By E. Navaras. Finlandia University.

This Endothelial cells purchase extra super cialis 100 mg mastercard, once thought to be passive conduits for special conduction system consists of the sinoatrial (SA) blood flow cheap extra super cialis 100 mg, are now known to perform two extremely impor- node, the atrioventricular node, bundle of His, right and left tant functions in maintaining homeostatic processes. The SA node, the nor- function is structural, in which the cells act as a permeability mal pacemaker of the heart, generates a burst of electrical barrier and regulate passage of molecules and cells across the energy approximately 60 to 100 times each minute under nor- blood vessel wall. The electrical current flows over the heart the cells secrete opposing mediators that maintain a balance in an orderly way to produce contraction of both atria, then between bleeding and clotting of blood (including activation both ventricles. Selected For example, the ventricles can beat independently, but at a mediators are listed in Table 50–1; some are discussed in rate of less than 40 beats per minute. In addition, the heart does not require nervous stimulation to contract. However, the autonomic nervous system does influence heart Endothelial Mediators That rate. Sympathetic nerves increase heart rate (through the re- TABLE 50–1 Regulate Cardiovascular lease of epinephrine and norepinephrine); parasympathetic Function nerves (by way of the vagus nerve) decrease heart rate. Promoting Factors Inhibiting Factors Vasomotor Tone Blood Supply Vasodilators Vasoconstrictors Endothelial-derived hyper- Angiotensin II The heart receives its blood supply from the coronary arter- polarizing factor (EDHF) Endothelin ies. Coronary arteries branch off the aorta at the aortic valve Nitric oxide (also called Endothelium-derived constricting endothelial-derived factor and fill during diastole, the resting or filling phase of the car- relaxing factor, or EDRF) Platelet-derived growth factor diac cycle. Coronary arteries branch into smaller arteries that Prostacyclin (prostaglandin I2) Thromboxane A2 supply specific parts of the myocardium, without an over- Blood Coagulation lapping supply from other arterial branches. However, artery- Procoagulants Anticoagulants to-artery anastomoses occur between many adjacent vessels. Tissue factor Heparin sulfate These anastomotic arteries may not supply sufficient blood to Von Willebrand factor Thrombomodulin the heart if a major artery is suddenly occluded, but they may Platelet activators Platelet inhibitors dilate to considerable size when disease (usually coronary Platelet-activating factor Nitric oxide Von Willebrand factor Prostacyclin atherosclerosis) develops slowly. The resultant collateral cir- Profibrinolytic factors Antifibrinolytic factor culation may provide a sufficient blood supply for myocar- Tissue plasminogen Plasminogen activator inhibitor-1 dial function, at least during rest.

In human subjects generic extra super cialis 100mg mastercard, the recruitment cal point of view extra super cialis 100 mg low price, only with relatively weak transcra- of D and I waves after transcranial electric stimu- nial electrical stimuli does the D wave arise at corti- lation seems very similar to that seen after direct cal level: with relatively strong stimuli, the shortest stimulationoftheexposedmotorcortexinmonkeys. At threshold, D leys produced by transcranial stimulation show ini- waveresponsestotranscranialelectricalstimulation tialrecruitmentofanearlyrapidlyconductedDwave probably arise from the axon, several nodes distant followed by a series of I waves that have a sim- to the cell body (see Rothwell, 1997). However, by electric stimulation the magnetic field can penetrate scalp and skull with minimal discomfort, perhaps only that due to the Figure 1. TMS is therefore now potential produced in the first dorsal interosseus usedexclusivelyforclinicalstudiesandalmostexclu- (FDI) muscle by anodal electrical stimulation of the sively in research studies. ThelatencyofEMGresponseselicitedbyelec- ulation is only used (with TMS) when testing the trical stimulation is short, and the calculated cen- excitability of corticospinal neurones (cf. Such short latencies suggest that General methodology at least the onset of the MEP is produced by activity in the fast-conducting corticospinal axons, probably The magnetic stimulation coil the monosynaptic component of the corticospinal This consists of coils of wire connected to a large tract. When the capacitance is dis- charged, a large but transient current flows through Changes induced in the PSTH of single units the coil, some several thousand amperes` within The PSTH of a FDI unit in Fig. Thecurrentproducesamagneticfieldofupto anodal electrical stimulation produces an early peak 3 tesla oriented perpendicular to the coil (see Barker, with a sharp increase and short duration, prob- Jalinous & Freeston, 1985). The skull presents a low ably due to the D wave activating the motoneurone impedance to magnetic fields of this frequency, and monosynaptically (Day et al. Increasing the the magnetic field induces eddy currents in superfi- stimulus intensity recruits further peaks which fol- cial layers of the brain at right angles to the field. This is illustrated in the neural tissue is stimulated electrically with both Fig. What differs is the later by a second peak due to the I2 wave (pyrami- method of delivery. Electrical currents induced by the magnetic field These flow parallel to the surface of the brain. The Disadvantages magneticfieldfallsoffrapidlywithdistancefromthe The major problem with transcranial electrical stim- coil: with a typical 12 cm-diameter round coil, ulation is that only a small fraction of the current the strength falls by half at a distance of 4–5 cm from flows into the brain.