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In this terload is unchanged purchase 200mg avana free shipping erectile dysfunction treatment london, the aortic valve opens at the same pres- idealized situation 50 mg avana fast delivery erectile dysfunction treatment yoga, there is no change in end-diastolic vol- sure (point 3). Afterload is also the same; therefore, the causes the ventricle to eject all of the extra volume that en- aortic valve opens at the same arterial pressure (point 3). This means that, when the aortic valve increased force of contraction causes the ventricle to eject closes at point 4, the volume and pressure of the ventricle are more blood and the aortic valve closes at a lower end-systolic identical to the values in Figure 14. This means that the mitral valve opens at a ume between points 3 and 4 is larger, representing the larger lower end-diastolic volume (point 1). This ation, the aortic valve opens (point 3) at a higher pressure and other changes, such as heart failure, are beyond the because aortic pressure is increased, as compared with Fig- scope of this text. The higher aortic pressure decreases stroke vol- ume, and the aortic valve closes (point 4) at a higher pres- Heart Rate Interacts With Stroke Volume sure and volume. Mitral valve opening and ventricular to Influence Cardiac Output filling (point 1) begin at a higher pressure and volume be- cause more blood is left in the ventricle at the end of sys- Heart rate can vary from less than 50 beats/min in a resting, tole. Filling of the ventricle proceeds along the same dias- physically fit individual to greater than 200 beats/min dur- tolic pressure-volume curve from point 1 to point 2. If stroke volume is held constant, in- 244 PART IV BLOOD AND CARDIOVASCULAR PHYSIOLOGY creases in heart rate cause proportional increases in cardiac inephrine by sympathetic nerves not only increases the output. However, heart rate affects stroke volume; changes heart rate (see Chapter 13) but also dramatically increases in heart rate do not necessarily cause proportional changes the force of contraction (see Fig. In considering the influence of heart rate epinephrine increases conduction velocity in the heart, re- on cardiac output, it is important to recognize that as the sulting in a more efficient and rapid ejection of blood from heart rate increases and the duration of the cardiac cycle the ventricles. Less filling of the ventricles leads to a re- an increase in sympathetic nervous system activity (as dur- duced end-diastolic volume and decreased stroke volume. A consequence of the reciprocal relationship between heart Influences on Stroke Volume and rate and the duration of diastole is that, within limits, de- Heart Rate Regulate Cardiac Output creasing the rate of a normal resting heart does not decrease cardiac output. The lack of a decrease in cardiac output is In summary, cardiac output is regulated by changing because stroke volume increases as heart rate decreases. Stroke volume is influenced Stroke volume increases because as the heart rate falls, the by the contractile force of the ventricular myocardium duration of ventricular diastole increases, and the longer and by the force opposing ejection (the aortic pressure or duration of diastole results in greater ventricular filling. Myocardial contractile force depends on ven- resulting elevated end-diastolic fiber length increases tricular end-diastolic fiber length (Starling’s law) and my- stroke volume, which compensates for the decreased heart ocardial contractility. This balance works until the heart rate is below 20 major factors: beats/min. At this point, additional increases in end-dias- 1) Norepinephrine released from cardiac sympa- tolic fiber length cannot augment stroke volume further be- thetic nerves and, to a much lesser extent, circulating cause the maximum of the ventricular function curve has norepinephrine and epinephrine released from the adre- been reached. At heart rates below 20 beats/min, cardiac nal medulla output falls in proportion to decreases in heart rate. If an electronic pacemaker is attached to the right atrium and the heart rate is increased by electrical stimula- tion, surprisingly little increase in cardiac output results. Sympathetic neural activity This is because as the heart rate increases, the interval be- tween beats shortens and the duration of diastole decreases. The decrease in diastole leaves less time for ventricular fill- β β β β 1 1 1 1 ing, producing a shortened end-diastolic fiber length, which Speed of Rate of rise subsequently reduces both the force of contraction and the Force of Conduction contraction velocity contraction and of pacemaker stroke volume. The increased heart rate is, therefore, offset relaxation potential by