Heart, autonomic nervous system activity

On the other hand, a decrease in blood pressure causes less than normal distension or stretch of the aorta and carotid arteries and a decrease in baroreceptor stimulation. Therefore, fewer afferent nerve impulses are transmitted to the vasomotor center. The vasomotor center then alters autonomic nervous system activity so that sympathetic stimulation of vascular smooth muscle and the heart is increased and parasympathetic stimulation of the heart is decreased. As a result, venous return, CO, and TPR increase so that MAP is increased back toward its normal value. The effects are summarized in Figure 15.5. 

The QT interval is a dynamic physiological variable depending on multiple factors such as cardiac cycle length (heart rate), autonomic nervous system activity, age, gender, plasma electrolyte concentrations, genetic variations in ion channels involved in cardiac repolarization. In addition, circadian and seasonal variations of the QT interval have been described [93]. 

Inhaled anesthetics change heart rate either directly by altering the rate of sinus node depolarization or indirectly by shifting the balance of autonomic nervous system activity. Bradycardia can be seen with halothane, probably because of direct vagal stimulation. In contrast, enflurane, and sevoflurane have little effect, and both desflurane and isoflurane increase heart rate. In the case of desflurane, transient sympathetic activation with elevations in catecholamine levels can lead to marked increases in heart rate and blood pressure when high inspired gas concentrations are administered. 

The baroreceptor reflex is a central reflex mechanism, which reduces heart rate following an increase in blood pressure. Each change in blood pressure is sensed by baroreceptors in the carotid arteries, which activate the autonomic nervous system to alter heart rate and thereby readjust blood pressure. 

The PNS is a part of the autonomic nervous system (see Chap. 22). It helps conserve body energy and is partly responsible for activities such as slowing the heart rate, digesting food, and eliminating body wastes. 

Many visceral organs are innervated by both divisions of the autonomic nervous system. In most instances, when an organ receives dual innervation, the two systems work in opposition to one another. In some tissues and organs, the two innervations exert an opposing influence on the same effector cells (e.g., the sinoatrial node in the heart), while in other tissues opposing actions come about because different effector cells are activated (e.g., the circular and radial muscles in the iris). 

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 rate of pacemaker discharge within these specialized myocytes is influenced by the activity of both divisions of the autonomic nervous system. Increased sympathetic nerve activity to the heart, the release of catecholamines from the adrenal medulla, or the exogenous administration of adrenomimetic amines will cause an increase in the rate of pacemaker activity through stimulation of -adrenoceptors on the pacemaker cells (Figure 16.3). 

It is well established that increased sympathetic nerve activity is associated with chronic heart failure (CHF) (Porter et al. 1990 Singh 2000 Olshansky 2005 Brodde et al. 2006 Watson et al. 2006). The increase in sympathetic activity is a compensatory mechanism that provides inotropic support to the heart and peripheral vasoconstriction. However, it promotes disease progression and worsens prognosis (Watson et al. 2006). The autonomic nervous system (ANS) is a very complex, balanced system that influences the initiation, termination, and perpetuation of atrial fibrillation (AF), and the AF affects the ANS (Olshansky, 2005). At rest, sympathetic and parasympathetic outflows are related reciprocally heart failure patients had high sympathetic and low parasympathetic outflows, and healthy subjects had low sympathetic and high parasympathetic outflows (Porter et al. 1990). 

From the influence of the autonomic nervous system it follows that all sympatholytic or sympathomimetic and parasympatholytic or parasympathomimetic drugs can produce corresponding effects on cardiac performance. These possibilities are exploited therapeutically for instance, p-blockers for suppressing excessive sympathetic drive (p. 96) ipratropium for treating sinus bradycardia (p. 108). An unwanted activation of the sympathetic system can result from anxiety, pain, and other emotional stress. In these cases, the heart can be protected from harmful stimulation by psychopharmaceuticals such as benzodiazepines (diazepam and others important in myocardial infarction). 

Norepinephrine Mostly excitatory, but inhibitory in some areas. Secreted by neurons in the locus ceruleus (subcortical area) to widespread areas of the brain, controlling wakefulness, overall activity, and mood. Also diffusely secreted in the sympathetic nervous system. Diffuse and widespread symptoms, including depression, changes in blood pressure, heart rate, and diffuse physiological responses, among many others. An important transmitter in the sympathetic branch of the autonomic nervous system. Diffusely affected by many medications. Several antidepressants work specifically on this neurotransmitter and its receptor sites. Many medications for general medical conditions affect this neurotransmitter as well. 

The other branch of the autonomic nervous system is the parasympathetic branch, which in general balances the actions of the sympathetic branch by exerting opposite effects. Parasympathetic activity reduces heart rate, bkxxl pressure, and so on. In contrast to sympathetic neurons, parasympathetic synapses arc primarily cholinergic. 

Stimulants are classic examples of sympathomimetic drugs that is, they act to stimulate or mimic activity in the sympathetic branch of the autonomic nervous system. Thus, many of their physiological effects arc the same as those seen during emotional arousal heart rate is up, blood pressure is up, respiratory rate is up, and sweating increases meanwhile, blood flow decreases to the internal organs and extremities but increases to the large muscle groups and the brain. Finally, body temperature is elevated and pupils arc dilated. 

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