Heart autonomic control

A, Naso C, et al. 2002. Effects of sildenafil citrate (viagra) on cardiac repolarization and on autonomic control in subjects with chronic heart failure. Am. Heart J. 143 703-10... 

Two recent epidemiological investigations have found associations between exposure to low levels of MeHg and adverse cardiovascular effects. A recent study by Sorensen et al. (1999) showed an association between prenatal exposure to MeHg and cardiovascular function at age 7. The study of 1,000 children from the Faroe Islands found that diastolic and systolic blood pressures increased by 13.9 and 14.6 mm Hg, respectively, as cord-blood Hg concentrations rose from 1 to 10 pg/L. In boys, heart-rate variabihty, a marker of cardiac autonomic control. 

Porges WL, Hennessy EJ, Quail AW, et al Heart-lung interactions The sigh and autonomic control in... 

Skill Keeper Autonomic Control of Heart Rate (see Chapter 6)... 

Possible Modes of Autonomic Control for Heart Rate... 

Backs, R. W. (1995). Going beyond heart rate Modes of autonomic control in the cardiovascular assessment of mental workload. The International Journal of Aviation Psychology, 5, 25-48. 

Lohmoller G, Matuschke A et al (1989) [False-positive test of autonomic neuropathy in HIV infection and AIDS Case control study of heart rate variability in 62 HIV positive patients]. Med Klin (Munich) 84(5) 242-245... 

The autonomic nervous system exerts the primary control on heart rate. Because the sympathetic and parasympathetic systems have antagonistic effects on the heart, heart rate at any given moment results from the balance or sum of their inputs. The SA node, which is the pacemaker of the heart that determines the rate of spontaneous depolarization, and the AV node are innervated by the sympathetic and parasympathetic systems. The specialized ventricular conduction pathway and ventricular muscle are innervated by the sympathetic system only. 

The concept of chemical neurotransmission originated in the 1920s with the classic experiments of Otto Loewi (which were themselves inspired by a dream), who demonstrated that by transferring the ventricular fluid of a stimulated frog heart onto an unstimulated frog heart he could reproduce the effects of a (parasympathetic) nerve stimulus on the unstimulated heart (Loewi Navratil, 1926). Subsequently, it was found that acetylcholine was the neurotransmitter released from these parasympathetic nerve fibers. As well as playing a critical role in synaptic transmission in the autonomic nervous system and at vertebrate neuromuscular junctions (Dale, 1935), acetylcholine plays a central role in the control of wakefulness and REM sleep. Some have even gone as far as to call acetylcholine a neurotransmitter correlate of consciousness (Perry et al., 1999). 

There are two classes of movements in the human body voluntary and involuntary. Voluntary movements are pretty clear they are the movements that we can control. Reaching for the French fries, swinging a baseball bat, turning on the TV, and typing at a computer keyboard provide obvious examples. Involuntary movements include those movements that we cannot readily control such as heart beats, vascular contraction, and movement of the gut muscles, and they basically control the internal environment of the body. Voluntary movements are controlled by the somatic nervous system. Involuntary movements are controlled by the autonomic nervous system, to which we now turn. 

Many neurons of both divisions of the autonomic nervous system are tonicaUy active that is, they are continually carrying some impulse traffic. The moment-to-moment activity of an organ such as the heart, which receives a dual innervation by sympathetic (noradrenergic) and parasympathetic (cholinergic) neurons, is controlled by the level of tonic activity of the two systems. 

The autonomic nervous system is divided into the sympathetic and parasympathetic components, which typically exert opposing effects. The sympathetic system is involved in the fight or flight reaction (increased blood pressure and heart rate, and accommodation for increased vision, for example) that prepares the organism for stressful situations. The parasympathetic system conversely establishes a more relaxed situation, for instance, the rest period after a meal. The autonomic nervous system that is responsible for the independent control of the mechanical and secretory functions of the gastrointestinal tract is sometimes called the enteric system. 

Irritable heart, cardiac neurosis, or DaCosta s syndrome, has been described since the Civil War. It is manifested by cardiac symptoms and autonomic adrenergic predominance (excess levels of or supersensitivity to peripheral catecholamines). Lactate-induced panic attacks in patients are also correlated with elevated epinephrine excretion in comparison with normal control subjects. 

Autonomic and hormonal control of cardiovascular function. Note that two feedback loops are present the autonomic nervous system loop and the hormonal loop. The sympathetic nervous system directly influences four major variables peripheral vascular resistance, heart rate, force, and venous tone. It also directly modulates renin production (not shown). The parasympathetic nervous system directly influences heart rate. In addition to its role in stimulating aldosterone secretion, angiotensin II directly increases peripheral vascular resistance and facilitates sympathetic effects (not shown). The net feedback effect of each loop is to compensate for changes in arterial blood pressure. Thus, decreased blood pressure due to blood loss would evoke increased sympathetic outflow and renin release. Conversely, elevated pressure due to the administration of a vasoconstrictor drug would cause reduced sympathetic outflow, reduced renin release, and increased parasympathetic (vagal) outflow. 

Physiologically, in both normal and hypertensive individuals, blood pressure is maintained by moment-to-moment regulation of cardiac output and peripheral vascular resistance, exerted at three anatomic sites (Figure 11-1) arterioles, postcapillary venules (capacitance vessels), and heart. A fourth anatomic control site, the kidney, contributes to maintenance of blood pressure by regulating the volume of intravascular fluid. Baroreflexes, mediated by autonomic nerves, act in combination with humoral mechanisms, including the renin-angiotensin-aldosterone system, to coordinate function at these four control sites and to maintain normal blood pressure. Finally, local release of vasoactive substances from vascular endothelium may also be involved in the regulation of vascular resistance. For example, endothelin-1 (see Chapter 17) constricts and nitric oxide (see Chapter 19) dilates blood vessels. 

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