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Past investigations antibiotics dental abscess discount zitrolab amex, including computed tomography antibiotics jeopardy buy zitrolab from india, computed evoked electroencephalographic studies, and postmortem studies of the brain have all provided evidence of a structural basis for dyslexia. Historically, Drake (1968) noted abnormally formed gyri in the parietal regions and ectopic neurons in the white matter, arrested during their migration to the cerebral cortex. The third brain examined showed only verrucose dysplasia almost exclusively confined to the left cerebral hemisphere (Kemper, 1984). Analysis of human malformations and animal models indicates that the focal and verrucose dysplasia probably arise during the later stages of neuronal migration to the cerebral cortex, and appear to result from the migration of neurons into focal areas of cortical destruction. In man, neuronal migration to the cerebral cortex occurs from the 8th to approximately the 16th week of gestation. Consistent with this timing is the presence of ectopic neurons in the white matter in two of the three brains examined. Geschwind and Behan (1982) Child reverses letters (writes "d" for "b") and sequence of letters in words (writes "saw" for "was") Verrucose dysplasia, with focal accumulation of neurons in layer 1 of cerebral cortex have noted a significant clustering of dyslexia, lefthandedness, autoimmune disease, and migraine in persons related to each other, indicating an association between these disorders. Currently, it is felt that developmental dyslexia is a disorder of network connections as demonstrated by Vandermosten (2012). Focal dysplasia, with abnormally large neurons extending into white matter of brain Treatment. Early identification and evaluation of dyslexia is essential for proper treatment. The optimal educational approach is multisensory, including phonemic awareness and enhanced phonologic processing, where virtually all respond. However, given that dyslexia is a lifelong disorder, many continue to struggle into adulthood when presented with new or less familiar words or in reading comprehension settings. Treatment should be aimed at enabling those to overcome deficits where possible and to learn strategies to circumvent and compensate for difficulties that cannot be overcome. The pathophysiology of these pervasive developmental disorders is not known, but it is believed to be an inherited abnormality in the structure and function of certain parts of the brain, including those that govern the development of social relatedness and language. There may be a complete lack of language, or, if language is present, it does not serve to initiate or sustain conversation with others. The child may not babble, point, smile, or make meaningful gestures; he or she may have poor eye contact, may appear to be hearing impaired, and may not know how to play with toys or engage in makebelieve play. The child often has very restricted patterns of interest and may show little interest in the environment. There may be inflexible routines that serve no function and repetitive behaviors such as hand-flapping or body-twisting. Autistic disorder is accompanied by intellectual disability in up to 60% of cases, and seizures are often also present in the children who have intellectual disability. A less severe type of these disorders, Asperger disorder, is characterized by all of the above problems, without language impairment and intellectual disability. Children with Asperger disorder who have stronger verbal skills are sometimes referred to as sounding like little adults and struggle to pick up on the normal "give and take of a conversation. If some of the above behaviors are present, but not enough to meet the diagnoses, the disorder is called pervasive developmental disorder, not otherwise specified. Genetic testing is recommended because single-gene disorders or genetic variations associated with autism are seen in 10%. If the child exhibits behaviors that are aggressive, destructive, or self-injurious, medications (such as atypical antipsychotics) may be helpful. Sometimes stimulant medication may be helpful in reducing hyperactivity and impulsivity, and antidepressants may be helpful in reducing compulsive behaviors. The majority of these treatments are designed to take place in the home or at school. Children with these disorders who are identified early, who have relatively intact language and intellectual abilities, and receive intensive treatment have the best outcomes. After a normal pregnancy and delivery, early development is apparently normal through age 6 months; however, retrospectively, subtle deviant patterns occur, including deceleration of acquired head growth. Developmental progress stalls between 6 and 18 months, followed by frank regression of fine motor skills and communication function, including loss of acquired language with poor visual and aural interactions suggesting autism. Concomitantly, stereotyped hand movements appear during wakefulness: hand-wringing, hand-mouthing, hand-clapping or patting, or unusual finger movements. Although gait is acquired in about 80%, approximately 20% of Rett children require assistance.

