Sympathetic Distribution & Functions PDF

35 Sympathetic Distribution& FUNCTIONS ILOs By the end of this lecture, students will be able to 1. Distribute the sympathetic nervous system innervation along the body organs and systems 2. Identify the general characteristics of sympathetic nervous system...

35 Sympathetic Distribution& FUNCTIONS ILOs By the end of this lecture, students will be able to 1. Distribute the sympathetic nervous system innervation along the body organs and systems 2. Identify the general characteristics of sympathetic nervous system 3. Correlate the functions of SNS with its distribution in different organs 4. Evaluate the significance of sympathetic functions 5. Interpret and relate the functions of the sympathetic nervous system to adaptation of the body during stress situations. 1- Segmental Distribution of the Sympathetic Nerve Fibers. The sympathetic pathways that originate in the different segments of the spinal cord are not necessarily distributed to the same part of the body as the somatic spinal nerve fibers from the same segments. Instead, the sympathetic fibers from cord segment T-1 generally pass up the sympathetic chain to terminate in the head; from T-2 to terminate in the neck; from T-3, T-4, T-5, and T-6 into the thorax; from T-7, T-8, T-9, T-10, and T-11 into the abdomen; and from T-12, L-1, and L-2 into the legs. This distribution is only approximate and overlaps greatly. Fibers that secrete norepinephrine are said to be adrenergic, a term derived from adrenalin, which is an alternate name for epinephrine. While those that secrete acetylcholine are said to be cholinergic. Most of the postganglionic sympathetic neurons are adrenergic. However, postganglionic sympathetic nerve fibers to the sweat glands and to the piloerector muscles of the hairs ( in animals) are cholinergic. 2- Adrenergic receptors: 1. Adrenergic receptors (adreno-receptors) a. α1 Receptors Are located on vascular smooth muscle of the skin and splanchnic regions, the gastrointestinal (GI) and bladder sphincters, and the radial muscle of the iris. Produce excitation (e.g., contraction or constriction). Are equally sensitive to norepinephrine and epinephrine. However, only norepinephrine released from adrenergic neurons is present in high enough concentrations to activate a1 receptors. b. α2 Receptors Page 1 of 7  Are located on sympathetic postganglionic nerve terminals to inhibit more release of neurotransmitter also in platelets, fat cells, and the walls of the GI tract Often produce inhibition c. ß1Receptors Are located in the sinoatrial (SA) node, atrioventricular (AV) node, and ventricular muscle of the heart. Produce excitation (e.g., increased heart rate, increased conduction velocity, increased contractility). Are sensitive to both norepinephrine and epinephrine, and are more sensitive than the α1 receptors. d. ß2 Receptors Are located on vascular smooth muscle of skeletal muscle, bronchial smooth muscle, and in the walls of the GI tract and bladder. Produce relaxation (e.g., dilation of vascular smooth muscle, dilation of bronchioles, relaxation of the bladder wall). Are more sensitive to epinephrine than to norepinephrine. Are more sensitive to epinephrine than the ß1 receptors. General characteristics of sympathetic nervous system As a generalization, the sympathetic nervous system is said to mediate stress responses, such as the classic fight or flight response, and the parasympathetic system mediates “vegetative” responses, such as digestion.The fight or flight response is a generalized reaction to extreme fear, stress, or physical activity and results in a patterned response in many organ systems. Functions of sympathetic Nervous system (figure 1) Organ Effects 1. Head and Neck -Motor to dilator pupillae muscle leading to dilatation of eye pupil (mydriasis). - Trophic secretory fibers to salivary glands (secretion of viscid saliva, small in volume and rich in organic matter). Page 2 of 7  - Vasoconstriction in both cutaneous and deep vessels 2. Thorax; Heart & Lung - Excitation of all properties of the cardiac muscle a. Increase heart rate. b.Increase force of contraction. - Vasodilator to coronaries, that is, increase the blood supply to the heart. - Dilatation to the bronchi. - Slight vasoconstriction of the pulmonary vessels. 3. Abdomen Liver Vasoconstriction to the blood vessels of the viscera. Stomach & intestine - Stimulation of glycogenolysis (increase blood glucose level) Gall Bladder - ↓Motility - ↓Secretion - Relaxation of the wall and contraction of sphincters. 4. Pelvis -Inhibitory to the smooth muscle of the distal half of the colon and rectum but motor to the internal anal sphincter leading to retention of faeces. - Inhibitory to the smooth muscle of urinary bladder and motor to the internal urethral sphincter leading to retention of urine – Vasoconstrictor to the blood vessels of pelvic viscera. - Motor to the smooth muscle of male sex organs (epididymis, ejaculatory ducts, seminal vesicles and prostate) leading to ejaculation of semen. 