the axolemma. Action potential propagation is blocked in demy- elinated axons because current fow dissipates through the denuded membrane before reaching the next cluster of Na+ channels. Action Potential Frequency Encodes result in the membrane remaining depolarized longer resulting in the repetitive Na+ infux and Information K+ effux cycles.
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