Although focal seizures may be seen with localized brain insults antimicrobial index buy zitrolab 250mg, such as neonatal strokes 5 infection control procedures buy zitrolab without prescription, they may also be seen in disorders that diffusely affect the brain, such as asphyxia, subarachnoid hemorrhage, hypoglycemia, and infection. In tonic seizures, the infant develops asymmetric posturing of the trunk or deviation of the eyes to one side. Myoclonic seizures are similar to those seen in older children, consisting of rapid jerks of muscles. The myoclonic seizures can consist of bilateral jerks, although occasionally unilateral or focal myoclonus can occur. Sick neonates often display repetitive, stereotyped behavior that may be confused with seizures. Myoclonus not associated with epileptiform discharges can also be seen in sick neonates. It is usually defined as a seizure or series of seizures without full recovery of consciousness between the seizures, which last at least 30 minutes. The most common causes in children are febrile seizures, meningitis, and preexisting neurologic disorders such as cerebral palsy. To minimize the risk of an adverse outcome, treatment should be initiated as soon as possible. Because most seizures begin in the community, medications such as rectal diazepam should be carried by emergency medical technicians, administered in the community setting, and the patient transported urgently to the nearest emergency department. Cardiorespiratory status and other vital functions should be assessed and support given if necessary. Blood samples should be drawn for analysis and an infusion of normal saline should be started. The initial in hospital, treatment is usually a benzodiazepine, such as lorazepam, administered intravenously. If benzodiazepines fail to terminate the seizure, then phenytoin or phenobarbitone may be administered. If seizures remain uncontrolled, then general anesthesia and artificial ventilation may be required. However, there is debate on whether aggressive therapy with anesthetic agents is warranted. Epilepsia partialis continua refers to the situation in which there is continuous focal motor activity that may last months to years. There are many genetic and neurometabolic causes that lead to seizures, typically beginning in childhood. Genetic disorders include disorders such as severe myoclonic epilepsy of childhood, tuberous sclerosis, Rett syndrome, Angelman syndrome, and fragile X. Neurometabolic disorders, which may also have a genetic cause, result in disturbances of metabolism and can lead to seizures. Disorders such as urea cycle defects, pyridoxine dependency, biotinidase deficiency, and glucose transporter deficiencies can cause severe seizures. The most common types of brain lesions causing seizures are tumors, vascular lesions, head trauma, infectious diseases, congenital malformation of the brain, and biochemical or degenerative disease processes affecting the brain. Brain tumor is an important cause of seizures, particularly in the adult patient, becoming an increasingly likely cause after the second decade of life and one of the main causes in the fourth and fifth decades. A brain tumor should be suspected in any person who has onset of seizures, especially focal seizures, after age 20 years. Head trauma is a major cause of seizures, which may occur shortly after the head injury or, more often, several months to several years later. Factors that increase the chance of development of post-traumatic seizures are a penetrating head injury, severe damage to the brain, prolonged periods of unconsciousness, posttraumatic amnesia, complications of wound healing, and a persistent neurologic deficit. Vascular disease is one of the most common causes of seizures in older persons, particularly after age 50 years. Seizures can occur transiently after an acute stroke (thrombotic, embolic, or hemorrhagic) or may develop later as a sequela of cerebrovascular disease. Although uncommon, arteriovenous malformations are frequently associated with seizures. Other vascular causes include subdural hematomas, venous thrombosis, and hypertensive encephalopathy. Seizures may occur with any acute infection of the nervous system or as a complication of damage to the nervous system by the inflammatory process. With improved neuroimaging, many patients who were thought to have idiopathic epilepsy have now been found to have brain malformations.