5. Limbs, thoracic and -Vasoconstrictor to cutaneous blood vessels. abdominal walls - Secretory to the sweat glands. - Motor to the erector pilae muscle causing erection of hair (in animals). - Vasodilatation to the blood vessels of the skeletal muscles, leading to increase of blood flow, increase of muscle glycogenolysis and increase force of contraction 6. General effects -Increase mental activity Page 3 of 7  - Increase basal metabolic rate - Increase blood coagulation - Increase blood glucose Function of the Adrenal Medullae Stimulation of the sympathetic nerves to the adrenal medullae causes large quantities of epinephrine and norepinephrine to be released into the circulating blood, and these two hormones in turn are carried in the blood to all tissues of the body. On the average, about 80 per cent of the secretion is epinephrine and 20 per cent is norepinephrine, although the relative proportions can change considerably under different physiologic conditions. The circulating epinephrine and norepinephrine have almost the same effects on the different organs as the effects caused by direct sympathetic stimulation, except that the effects last 5 to 10 times as long because both hormones are removed from the blood slowly over a period of 2 to 4 minutes. The circulating norepinephrine causes constriction of essentially all the blood vessels of the body; it also causes increased activity of the heart, inhibition of the gastrointestinal tract, dilation of the pupils of the eyes, and so forth. Epinephrine causes almost the same effects as those caused by norepinephrine, but the effects differ in the following respects: First, epinephrine, because of its greater effect in stimulating the beta receptors, has a greater effect on cardiac stimulation than does norepinephrine. Second, epinephrine causes only weak constriction of the blood vessels, in comparison with much stronger constriction caused by norepinephrine. norepinephrine greatly increases the total peripheral resistance and elevates arterial pressure, whereas epinephrine raises the arterial pressure to a lesser extent but increases the cardiac output more. A third difference between the actions of epinephrine and norepinephrine relates to their effects on tissue metabolism. Epinephrine has 5 to 10 times as great a metabolic effect as norepinephrine. Indeed, the epinephrine secreted by the adrenal medullae can increase the metabolic rate of the whole body often to as much as 100 per cent above normal, in this way increasing the activity and excitability of the body. It also increases the rates of other metabolic activities, such as glycogenolysis in the liver and muscle, and glucose release into the blood. Page 4 of 7 In summary, stimulation of the adrenal medullae causes release of the hormones epinephrine and norepinephrine, which together have almost the same effects throughout the body as direct sympathetic stimulation, except that the effects are greatly prolonged, lasting 2 to 4 minutes after the stimulation is over. Figure 1: Functions of sympathetic Nervous system Mass Discharge of sympathetic system. In many instances, almost all portions of the sympathetic nervous system discharge simultaneously as a complete unit, a phenomenon called mass discharge. This frequently occurs when the hypothalamus is activated by fright or fear or severe pain. The result is a widespread reaction throughout the body called the alarm or stress response. At other times, activation occurs in isolated portions of the sympathetic nervous system. For example, during the process of heat regulation, the sympathetic control sweating and blood flow in the skin without affecting other organs innervated by the sympathetic. “Alarm” or “Stress” Response of the Sympathetic Nervous System Page 5 of 7 When large portions of the sympathetic nervous system discharge at the same time—that is, a mass discharge this increases in many ways the ability of the body to perform vigorous muscle activity. Let us summarize these ways: 1. Increased arterial pressure 2. Increased blood flow to active muscles concurrent with decreased blood flow to organs such as the gastrointestinal tract and the kidneys that are not needed for rapid motor activity 3. Increased rates of cellular metabolism throughout the body 4. Increased blood glucose concentration 5. Increased glycolysis in the liver and in muscle 6. Increased muscle strength 7. Increased mental activity 8. Increased rate of blood coagulation The sum of these effects permits a person to perform far more strenuous physical activity than would otherwise be possible. Because either mental or physical stress can excite the sympathetic system, it is frequently said that the purpose of the sympathetic system is to provide extra activation of the body in states of stress: this is called the sympathetic stress response. The sympathetic system is especially strongly activated in many emotional states. For instance, in the state of rage, which is elicited to a great extent by stimulating the hypothalamus, signals are transmitted downward through the reticular formation of the brainstem and into the spinal cord to cause massive sympathetic discharge; most aforementioned sympathetic events ensue immediately. This is called the sympathetic alarm reaction. It is also called the fight or flight reaction because anyone in this state decides almost instantly whether to stand and fight or to run. In either event, the sympathetic alarm reaction makes the subject’s subsequent activities vigorous. Page 6 of 7 Page 7 of 7