the decrease in stroke volume. When the rate increases above 180 beats/min secondary to an abnormal pacemaker, stroke volume begins to fall as a result of poor diastolic fill- Duration of systole Heart (small effect) rate ing. Increase Events in the myocardium compensate to some degree Decrease for the decreased time available for filling. First, increases in Stroke volume heart rate reduce the duration of the action potential and, Decrease Treppe thus, the duration of systole, so the time available for dias- Increase (small effect) tolic filling decreases less than it would otherwise. Second, faster heart rates are accompanied by an increase in the Cardiac force of contraction, which tends to maintain stroke vol- output ume. The increased contractility is sometimes called treppe or the staircase phenomenon. Various effects of norepinephrine on the heart com- pensate for the decreased duration of diastole and hold stroke Effects of Increased Heart Rate as a Result of Changes in volume relatively constant, so that cardiac output increases with Autonomic Nerve Activity. The words “Increase” and “Decrease” in occurs because of decreased parasympathetic and in- small type denote quantitatively less important effects than the creased sympathetic neural activity. CHAPTER 14 The Cardiac Pump 245 3) Disease states, such as coronary artery disease, my- ocarditis (see Chapter 10), bacterial toxemia, and alter- ations in plasma electrolytes and acid-base balance 4) Intrinsic changes in contractility with changes in heart rate and/or afterload Heart rate is influenced primarily by sympathetic and parasympathetic nerves to the heart and, by a lesser extent, by circulating norepinephrine and epinephrine. The effect A of heart rate on cardiac output depends on the extent of concomitant changes ventricular filling and contractility. C Heart failure is a major problem in clinical medicine (see Clinical Focus Box 14. A V = C THE MEASUREMENT OF CARDIAC OUTPUT mg mL = The ability to measure output accurately is essential for per- mg/mL forming physiological studies involving the heart and man- FIGURE 14. Cardiac output is measured either by one of The volume (V) of liquid in the beaker equals the amount (A) of several applications of the Fick principle or by observing dye divided by the concentration (C) of the dye after it has dis- changes in the volume of the heart during the cardiac cycle.
The center section shows the effects of a shift from the prone to the upright position with quiet standing purchase avana 200mg line erectile dysfunction age 32. The right panel shows the effect of activating the muscle pump by contracting leg muscles cheap 100mg avana amex erectile dysfunction 25. Note that the muscle pump restores central blood volume and cardiac output to the levels in the prone position. The fall in heart rate and rise in peripheral blood flow (forearm, splanchnic, and renal) associated with activation of the muscle pump reflect the reduction in baroreceptor reflex activity associated with increased cardiac output. RVEDP, right ventricular end-diastolic pressure; SVR, systemic vascular resistance. Small type represents compensatory changes that return variables toward the original values. These factors, together with neural and 30 minutes, a 10% loss of blood volume into the interstitial myogenic responses and the muscle and respiratory pumps, space can occur. This loss, coupled with the 550 mL dis- play a significant role during the seconds and minutes fol- placed by redistribution from the central blood volume into lowing standing (Fig. The combination of all of the legs, causes central blood volume to fall to a level so low these factors minimizes net capillary filtration, making it that reflex sympathetic nerve activity cannot maintain car- possible to remain standing for long periods. Diminished cerebral blood flow and a loss of consciousness (fainting) result. Arteriolar constriction due to the increased reflex sym- Long-Term Responses Defend Venous pathetic nerve activity tends to reduce capillary hydrostatic Return During Prolonged Upright Posture pressure. However, this alone does not bring capillary hy- In addition to the relatively short-term cardiovascular re- drostatic pressure back to normal because it does not affect sponses, there are equally important long-term adjustments the hydrostatic pressure exerted on the capillaries from the to orthostasis. The muscle pump is the most important factor bed (or astronauts not subject to the force of gravity). The people who are bedridden, intermittent upright posture alternate compression and filling of the veins as the muscle does not shift the distribution of blood volume from the pump works means the venous valves are closed most of the thorax to the legs. When