Sympathetic/parasympathetic balance

Sympathetic and parasympathetic nerves innervate the penis. In the flaccid state, OC2-adrenergic receptors mediate tonic contraction of the arterial and corporal smooth muscles. This maintains high penile arterial resistance and a balance exists between blood flow into and out of the corpora. With sexual stimulation, nerve impulses from the brain travel down the spinal cord to the thoracolumbar ganglia.3 A decrease in sympathetic tone and an increase in parasympathetic activity then occurs, causing a net increase in blood flow into the erectile tissue. Erections may also occur as a result of a sacral nerve reflex arc while patients are sleeping (nocturnal erections). 

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 resting heart rate of 60-80 bpm results from dominant vagal tone. The intrinsic rate generated by the sinoatrial (SA) node is 110 bpm. Control of heart rate is, therefore, through the balance of parasympathetic and sympathetic activity via the vagus and cardioaccelerator (T1-T5) fibres, respectively. 

Since the main clinical use for antisympathotonics is in the treatment of essential hypertension, such drugs will be discussed in Chapter 20 in more detail. The alkaloid reserpine from Rauwolfia serpentina was the first drug used clinically to reduce sympathetic tone. Reserpine reduce the ability of storage and release of various transmitters (adrenaline, noradrenaline, serotonine and dopamine) by an irreversible destruction of the axonal vesicle membranes. The duration of the reserpine effect is actually determined by the de novo synthesis of these structure. Beside various central side effects like sedation, depression, lassitude and nightmares the pattern of unwanted effects of reserpine is determined by the shift of the autonomic balance towards the parasympathetic branch myosis, congested nostrils, an altered saliva production, increased gastric acid production, bardycardia and diarrhea. As a consequence of the inhibition of central dopamine release, reserpine infrequently shows Parkinson-like disturbances of the extrapyramidal system. 

Figure. 2—4. The two major components of the ANS. The parasympathetic neurons release acetylcholine the sympathetic neurons release norepinephrine. These two systems provide a balance of control of the function of the organs and structures...

Recall that scopolamine, an ingredient in henbane, blocks muscarinic acetylcholine receptors. This blockade essentially removes the influence of the parasympathetic nervous system on the body. In the absence of this influence, the balance of forces is upset and the sympathetic nervous system gains the upper hand thus, your heart rate increases, your pupils dilate, salivation stops, your ability to urinate is impaired, and you become constipated overall, things get very uncomfortable. But none of this is directly lethal (unless the constipation makes one commit suicide). If you do die from an overdose of henbane, it is believed to result from either a complex series of events in your brain that lead to the loss of control of your diaphragm, causing death from asphyxiation, or from cardiac arrest. This is why the deadly nightshade is so deadly and how Shakespeare chose to kill King Hamlet with henbane. 

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). 

The parasympathetic division maintains essential bodily functions, such as digestive processes and elimination of wastes, and is required for life (see Figure 3.3). It usually acts to oppose or balance the actions of the sympathetic division and is generally dominant over the sympathetic system in "rest and digest" situations (see Figure 3.4). The parasympathetic system is not a functional entity as such and never discharges as a complete system. If it did, it would produce massive, undesirable, and unpleasant symptoms. Instead, discrete parasympathetic fibers are activated separately, and the system functions to affect specific organs, such as the stomach or eye. 

Disturbed balance between the parasympathetic and the sympathetic systems can result in disturbances in cardiac function. As discussed later, chemical warfare agents play an important role in disturbing this balance. 

The electrical activity of the heart is modulated by hormones and neurotransmitters. Xenobiotics disturb their balance. The parasympathetic system releases ACh and the sympathetic system releases catecholamines (norepinephrine and epinephrine). These bind to a and p types of receptors. ai-receptors are present on the post-synaptic member of the organ and mediate vasoconstriction and stimulation of Na" /K -ATPase, the Na" /Ca exchanger, and the Na /H exchanger. This affects the Ikatp and inhibits the Ij,ia+ and Ito. The a-receptor stimulation thus effectuates depolarization, and the a2-receptor inhibits norepinephrine release. 

Pupil size and function can be affected by peripheral autonomic action and by centrally initiated impulses.The iris is an excellent indicator of autonomic activity because of the delicate balance between adrenergic and cholinergic innervation to the iris dilator and iris sphincter muscles, respectively. By acting directly on these muscles, both sympathetic and parasympathetic agents can influence pupil size and activity. 

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. 

Organs controlled by the autonomic nervous system usually are innervated by both the sympathetic and the parasympathetic systems. There is a continual state of dynamic balance between the two. systems. Theoretically, one should achieve the same end result by either stimulation of one of the sy.s-tems or blockade of the other. Unfortunately, there is usually a limitation to this type of generalization. There are. however, three predictable and clinically useful results from blcK king the muscarinic effects of ACh. 

The size of the pupil is determined by the balance of forces exerted by the dilator muscles fibers (sympathetically innervated and radially arranged) and the constrictor muscle fibers (parasympathetically innervated and circularly arranged) of the iris. Normally both sets of muscle fibers have a constant degree of tonus and act reciprocally to dilate or constrict the pupil. Any substance that paralyzes the constrictor muscle fibers (parasympathol3d ic) allows the unopposed tone of dilator muscle fibers to widen the pupil. 

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. 

Atropine is an antimuscarinic agent that blocks the depressant effect of acetylcholine on both the sinus and atrioventricular nodes, thus decreasing parasympathetic tone. During asystole, parasympathetic tone may increase because of the vagal stimulation that occurs secondary to intubation, the effects of hypoxia and acidosis, or alterations in the balance of parasympathetic and sympathetic control. Unfortunately, there are no large randomized trials showing benefit from atropine for the treatment of asystole. Evidence is limited to small case series or retrospective reviews. " ... 

Previously in this chapter you learned that the autonomic nervous system has two branches. These are the sympathetic branch and parasympathetic branch. Both branches act on the same organ cells but in an opposite way. The sympathetic branch stimulates a response and the parasympathetic branch depresses a response by the organ cell. Together, they keep the organ in balance (homeostasis). 

To maintain homeostasis, the continuous transudation of fluid and solutes from the pulmonary capillary bed into the surrounding inteistitium and alveolar space is balanced by lymphatic drainage out of the lung. The lymphatic flow is directed toward the hilum from the pleural surfaces. From lymph nodes in the hilum, the lymph travels to the paratracheal nodes and then eventually into the venous system via the thoracic duct. The lung has nerve fibers from both the vagal nerves parasympathetic) and the sympathetic nerves. The efferent fibers go to the bronchial musculature and the afferents come from the bronchi and alveoli. 

Osteopathic intervention should address the autonomic nervous system and the lymphatics. The sympathetic and parasympathetic systems must be balanced. Lymphatic techniques are important but should be used with caution so that an overload of fluid from the extremities is not... 

Osteopathic manipulation is aimed at the autonomic nervous system, particularly seeking to balance the sympathetic and parasympathetic arms. Removal of somatic problems makes the patient more comfortable and thus more able to cope with the other issues in life. 

The autonomic nervous system provides access to most of the viscera of the body through either the sympathetic or parasympathetic systems, or by balancing the two systems. The autonomic nervous system is discussed in detail in Chapter 109. 

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