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In contrast bacteria from bees possible alternative to antibiotics buy cheap zitrolab 500 mg on line, acute bacterial rhinosinusitis presents with more than 7 days of purulent rhinorrhea antibiotics for dogs at feed store zitrolab 100mg line, nasal congestion, facial or dental pain/pressure, an accompanying cough, halitosis, and, if severe, fever (50% of adults). Fungal sinusitis may be acute or chronic (lasting more than 12 weeks) and is of particular concern in patients who are immunocompromised. Sphenoid sinusitis is an uncommon infection that may manifest as an acute or subacute headache associated with nausea and vomiting. It may accompany pan sinusitis but, when isolated, may not have associated nasal symptoms. It can mimic many other causes of headache, including aseptic meningitis, migraine, and trigeminal neuralgia. Excessive tearing, photophobia, and paresthesias in the trigeminal nerve distribution may accompany sphenoid rhinosinusitis. This should be considered in patients with a severe, intractable new-onset headache that worsens with coughing, bending, or walking; interferes with sleep; is progressive in severity; and does not respond well to analgesics. Initial treatment is conservative, with an oral appliance or bite plate and possibly physical therapy. Medication such as nonsteroidal anti-inflammatory drugs, muscle relaxants, and tricyclic antidepressants can also be tried. Occasionally, infection of the dental pulp or apical root may cause neuralgic type pain in the second and third trigeminal divisions, which is difficult to distinguish clinically from trigeminal neuralgia. For this reason, patients with trigeminal neuralgia may undergo one or more unnecessary dental procedures before the correct diagnosis is made. Conversely, it is important to exclude true dental disease before making a diagnosis of trigeminal neuralgia. This creates severe eye pain sometimes associated with a unilateral headache, nausea/vomiting, conjunctival injection, and a mid-dilated nonreactive pupil. Patients may describe intermittent visual blurring while "seeing halos around objects. Chronic open-angle glaucoma, the more common form of glaucoma, is not a cause of headaches. Nevertheless, an emergency medicine physician must evaluate for the possibility of an underlying, secondary cause for the pain. New headaches beginning during vigorous exertion or after head/neck trauma require consideration of an intracranial hemorrhage or cervicocephalic arterial dissection. Most concerning are those patients presenting with an explosive, debilitating, or "thunderclap" headache often referred to as the "worst headache of my life" or "I felt like I was hit with a sledge hammer. If either type hemorrhage is ruled out, other pathologic mechanisms for an acute severe headache require consideration (see table at right). Equally important is the patient presenting with an acute headache and new-associated neurologic symptoms, particularly focal motor or sensory loss, language dysfunction, or encephalopathic symptomatology, such as confusion or seizures. Other historical details in the acute headache patient also cause significant concern warranting further and immediate evaluation. These include older age, immunocompromise, recent infection or fever, history of cancer, clotting or bleeding disorders (particularly including therapeutic anticoagulation), progressively worsening headache severity, or symptoms of systemic illness, that is, weight loss, fatigue, myalgia, or unexplained anemia. Any patient whose clinical presentation with headache includes fever, alteration in consciousness or mentation, or an overall toxic appearance requires an urgent evaluation for a possible underlying infection. Nuchal rigidity usually indicates meningeal irritation, which can be seen with either subarachnoid hemorrhage or meningitis. Papilledema reflects increased intracranial pressure and warrants further investigation for disorders causing mass effect, such as tumor, infection, hemorrhage, or idiopathic intracranial hypertension. Intraparenchymal hemorrhage is more likely to cause relatively rapid evolution of focal neurologic symptoms as well as seizures and altered mentation, depending on the size and location of the hematoma. A history of anticoagulation, especially in an older patient presenting with headache is particularly concerning for hemorrhage.

