Blood pressure autonomic control

Noradrenaline - the EEG is aroused by stimulants such as the amphetamines and methylphenidate whereas drugs such as reserpine which deplete brain noradrenaline have the opposite effect. Similar effects to the stimulants may be obtained by the electrical stimulation of the locus coeruleus which has been shown to decrease in activity during the REM sleep phase of the sleep cycle. The precise role that noradrenaline plays in sleep is uncertain. While it may be involved in sleep induction, noradrenaline also has many other physiological functions including control of the heart rate, blood pressure, autonomic activity, etc. which play a role in the entraining process. 

Blood pressure is under the control of the autonomic (sometimes called the involuntary or reflex) nervous system. 

Patients with the following underlying conditions can be particularly sensitive to the actions of vasodilators, including sildenafil, tadalafil, and vardenafil Those with left ventricular outflow obstruction (eg, aortic stenosis, idiopathic hypertrophic subaortic stenosis) and those with severely impaired autonomic control of blood pressure. 

Nowadays a broad spectrum of quite specific blood pressure lowering drugs is available which restricts the use of ganglion blockade. There are only a few situations in which the pharmacological blockade autonomous ganglia is clinically useful hypertensive emergencies, controlled hypotension in neurosurgery and in the treatment of pulmonary edema. 

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. 

Given orally at normal therapeutic doses BDZs have little effect on cardiovascular, respiratory or autonomic function. Respiratory depression and reduced systolic blood pressure may occur but this is seen principally with intravenous administration or overdose. Leucopenia and eosinophilia are rare. There was a suggestion in the early 1980s of increased risk of breast cancer but a subsequent large case-control study refuted this. 

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. 

In addition to the somatic motor system that operates the voluntary (striated) muscles via the pyramidal tract, there is the autonomic system, which controls the involuntary (smooth) muscles, glands, heartbeat, blood pressure, and body temperature. This system has its origins in both the cerebral cortex and... 

The human nervous system can be divided into two major functional areas the somatic nervous system and the autonomic nervous system (ANS). The somatic division is concerned primarily with voluntary function—that is, control of the skeletal musculature. The ANS is responsible for controlling bodily functions that are largely involuntary, or automatic, in nature. For instance, the control of blood pressure (BP) and other aspects of cardiovascular function is under the influence of the ANS. Other involuntary, or vegetative, functions such as digestion, elimination, and thermoregulation are also controlled by this system. 

Methaqualone also affects involuntary body functions that are controlled by the autonomic nervous system, lowering blood pressure, breathing rate, and pulse and bringing about a state of deep relaxation. Though thought to be an aphrodisiac because it lowers inhibitions, methaqualone, as a CNS depressant, usually impairs sexual performance, inhibiting arousal and climax. 

The afferent neurons of the autonomic nervous system are important in the reflex regulation, for example, by sensing pressure in the carotid sinus and aortic arch and signaling the CNS to influence the efferent branch of the system to respond. Conditions such as trauma, fear, hypoglycemia, cold, or exercise activate the sympathetic neurons. Both sympathetic and parasympathetic neurons emerge from the brain stem or spinal cord. Blood pressure is regulated largely by sympathetic control of vascular tone. 

This is quite different from the more typical experience that most of us have had at one time or another - to awaken from a dream in which we were trying to escape from imaginary pursuers, absolutely terrified. In the second case, which is more likely to occur in REM sleep, we have formed the perceptual scenario of an attack situation from which we are attempting to flee, and our emotion is appropriate to the dreamed action. Figure 7 shows the activation that is not in our control (i.e. autonomic activation) which is normally associated with REM sleep. As can be seen, increases in heart rate, blood pressure, and respiratory rate can begin in NREM sleep. 

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. 

A 39-year-old quadriplegic man with poorly controlled pain had many features consistent with autonomic dysfunction (for example a C4 spinal lesion, orthostatic hypotension, hypertension). He routinely used trans-dermal clonidine and transdermal glyceryl trinitrate as needed for control of acute hypertensive episodes. The clonidine was discontinued, after which his blood pressure fell (maximum systolic and diastolic pressures by about 50 and 25 mmHg respectively). 

Because capillary walls are thin (to permit diffusion) the blood that is delivered to them must be delivered under reduced pressure. This is accomplished by the arterioles, which combine relatively muscular walls with a narrow lumen. The arterial blood pressure is a function of cardiac output and the total peripheral vascular resistance, which is primarily a function of the degree of normal tension (tonus) of the smooth muscle cells in the walls of the arterioles. If this tonus increases above the normal range for extended periods of time, hypertension (high blood pressure) will result. This tonus is under the control of the autonomic nervous system and of adrenergic hormones (catecholamines). 

Central mechanisms. Some agents may act within the CNS to modify autonomic control of sympathetic tone and blood pressure. Clonidine inhibits release of noradrenaline by an agonist action at the autoinhibitory a2-adrenoceptors on sympathetic nerve endings. Methyldopa is thought to work, at least in part, centrally, acting both as an inhibitory false substrate in the biosynthetic pathway, also producing an active metabolite with actions at aj-adrenoceptors. 

In the peripheral nervous system, norepinephrine is an important neurotransmitter in the sympathetic branch of the autonomic system. Sympathetic nerve transmission operates below the level of consciousness in controlling physiological function of many organs and tissues of the body. The sympathetic system plays a particularly important role in regulating cardiovascular function in response to postural, exertional, thermal, and mental stress. With sympathetic activation, the heart rate is increased, peripheral arterioles are constricted, skeletal arterioles are dilated, and the blood pressure is elevated. In addition, sympathetic nerve stimulation dilates pupils inhibits smooth muscles of the intestines, bronchi, and bladder and closes the sphincters. Sympathetic signals work in balance with the parasympathetic portion of the autonomic nervous system to maintain a stable internal environment. 

Dysautonomias also range from mechanistically straightforward disorders in which altered autonomic function plays a primary pathophysiological role (e.g., pure autonomic failure), to conditions in which altered autonomic fimction worsens an independent pathophysiological state (e.g., cardiac failure), and to more mysterious disorders in which the involvement of the autonomic nervous system is less clear (e.g., chronic fatigue syndrome). Abnormalities of blood pressure control represent the common presenting cfinical features of the dysautonomias. In those involving... 

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. 

The ANS is the major involuntary portion of the nervous system and is responsible for automatic, unconscious bodily functions, such as control of heart rate and blood pressure and both gastrointestinal and genitourinary functions. The ANS is divided into two major subcategories the parasympathetic autonomic nervous system (PANS) and the sympathetic autonomic nervous system (SANS). 

Blood pressure is the product of total peripheral resistance (TPR) and cardiac output (CO). Both branches of the ANS are involved in the autonomic (or neural) control of blood pressure via feedback mechanisms. Changes in mean blood pressure are detected by baroreceptors, which relay information to the cardiovascular centers in the brainstem controlling PANS and SANS outflow. For example, an increase in mean blood pressure elicits baroreceptor discharge,... 

Blood pressure (BP) is a product of the total peripheral resistance (TPR) times the cardiac output (CO). The CO is equal to the heart rate (HR) times the stroke volume (SV). The autonomic (neural) system helps regulate the BP through feedback control involving the baroreceptors, the cardiovascular centers in the brain stem, and the PANS and SANS, which act in an opposing but coordinated manner to regulate the pressure. 

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