Sympathetic system

As we have had occasion to note more than a few times previously, the guanidine function forms the basis of a family of hypotensive agents active by reason of their activity as blockers of the peripheral sympathetic system. Condensation of tetra-hydroisoquinoline with the S-methyl ether of thiourea affords the antihypertensive drug debrisoquin (135). ... 

The small molecular transmitter characteristic of the sympathetic nervous system is norepinephrine. However, epinephrine from the adrenal medulla is an important partner in systemic sympathetic actions. The actions of the sympathetic system are unique because in different smooth muscles responses are diametrically opposed they may be either by contraction or inhibition of contraction and... 

Because of its motor, i.e., activating effect on vascular smooth muscle and its inhibitory effect on intestinal smooth muscle, the sympathetic nervous system has been cast into the role of the component of the nervous system that executes control of visceral function in times of physical emergency for the organism. The phrase fight or flight has been often used to describe the circumstances in which the adrenergic transmitters of the sympathetic system are dominant over the cholinergic parasympathetic system. This concept is perhaps oversimplified but it has the utility of a first approximation of how the two components of the ANS interact in the periphery. Sensory inputs which lead to increased blood pressure, for example, activate the sympathetic pathways. 

Psychogenic dysfunction occurs if a patient does not respond to psychic arousal. It occurs in up to 30% of all cases of ED. Common causes include performance anxiety, strained relationships, lack of sexual arousability, and overt psychiatric disorders such as depression and schizophrenia.5 It is postulated that the anxious or nervous man will have excessive stimulation of the sympathetic system, leading to smooth muscle contraction of arterioles and vascular spaces within erectile tissue.6 O Many patients may initially have organic dysfunction, but develop a psychogenic component as they try to cope with their inability to achieve an erection. It has been estimated that up to 80% of ED cases have an organic cause, with many having a psychogenic component as well.1... 

Describe the overall and specific functions of the sympathetic system... 

The ANS is composed of two anatomically and functionally distinct divisions the sympathetic system and the parasympathetic system. Two important features of these divisions include ... 

Table 9.2 Distinguishing Features of Sympathetic and Parasympathetic Systems Sympathetic system Parasympathetic system...

Finally, the preganglionic neuron may travel to the adrenal medulla and synapse directly with this glandular tissue. The cells of the adrenal medulla have the same embryonic origin as neural tissue and, in fact, function as modified postganglionic neurons. Instead of the release of neurotransmitter directly at the synapse with an effector tissue, the secretory products of the adrenal medulla are picked up by the blood and travel throughout the body to all of the effector tissues of the sympathetic system. 

Because the terminal ganglia are located within the innervated tissue, there is typically little divergence in the parasympathetic system compared to the sympathetic system. In many organs, the ratio of preganglionic fibers to postganglionic fibers is 1 1. Therefore, the effects of the parasympathetic system tend to be more discrete and localized, with only specific tissues stimulated at any given moment, compared to the sympathetic system in which a more diffuse discharge is possible. 

As previously mentioned, the cells of the adrenal medulla are considered modified sympathetic postganglionic neurons. Instead of a neurotransmitter, these cells release hormones into the blood. Approximately 20% of the hormonal output of the adrenal medulla is norepinephrine. The remaining 80% is epinephrine (EPI). Unlike true postganglionic neurons in the sympathetic system, the adrenal medulla contains an enzyme that methylates norepinephrine to form epinephrine. The synthesis of epinephrine, also known as adrenalin, is enhanced under conditions of stress. These two hormones released by the adrenal medulla are collectively referred to as the catecholamines. 

Figure 9.2 Autonomic nerve pathways. All preganglionic neurons release acetylcholine (Ach), which binds to nicotinic receptors (N) on the postganglionic neurons. All postganglionic neurons in the parasympathetic system and some sympathetic postganglionic neurons innervating sweat glands release Ach that binds to muscarinic (M) receptors on the cells of the effector tissue. The remaining postganglionic neurons of the sympathetic system release norepinephrine (NE), which binds to alpha (a) or beta (P) receptors on cells of the effector tissue. The cells of the adrenal medulla, which are modified postganglionic neurons in the sympathetic system, release epinephrine (EPI) and NE into the circulation.

Sympathetic nerve activity causes an increase in blood pressure through many mechanisms, including an increase in cardiac activity and vasoconstriction. Activation of the sympathetic system also causes the stimulation of Pi-adrenergic receptors on the renin-producing cells, which promotes renin release. 