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Platelets are particularly relevant in the high-pressure arterial circulation virus in us purchase zitrolab 100 mg amex, where minor vascular damage can rapidly lead to major hemorrhage virus scan software cheap 500 mg zitrolab free shipping. Platelets assume a critical role in this response, because they initially contain blood loss and, as a second step, provide an active surface for rapid fibrin and, ultimately, clot formation. Platelets then release thromboxane A2 and products of their storage granules that lead to aggregation and recruitment of additional platelets. As more platelets aggregate, a fibrin network develops and stabilizes the mass into a "white thrombus. Either platelet-fibrin aggregates or more fully formed clots may break off, leading to embolization in distal arteries. Fibrin White thrombus Platelets attach to the injured endothelium (adhesion) and to other platelets (aggregation) via specific surface glycoproteins. The content of alpha and dense granules is released, contributing to further growth of the platelet plug. These pathophysiologic differences define the antithrombotic or anticoagulant agents used in each situation. Antiplatelet agents are the treatment of choice to prevent coronary artery disease or arterial ischemic stroke, whereas antithrombin-based interventions, such as heparin and warfarin, are used for prophylaxis and treatment of systemic and cerebral venous thrombosis. Individuals with increased tendency to thrombosis are designated as having thrombophilia, either acquired. Clinically, inherited thrombophilia is characterized by one or more of the following: (1) thrombotic events occurring before age 45 to 50 years; (2) spontaneous, recurrent, or life-threatening events; (3) thrombosis occurring at unusual sites, including the central nervous system; and (4) family history of thromboembolic events. Secondary hemostasis, or blood coagulation, is initiated by interaction of blood with vascular subendothelium or tissue factor exposed on cell surfaces after cellular injury. Intrinsic and extrinsic coagulation pathways converge through a series of steps to form a common pathway, ultimately leading to thrombin generation. The coagulation cascade rapidly transduces small initiating stimuli into large fibrin clots. Endogenous anticoagulant mechanisms offset the potentially explosive nature of this cascade by carefully regulating extent of coagulation serine protease generation. The natural anticoagulants permit coagulation to proceed locally while preventing it from becoming a systemic process. Congenital and acquired hypercoagulable states arise when imbalance develops between prothrombotic and anticoagulant plasma activities in favor of thrombosis. In most inherited thrombophilias, genetic variations of proteins regulating hemostasis ultimately lead to increased generation, or impaired neutralization of thrombin, predisposing to thrombotic events. In the presence of heparin sulfate, the rate of inactivation is increased by several 1000-fold. Although gene mutations in these natural anticoagulants are uncommon, when present they lead to venous and arterial thrombosis in early adulthood. If these occur in homozygosity, severe thrombogenesis occurs during infancy and childhood that is often incompatible with life. This is the most common genetic defect related to venous thrombosis, present in 10% to 50% of affected individuals. Worldwide carrier frequencies range from 1% to 15%; it is highly prevalent among Caucasians. This point mutation in the coagulation factor V gene renders the mutant factor V resistant to proteolytic degradation by activated protein C, a characteristic denominated activated protein C resistance. Because it interacts synergistically with smoking, oral contraceptives, and other inherited thrombophilias, it is a potential risk factor for ischemic stroke in young patients with additional vascular risk factors. A prothrombin gene G-to-A substitution in the 3untranslated region is associated with elevated plasma prothrombin levels and increased thrombotic risk. The relationship with arterial thrombosis and stroke remains controversial; although this mutation is associated with a moderate increase in arterial thrombotic disease, it assumes particular importance in certain subgroups, including young women taking oral contraceptives and children. Homocysteine is a sulfurcontaining amino acid formed as an intermediary compound during methionine metabolism and metabolized by both remethylation and trans-sulfuration. Deleterious effects of hyperhomocysteinemia include endothelial dysfunction, platelet activation, and arterial and venous thrombus formation.