the valves are closed, the hydrostatic column tral blood volume (and pressure) is greater than in a person of blood in the leg veins at any point is only as high as the who is periodically standing up in the presence of gravity. The average increase in central blood volume caused by ex- The myogenic response of arterioles to increased trans- mural pressure also acts to oppose filtration. As discussed earlier, raising the transmural pressure stretches vascular smooth muscle and stimulates it to contract. This is espe- Blood cially true for the myocytes of precapillary arterioles. The volume Atrial Arterial elevated transmural pressure associated with standing causes pressure volume a myogenic response and decreases the number of open cap- illaries. With fewer open capillaries, the filtration rate for a AVP ANP Medullary cardiovascular center: increased given capillary hydrostatic pressure imbalance is less. Blood loss in- fluences sodium and water excretion by the kidney via several pathways. All these pathways, combined with an increased intake of salt and water, restore the extracellular fluid FIGURE 18. These responses occur longed standing, capillary filtration reduces ve- later than those shown in Figures 18. Without the compensatory events that result in the pathways responsible for stimulating an increased intake of salt changes shown in small type, prolonged standing would in- and water are not shown. CHAPTER 18 Control Mechanisms in Circulatory Function 303 tended bed rest results in reduced activity of all of the path- Aldosterone acts on the distal nephron to cause in- ways that increase blood volume in response to standing. Aldosterone released from the adrenal cortex day and is quantitatively significant after a few days. Wa- point, standing becomes difficult because blood volume is ter intake is determined by thirst and the availability of not adequate to sustain a normal blood pressure. This Increased plasma osmolality, sensed by the hypothalamus, increase, proportioned between the extrathoracic and in- results in both thirst and increased AVP release. Thirst and trathoracic vessels, augments stroke volume during stand- AVP release are also increased by decreased stretch of ing. If blood volume is not maintained by intermittent erect baroreceptors and cardiopulmonary receptors. Renal arteriolar vasoconstric- The long-term regulation of blood volume is driven by tion associated with increased sympathetic nerve activ- changes in plasma volume accomplished by sympathetic ity produced by standing reduces the glomerular filtra- nervous system effects on the kidneys; hormonal changes, tion rate. This results in a decrease in filtered sodium and including RAAS, AVP, and ANP; and alterations in pressure tends to decrease sodium excretion. Regulation of the precise quantities of wa- control of extracellular fluid volume is determined by the bal- ter and sodium that are excreted maintains the correct ance between the intake and excretion of sodium and water. Sodium excre- The distribution of extracellular fluid between plasma tion is much more closely regulated than sodium intake. Ex- and interstitial compartments is determined by the balance cretion of sodium is determined by the glomerular filtration of hydrostatic and colloid osmotic forces across the capil- rate, the plasma concentrations of aldosterone and ANP, and lary wall. Retention of sodium and water tends to dilute a variety of other factors, including angiotensin II.
Hence purchase avana 50mg mastercard xarelto impotence, these nerves widening of thecentralcanal intothefourth haveoriginallybeenvisceromotornerves safe 200 mg avana erectile dysfunction treatment san francisco. Unlike viscerosensory (A6) regions, and sensory genuine striated muscles, they are not alarplate(A7)changestoamediolateralar- completely voluntary (e. The cranial nerve nu- clei in the medulla oblongata are arranged The trigeminal nerve (V) emerges from the according to this blueprint (A3) (p. The facial Cranial Nerves (B) nerve (VII) and the vestibulocochlear nerve According to classical anatomical nomen- (VIII) leave the medulla oblongata at the clature, there are 12 pairs of cranial nerves, cerebellopontine angle. The taste fibers of although the first two pairs are not really the facial nerve emerge as an independent peripheral nerves. The consists of the olfactory fibers, the bundled glossopharyngeal nerve (IX) and the vagus processes of sensory cells in the olfactory nerve (X) emerge dorsal to the olive. Supe- epithelium which enter the olfactory bulb rior ganglion of the vagus nerve (B15). The optical nerve (II) is a cervical roots of the accessory nerve (XI) cerebral pathway; the origin of the optical unite to form the spinal root (B16). The fibers, the retina, together