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 sympathetic system innervates most tissues in the heart including the SA node, AV node, and ventricular muscle. Sympathetic stimulation causes an increase in HR as well as an increase in ventricular contractility, which... 

The sympathetic system also innervates vascular smooth muscle and regulates the radius of the blood vessels. All types of blood vessels except capillaries are innervated however, the most densely innervated vessels include arterioles and veins. An increase in sympathetic stimulation of vascular smooth muscle causes vasoconstriction and a decrease in stimulation causes vasodilation. Constriction of arterioles causes an increase in TPR and therefore MAP. Constriction of veins causes an increase in venous return (VR) which increases end-diastolic volume (EDV), SV (Frank-Starling law of the heart), CO, and MAP. 

Sympathetic stimulation of veins. The smaller, more compliant veins that serve generally as blood reservoirs as well as specific blood reservoirs are densely innervated by the sympathetic system. Stimulation of the vascular smooth muscle in the walls of these vessels causes vasoconstriction and a decrease in venous compliance. Vasoconstriction increases venous pressure in the veins the blood is squeezed out of the veins and, due to the presence of one-way valves, moves toward the heart so that VR increases. A decrease in sympathetic stimulation allows the veins to relax and distend. The vessels become more compliant and capable of holding large volumes of blood at low pressures. In this case, VR decreases. 

Resistance in the arterioles of the working muscles is regulated locally. As discussed previously, active hyperemia results in production of several factors that cause metabolic vasodilation. Exercising muscles generate COz, H+ and K+ ions, heat, and adenosine. The vasodilator effect of these locally produced substances overrides the vasoconstrictor effect of the sympathetic system in the muscle. As a result, local vascular resistance is decreased. The combination of increased driving pressure and decreased local vascular resistance causes an increase in blood flow to the working muscles. 

Only the sympathetic system supplies the ureters, uterus, uterine tubes, vasa deferentia, blood vessels (which are constricted), sweat glands (actually cholinergic, however, fig. 3) and ihe arrectores pilorum which are contracted. 

The pupil is supplied with constrictor fibres from the parasympathetic system (via the Illrd or oculomotor nerve and the ciliary ganglion) and with dilator fibres from the sympathetic system (the upper thoracic nerves to the sympathetic and to the inferior and superior cervical ganglia fig. 8). 

In neonates and children the sympathetic system is relatively underdeveloped while the parasympathetic supply is relatively well formed. Despite a high resting heart rate in this population, many insults may, therefore, result in profound bradycardia. The most serious of these insults is hypoxia. 

There is evidence that stressful conditions decrease the effectiveness of the immune system the loss of a job, a divorce or a bereavement increases the risk of development of cancer. This is due to impairment of the process of immune surveillance carried out by the neutrophils and other immune cells. They kill tumour cells that are migrating from a primary tumour to establish another tumour in a different tissue. The impairment may be due to chronic activation of the sympathetic system, which increases the... 

Figure 19.17 The biochemistiy and physiology responsible for penile erection. Sexual activity itself begins with a state of arousal that leads to erection. Arousal results in part from stimulation of the sense organs. The hypothalamus coordinates the sensations and activates the autonomic nervous system. Sensory nerves from the skin of the penis and other erogenous zones stimulate the parasympathetic system. This activates nitric oxide synthase and the resultant nitric oxide, via cyclic GMP, causes vasodilation of the arterioles. This increases blood flow through the corpora cavernosa which then expands producing an erection. Pheromones secreted by the female can stimulate the odour detecting system in the nasal cavity of the male (Chapter 12 and see above). Stress, however, activates the sympathetic system releases cyclic AMP which can result in vasoconstriction of the arterioles. Other factors that can interfere with an erection are physical fatigue and alcohol.

Phenylephrine is a nasal decongestant that mimics the sympathetic system, thereby increasing the heart rate and blood pressure. It may aggravate conditions such as diabetes, hypertension and glaucoma. Patients with hypertension, ischaemic heart disease, hyperthyroidism, diabetes and glaucoma are therefore given topical nasal sympathomimetics rather than systemic sympathomimetics. Both topical and systemic sympathomimetics are contraindicated in patients taking monoamine oxidase inhibitors, because concurrent administration of the two products may lead to a hypertensive crisis. 

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