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Diseases

  • Hypogonadism, isolated, hypogonadotropic
  • Angel shaped phalangoep
  • Borjeson syndrome
  • Anterior pituitary insufficiency, familial
  • Mulliez Roux Loterman syndrome
  • Intestinal malrotation facial anomalies familial type
  • Kosztolanyi syndrome
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Complete neuronophagia Paralytic residual of spinal poliomyelitis Multiple crippling deformities; contractures infection of the blood discount 500 mg zitrolab with visa, atrophy antibiotic resistance effects on society purchase zitrolab 500mg visa, severe scoliosis, and equinovarus Genu recurvatum, atrophy of limb Scoliosis Today, poliovirus is the least common cause of an asym metric flaccid lower extremity weakness. The other enteroviruses (the coxsackieviruses, the echoviruses, and the numbered enteroviruses), and the flaviviruses, most notably the West Nile virus, are the much more common etiologic agents of a flaccid paralysis. When poliovirus infection is suspected, at least two stools specimens and two throat swabs should be obtained 24 hours apart. As with all suspected enteroviral infections, acute and convalescent serology should be sent 4 weeks apart to detect a fourfold increase in immunoglobulin G (IgG). The treatment of poliovirus infection is primarily supportive, and prevention with mass vaccination of all children is essential. Initial infection occurs in the upper respi ratory tract, followed by a viremia and the appearance of the characteristic vesicular lesions of chickenpox. Varicellazoster virus establishes latency in the cranial nerve ganglia and dorsal root ganglia along the neur axis. Reactivation of the virus causes shingles, which presents with severe localized pain followed within 3 to 4 days by the appearance of a vesicular rash on an erythematous base in one to three dermatomes. Zoster has a predilection for the mid to lower thoracic, upper lumbar, and ophthalmic (V1) dermatomes. The neurologic complications of varicella, or chickenpox, include encephalitis (which most often manifests as an acute cerebellar ataxia), aseptic meningitis, polyneuritis, multiple cranial neuropathies, or Reye syndrome. The neurologic complications of zoster include meningitis, encephalitis, vasculopathy, cerebellitis, Ramsay Hunt syndrome, postherpetic neuralgia, myelopathy, and chronic radicular pain without rash (zoster sine herpete). Varicellazoster virus encephalitis can develop associated with zoster, follow zoster by days to months, or may develop without any history of a vesicular rash. The symptoms of encephalitis include fever, headache, seizures, focal neurologic deficits, and an altered level of consciousness. Varicellazoster virus encephalitis is due to ischemic and hemorrhagic infarctions in both cortical and subcortical gray matter and white matter. Neuro imaging in patients with varicellazoster virus ence phalitis may demonstrate ischemic and hemorrhagic infarctions and demyelinative lesions. Zoster reactiva tion may also cause a ventriculitis and periventriculitis with hydrocephalus, altered mental status, and gait abnormalities. Patients with reactiva tion of varicellazoster virus in the trigeminal ganglion develop vesicular lesions in the ophthalmic division of the trigeminal nerve and are at risk for infarction in the distribution of the carotid, anterior, or middle cerebral arteries due to varicella zoster virus vasculopathy. An acute cerebellar ataxia can com plicate childhood varicella but may also occur in adulthood. Ramsay Hunt syndrome is due to the reactivation of varicellazoster virus in the geniculate ganglion, resulting in a peripheral facial nerve palsy. Postherpetic neuralgia is the most common neurologic complication of varicella zoster virus. The pain of zoster tends to resolve as the lesions heal but may be associated with or followed by postherpetic neuralgia. Postherpetic neuralgia is defined as the presence of pain in the dermatomal dis tribution of the vesicular rash for more than 1 month after the onset of zoster, after the lesions have healed. Zoster sine herpete is pain in a dermatomal distribution without the appearance of a vesicular rash. Postherpetic neural gia is treated with a combination of amitriptyline and gabapentin. The routine use of the varicella vaccine in childhood has decreased the incidence of chicken pox. First exposure is usually asymptomatic, but in some individuals, vesicular lesions develop in the mouth. The virus spreads by retrograde and anterograde transport to the trigeminal ganglion, where it establishes latent infection. Herpes simplex virus encephalitis is due to reactivation of latent herpes simplex virus infection and presents with a subacute progression of fever, hemicranial headache, behavioral abnormalities, focal seizure activity, and focal neuro logic deficits, most often dysphasia or hemiparesis. Spinal fluid analysis demonstrates a lymphocytic pleocytosis with a normal or rarely mildly decreased glucose concentration. Herpes simplex virus encephalitis is treated with intravenous acyclovir for 3 weeks. Common animal disseminators Foxes Humans acquire rabies from the bite of a rabid animal or from inhalation from aerosolized virus in caves inhabited by rapid bats.