with the pig- upper fibers originating from the medulla mented epithelium of the eyeball repre- oblongata form the cranial root; they run a sents an evagination of the diencephalon short course in the nerve and change over to (p. Optic chiasm (B9), optical tract the vagus nerve as internal branch (B17). The oculomotor nerve (III) it represents the remnants of several cervi- leaves the brain on the floor of the inter- cal nerves that have become included in the peduncular fossa (B11); the trochlear nerve brain region secondarily and now have only (IV) emerges at the dorsal surface of the rudimentary sensory roots. IV); the abducens nerve (VI) emerges from B20 Anterior perforated substance. Five nerves have developed from the B22 Choroid plexus (flower spray of Boch- branchial arch nerves of lower vertebrates: dalek) (p. Longitudinal Organization, Cranial Nerves 103 1 2 3 7 6 5 4 A Longitudinal organization of the medulla oblongata (according to Herrick) 8 I 18 9 19 20 II 21 10 III IV 13 V 11 12 VI VII VIII 14 17 X XII 15 IX XI 22 16 B Cranial nerves, base of the brain Kahle, Color Atlas of Human Anatomy, Vol. Three the wall of the cavernous sinus, the ophthal- bony depressions on each side correspond mic nerve (A14) extends with its branches to the basal aspects of the brain; the basal through the orbital fissure, the maxillary aspect of the frontal lobe lies in the anterior nerve (A15) through the round foramen, and cranialfossa(A1),thatofthetemporallobein the mandibular nerve (A16) through the oval the middle cranial fossa (A2), and the basal foramen. The brain stem rests on the clivus, its two layers form both a cover for the brain and the cerebellar hemispheres fit into the and the periosteum. From the confluence of these two layers are large venous sinuses sinuses (A19), the transverse sinus (A20) em- (p. Nerves and blood vessels pass braces the posterior cranial fossa and opens through numerous foramina in the base of intotheinternaljugularvein(A21). Basal to the meatus, the close to the midline, the olfactory nerves glossopharyngeal nerve (A24), vagus nerve pass through the openings of the thin (A25), and accessory nerve (A26) pass lamina cribrosa to the olfactory bulb (A4). The fiber bundles of the hypoglossal middle cranial fossae; its depression con- nerve (A27) pass as a single nerve through tains the hypophysis (A5), which is attached the hypoglossal canal. Lateral to the sella turcica, the internal carotid artery (A6) passes through the carotid canal into the cranial cavity. The optic nerve (A8) enters the cranial cavity through the optic canal in the medial area of the fossa, while the eye-muscle nerves leave the cavity through the superior orbital fissure (see vol. The paths of the abducens nerve (A9) and the trochlearnerve (A10) are charac- terized by their intradural position. The ab- ducens nerve enters the dura at the middle level of the clivus, and the trochlear nerve enters at the edge of the clivus at the attach- ment of the tentorium. The oculomotor nerve (A11) and the trochlear nerve run through the lateral wall of the cavernous sinus, and the abducens nerve through the laterobasal sinus of the internal carotid artery (see vol. The trigeminal nerve (A12) reaches below a dural bridge into the middle cranial fossa where the trigeminal ganglion (A13) lies in a pocket formed by the two dural layers, the trigeminal cavity. Base of the Skull 105 1 4 8 14 10 5 7 15 6 16 13 10 13 2 11 9 12 18 9 22 23 27 24 25 21 26 17 3 20 19 A Base of the skull, viewed from above (preparation by Professor Platzer) Kahle, Color Atlas of Human Anatomy, Vol. Its fibers run in a curve directed dorsally, extend on the floor As in the spinal cord, where the anterior of the rhomboid fossa (facial colliculus) horn represents the area of origin of motor around the abducens nucleus (internal genu fibers and the posterior horn the area of ter- of facial nerve) (A12), and then descend mination of sensory fibers, the medulla ob- again to the lower border of the pons where longata contains the nuclei of origin (with they emerge from the medulla oblongata. The sensory nuclei are located laterally; The somatomotor nuclei lie close to the most medially lies the viscerosensory soli- midline: tary nucleus (AB14), in which the sensory fibers of the vagus nerve and the glos-! The nucleus of the hypoglossal nerve (AB1) sopharyngeal nerve, as well as all taste fibers, (tongue muscles) terminate. The nucleus of the abducens nerve (AB2) clear area of the trigeminal nerve, which has! The nucleus of the trochlear nerve (AB3) the largest expanse of all cranial nerves and! The pontine nucleus of the trigeminal nerve The visceromotor nuclei follow laterally, (principal sensory nucleus) (AB15) namely, the genuine visceromotor nuclei!