Progesterone stimulates secretion from the uterine glands (called the secretory phase of the uterus) treatment for uti resistant to cipro buy zitrolab without prescription. In the absence of an implanting embryo the corpus luteum dies antibiotic yeast infection symptoms discount generic zitrolab canada, progesterone production ceases, and the uterine endometrium is sloughed (called the menstrual phase, or period, of the uterus-this corresponds to days 1 to 5 of the follicular phase of the ovary). Cervical mucus is hormonally regulated so that at midcycle in response to estrogen, cervical mucus promotes entry of sperm into the uterus from the vagina. During the luteal phase in response to progesterone, cervical mucus becomes thick and poses a barrier to entry of sperm and microbes into the uterus. Fertilization is a complex series of events that occur in the oviduct and lead to penetration of the oocyte by sperm. Early embryogenesis (up to day 6 after fertilization) occurs in the oviduct and gives rise to a blastocyst that hatches from the zona pellucida. Estrogen production requires placental cells (syncytiotrophoblasts) as well as the fetal adrenal and liver-collectively called the fetoplacental unit. Pregnancy and the hormones of pregnancy induce major changes in maternal physiology, including an increase in insulin resistance, an increase in the use of free fatty acids by the mother, and development of the mammary glands. Mammary gland development (but not lactation) is promoted by estrogen, progesterone, and placental lactogen but also by maternal pituitary prolactin, whose secretion is stimulated by placental estrogens. Oxytocin is a pituitary hormone that promotes contraction of certain smooth muscles, including myometrial contractions during labor and myoepithelial contractions in the breasts that lead to let-down of milk in response to suckling. Menopause results from exhaustion of the ovarian reserve and is characterized by low ovarian hormone and elevated gonadotropin levels. Cardiac and metabolic effects of anabolic-androgenic steroid abuse on lipids, blood pressure, left ventricular dimensions, and rhythm. Oocyte maturation: gamete-somatic cells interactions, meiotic resumption, cytoskeletal dynamics and cytoplasmic reorganization. Stops and starts in mammalian oocytes: recent advances in understanding the regulation of meiotic arrest and oocyte maturation. Enzymes involved in the formation and transformation of steroid hormones in the fetal and placental compartments. The mammary stem cell hierarchy: a looking glass into heterogeneous breast cancer landscapes. Index Page numbers followed by "f " indicate figures, "t" indicate tables, and "b" indicate boxes. Access additional resources online at no extra charge by scratching the pin code at the bottom right and redeeming the code at ebooks. An email will then be sent to you with final instructions for accessing your online-only materials. Aqueous diffusion (1) Passage through central pores in cell membranes (2) Possible for low-molecular-weight substances. Open circles represent molecules that are moving down their electrochemical gradient by simple or facilitated diffusion. Shaded circles represent molecules that are moving against their electrochemical gradient, which requires an input of cellular energy by transport. Primary active transport is unidirectional and utilizes pumps, while secondary active transport takes place by cotransport proteins. Process in which a cell engulfs extracellular material within membrane vesicles b. Drugs in aqueous solutions mix more readily with the aqueous phase at absorptive sites, so they are absorbed more rapidly than those in oily solutions. Drugs in suspension or solid form are dependent on the rate of dissolution before they can mix with the aqueous phase at absorptive sites.

Mitsunari M virus 52 generic zitrolab 500mg with mastercard, Yoshida S antimicrobial activity of medicinal plants order zitrolab australia, Shoji T, et al: Macrophage-activating lipopeptide-2 induces cyclooxygenase-2 and prostaglandin E(2) via 182. These remarkable adaptations begin soon after conception and continue as gestation advances, yet most are almost totally reversible within weeks to months after delivery. These physiologic adaptations are usually well tolerated by the pregnant patient, but they must be understood so that normal can be distinguished from abnormal. This is supported by studies of systolic time intervals in pregnancy11,12 and echocardiographic demonstration of a decreased ratio of the loadindependent wall stress to the velocity of circumferential fiber shortening. This results in increased compliance of capacitive (predominantly elastic wall) and conductive (predominantly muscular wall) arteries and veins that is evident as early as at 5 weeks of the beginning of amenorrhea. Cardiac output may be calculated by invasive heart catheterization using dye dilution or thermodilution, or by noninvasive methods such as impedance cardiography and echocardiography. Limited data have been obtained from normal pregnant women by means of an invasive method. These validation studies have not been performed in healthy pregnant women, and reports are limited to critically ill patients. Yet to be determined is the most appropriate echocardiographic technique (pulsed-wave or continuous Doppler) and the most reproducible site through which to measure blood flow. This rise reflects increased production of red blood cells rather than prolongation of red blood cell life.

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Overwhelming evidence suggests a role for prostaglandins in the process of labor virus x 2010 buy cheap zitrolab 250 mg, both at term and before term bacteria 2 in urine test effective 250 mg zitrolab,35,36 which is probably common to all mammalian viviparous species. These compounds are structurally similar but can have different and often antagonistic actions. Regulation of prostaglandin synthesis occurs at several different levels of the arachidonic acid cascade. These hormones act locally in a paracrine or autocrine fashion (or both) by binding to specific prostaglandin receptors on adjacent cells. In addition, unesterified arachidonic acid can diffuse into the cell and interact directly with nuclear transcription factors to regulate the transcription of target genes, including cytokines and other hormones. In this way, the chorion serves as a protective barrier, preventing transfer of the primary prostaglandins from the fetoplacental unit to the underlying decidua and myometrium. It is rapidly inactivated in the liver and kidney, resulting in a biologic half-life of 3 to 4 minutes in the maternal circulation. Concentrations of oxytocin in the maternal circulation do not change significantly during pregnancy or before the onset of labor, but they do rise late in the second stage of labor. Specific receptors for oxytocin are present in the myometrium, and there appear to be regional differences in oxytocin receptor distribution, with large numbers of receptors in the fundal area and few receptors in the lower uterine segment and cervix. It may act directly through both oxytocin receptor-mediated and nonreceptor, voltage-mediated calcium channels to affect intracellular signal transduction pathways that promote uterine contractions. It may also act indirectly through stimulation of amniotic and decidual prostaglandin production. As with other types of muscle, action potentials must be generated and propagated in the myometrium to effect contractions, a process known as electromechanical coupling. These cells have a higher resting transmembrane potential and spontaneously initiate action potentials. The action potential results in a rapid rise in intracellular calcium derived from both extracellular and intracellular sources, which triggers myometrial contractions by encouraging the relative movement of thick (myosin) and thin (actin) filaments within the contractile apparatus, resulting in shortening of the contractile unit. In this way, the electrical activity is translated into mechanical forces that are exerted on the intrauterine contents. The frequency of contractions correlates with the frequency of action potentials; the force of contractions correlates with the number of spikes in the action potential and the number of cells activated together; and the duration of contractions correlates with the duration of the action potentials. As labor progresses, electrical activity becomes more organized and increases in amplitude and duration. The strength of contractions, which is best measured as intrauterine pressure in millimeters of mercury (mm Hg), varies with the stage of labor. Early labor contractions have a peak intensity of 25 to 30 mm Hg, and this increases to 60 to 65 mm Hg during active labor. For example, the more rapid labor observed in multiparous compared with nulliparous women is caused not by increased intrauterine pressures during labor (indeed, multiparous women have lower intrauterine pressures than nulliparas)181 but by a reduction in the resistance of the pelvic floor. Although this movement is similar in all muscles, several structural and regulatory features are unique to smooth muscle including the myometrium. The thin filaments insert into dense bands linked by the cytoskeletal network, thereby allowing the generation of force in any direction within the cell. This allows smooth muscle cells to generate greater force (greater shortening) than striated muscle cells and with relatively little energy expenditure. Smooth muscle myosin is a hexamer consisting of two heavy-chain subunits (approximately 200 kDa) and two pairs each of 20- and 17-kDa light chains. A neck region connects the globular head to the -helical tail, which interacts with the tail of the other heavy-chain subunits. In this way, multiple myosin molecules interact through their -helical tails to make a coiled-coil rod, which forms the thick filament. Thin filaments are composed of actin, which polymerizes into a double-helical strand in association with a number of proteins. The myosin head then detaches and, when reactivated, can reattach at another site on the actin filament. Actin-myosin interaction is regulated by the intracellular calcium concentration, which is mediated through the calciumbinding protein, calmodulin (CaM). The phosphatase group of enzymes also plays an important role in determining the sensitivity of the contractile apparatus to electrical stimuli and changes in intracellular calcium concentrations.

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