This measures the distribution of a previously administered positron-emitting isotope 100mg avana visa erectile dysfunction caused by lisinopril. PETcould be regarded as a form of in vivo autoradiography except that the radioligand is not [3H] but [15O] generic 200 mg avana erectile dysfunction age 70, [13N], [11C] or [18F], all of which have short half-lives (2, 10, 20 and 110 min respectively) and so the labelled ligand can only be prepared just before use. The positron emitted from the proton of the isotope collides with an electron in the atomic orbit so that two gamma-rays are given out simultaneously at 1808 to each other. Since the detectors only respond when they make two simultaneous detections, i. The intensity of the detected emission is colour coded and reflects the concentration of ligand (Fig. Of course, a low level of emission will be detected throughout the brain from the presence of the labelled substance and its metabolites in the blood and extracellular fluid, as well as that non-specifically located in all neuronal and glial tissue and such background activity must be distinguished from the more specific labelling. After its injection a labelled precursor should be taken up and detected in appropriate nerve terminals (and possibly cell bodies) so that the intensity of emission reflects the density of nerve terminals and the innervation. Using this procedure it has been possible to show that very little [18F] fluorodopa is concentrated in the striatum of Parkinsonian patients, compared with normals (Fig. Whether the label remains on dopa or is transferred to dopamine will not greatly affect the result since both will label DA neurons although some will occur in noradrenergic nerve terminals. The injection and subsequent detection of an appropriately labelled ligand can give an indication of the density of the receptors to which it is bound. Various ligands have been used in this way to label and measure the number of DA and 5-HTreceptors in schizophrenics (see Chapter 17) and the extent to which they are occupied by drugs used to treat the disorder. As with any binding study the validity of the approach depends on the specificity of the ligand for its receptor. Of course, there will always be the background activity mentioned above, but the extent of specific binding may be gauged by comparing the density of emission in any area where the NTis likely to function (e. The difference between these two levels should in fact increase as unbound drug is lost (excreted). To determine the precise number of receptors and see if that varies from brain to brain (e. Normally the estimation of receptor number requires a measure of specific binding at two or more ligand concentrations under equilibrium conditions (see Chapter 3), which will clearly be difficult in vivo, not least because the effect of different doses may be unacceptable to the patient or subject. It must also be remembered that much of the in vivo binding can be to presynaptic receptors and uptake sites as well as postsynaptic receptors, although drugs specific for those sites can be used to label nerve terminals. IMPLICATIONS OF NEUROTRANSMITTER MALFUNCTION FROM DRUG STUDIES It is perhaps not surprising, in view of all the problems associated with studying NT function directly in humans, that much attention has been focused on evaluating how 292 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION drugs which are therapeutically effective in a particular disorder may modify the function of a NT. That NT may be implicated in the disorder if it can be shown that the relative therapeutic effectiveness of a range of drugs correlates significantly with some action they have on it. This requires that all the compounds must have a similar chance of reaching the CNS, or at least of providing appropriate plasma levels, and also that compounds with little or no clinical efficacy in the disorder do not modify the NTin the same way as effective drugs. Thus there is a clear correlation between DA receptor antagonism, as measured by ligand binding studies and the anti-schizophrenic activity of a wide range of phenothiazine and butyro- phenone derivatives (see Chapters 7 and 17). Good correlations between the analgesic potency of morphine derivatives and displacement of the labelled morphine antagonist, naloxone, helped not only to formulate the concept of opioid receptors and hence of endogenous opioids to occupy them but also the actual discovery of the enkephalins. Displacement of labelled diazepam by a wide range of other benzodiazepines, in an order in keeping with their clinical efficiency as anxiolytics, led to the realisation of endogenous benzodiazepine receptors. Unfortunately a significant correlation between the clinical efficacy and a particular pharmacological effect of a range of drugs may give the impression that that is the only way in which that disorder may be treated. When drugs are evaluated for therapy in a peripheral malfunction, the tendency is to try to show that they work through different mechanisms. Indeed it is considered desirable, if not essential, to develop anti-hypertensive drugs with different actions, e. Yet remarkably, when dealing with the CNS, the tendency has been to try to treat a disorder by manipulating just one NT. Consequently the assumption arose that all anti-depressant drugs must augment NA function and all neuroleptic anti-schizophrenic drugs must be DA antagonists, etc. Certainly the intro- duction of some drugs that are effective in depression without blocking NA uptake, which was the conceived mode of action of all the earlier tricyclic anti-depressants, not only gave a new approach to therapy but also questioned the long-held view that depression is due only to reduced NA function. There could be as many ways of controlling depression as there are of reducing blood pressure and a number of different NTs involved. NEUROTRANSMITTER FUNCTION IN HUMANS 293 All such animal procedures suffer from the obvious and basic problem that laboratory animals do not behave like humans and that humans cannot reliably interpret their reactions and behaviour. Indeed there are some tests, as well as animals with varied spontaneous seizures, that are even predictive of particular forms of epilepsy. But then convulsions are a very basic form of activity common to most species and epileptic seizures that are characterised by behavioural rather than motor symptoms are more difficult to reproduce in animals. It is a statement of the obvious to say that we cannot tell when a rat is anxious or depressed, assuming that they can even experience such human reactions, but they can be subjected to conditions that would make us anxious or depressed. Whether animals then react in the same way as humans is not certain but we have to assume that to be the case if the animal is to be used to detect changes in NTfunction that can be related to humans.
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