Chapter 18 Circulation PDF

Chapter 18: Nervous regulation of the circulation, and the rapid control of arterial pressure As discussed in Ch. 17, local blood flow regulation occurs by metabolic mechanisms. control of This chapter shows that nervous the circulation has more “global” functions, such as redistribution of flo...

Chapter 18: Nervous regulation of the circulation, and the rapid control of arterial pressure As discussed in Ch. 17, local blood flow regulation occurs by metabolic mechanisms. control of This chapter shows that nervous the circulation has more “global” functions, such as redistribution of flow, changing pumping activity of the heart, and the rapid control of arterial pressure. Slide 65 The Autonomic Nervous System “The effectors” ANS Branches Sympathetic Parasympathetic 1. Sympathetic – Most important!!! powerful influence on vascular tone and cardiac function. ­ HR, ­ Cardiac contractility, ­ vasoconstriction (­ resistance) 2. Parasympathetic – ¯ cardiac function (¯ HR, contractility-small effect) Slide 66 Autonomic innervation of the circulation. No innervation in capillaries, Pre-capillaries or meta-arterioles. Sympathetic Parasympathetic Effect of the Sympathetic nervous system: Veins↓ volume Small arteries and arterioles↑Resistance, ↓ Blood flow Figure 18-1 Slide 67 Central Vasomotor Centers Distinct vaso-,cardio-motor centers in the brain stem (medulla). Vasomotor Center: Transmits parasympathetic via vagus nerves to heart. And, sympathetic impulses through the cord and peripheral sympathetic nerves to blood vessels. Figure 18-1 Slide 68 Central Vasomotor Centers Vasoconstrictor Area. Continual firing. Excites preganglionic sympathetic neurons. Maintains vasomotor tone of blood vessels. Vasomotor tone: Continuous partial state of contraction Figure 18-1 Slide 69 Central Vasomotor Centers Vasodilator Area Inhibits activity of vasoconstrictor area Figure 18-1 Slide 70 Central Vasomotor Centers Sensory Area (Tractus Solitarius) Receives signals from “pressure” receptors (baroreceptors, atrial receptors) Helps control vasoconstrictor and vasodilator areas Figure 18-1 Slide 71 Parasympathetic innervation Target organ innervation: Heart Vagus nerve carries parasympathetic nerve activity to the heart. Parasympathetic Stimulation ¯ ¯ HR ¯Contractility No effect on vasomotor tone Figure 18-1 Slide 72 Sympathetic innervation Target organ innervation: Heart HEART ↑ HR ↑ Contractility Figure 18-1 Slide 73 Sympathetic innervation of blood vessels. Only certain blood vessels are innervated. Small arteries NOT!!! Figure 18-2 Slide 74 Sympathetic innervation of blood vessels. Target organ innervation: Veins Figure 18-2 Sympathetic stimulation------>venoconstriction------>pushes blood to heart Slide 75 Sympathetic innervation of blood vessels. Target organ innervation: Arteries “Vasoconstrictor” vs. “vasodilator” fibers. Small arteries Vast majority are vasoconstrictor fibers Neurotransmitter = Norepinephrine (NE, Noradrenaline) Receptor = alpha adrenergic receptor Powerful vasoconstriction in kidney, intestines, spleen, skin. Less potent in brain, skeletal muscle. Slide 76 Sympathetic innervation of blood vessels. Target organ innervation: Arteries “Vasoconstrictor vs. vasodilator fibers. Small arteries Few are vasodilator fibers. Importance? Neurotransmitter = Epinephrine (Epi, Adrenaline) Receptor = beta adrenergic receptor Tissue = Skeletal Muscle Slide 77 Sympathetic innervation of blood vessels. Target organ innervation: Arteries “Vasoconstrictor vs. vasodilator fibers. Small arteries Vasodilatory fibers may be involved in syncope (fainting). What causes syncope? Emotional response 1. activation of cardioinhibitory centers (sudden ¯HR ® ¯CO ® ¯BP) 2. activation of sympathetic vasodilatory fibers (vasodilation in skeletal muscle vessels® ¯R ® ¯BP) ¯BP ® ¯blood flow to brain ® loss of consciousness Slide 78 Sympathetic tone maintains basal blood pressure. Anesthesia blocks sympathetic nerve activity. ¯ SNA ® ¯vasoconstrictor tone ® ¯ resistance ® ¯ BP Figure 18-4 Demonstrates importance of sympathetic tone on maintaining basal blood pressure. Slide 79 Sympathetic tone maintains basal blood pressure. NE bolus ® raises BP Figure 18-4 Slide 80 Role of nervous system in rapid control of blood pressure. An important function of nervous control of circulation is the capability to cause rapid increases in pressure when needed. When are pressure increases needed? Exercise, extreme muscle exercise Stress, alarm reaction Slide 81 Role of nervous system in rapid control of blood pressure. How are pressure increases accomplished? Vasoconstriction - Arterioles (almost all, increases TPR ® ­BP ) - Veins (pushes blood to heart, increases blood volume, stretch heart, ­HR ® ­BP ) Cardiac Stimulation - increase pumping capability - sympathetic stimulation increase contractile force of the heart muscle Slide 82 Role of nervous system in rapid control of blood pressure. How quick does it occur? 5 to 10 seconds Slide 83 Example of ability of nervous system to raise pressure: exercise Why does pressure need to increase with exercise? An increase in blood pressure helps drive the increased blood flow required for exercising muscle. £ in P Vasodilation in skeletal muscle also helps augment flow. ¤R Remember: P = F x R; F = P/R so… £ in P and ¤ R will £ F Slide 84 Example of ability of nervous system to raise pressure: exercise Why does pressure need to increase with exercise? How does pressure increase during exercise? Results from sympathetic activation and simultaneous parasympathetic withdrawal at exercise onset… increases cardiac function (contractility, HR) vasoconstriction to non-exercising beds (gut) Slide 85 Arterial Baroreflex Control System Q: What is the baroreflex? A: Neural reflex response that helps to maintain or buffer changes in MAP. Negative feedback reflex mechanism. The most important short term mechanism to prevent fluctuations in arterial blood pressure with posture, emotion, hemorrhage, etc. Slide 86 Baroreflex Control Baroreceptor Anatomy Stretch receptor 3 Components •Sensor/detector •Central sensory center (NTS) •Effector (autonomic nervous system) Sensor •Detect arterial pressure (wall stretch) ­BP stretches BR sensor •Located in carotid sinuses and aortic arch Figure 18-5 Slide 87 Baroreflex Control Baroreceptor Anatomy Sensory Region NTS (nucleus tractus solitarius) Pathway of signal transmission: Carotid baroreceptors to Hering’s nerves to glossopharyngeal nerves to nucleus tractus soltarius Figure 18-5 Slide 88 Baroreflex Control Baroreceptor Anatomy Sensory Region NTS (nucleus tractus solitarius) Pathway of signal transmission: Aortic baroreceptors to vagus nerve to nucleus tractus soltarius Figure 18-5 Slide 89 Baroreflex Control Baroreceptor Activation Baroreceptors most sensitive within the normal BP range Baroreceptor system primary function is to reduce excess, rapid fluctuations in arterial pressure. Orthostatic hypotension Slide 90 Figure 18-6 Baroreflex Control Baroreceptor Activation with Pressure Baroreceptors are silent at low pressures (about 40 - 50 mm Hg). SLOPE = sensitivity Near the normal MAP (100 mm Hg), small changes in BP lead to large changes in receptor discharge. Slope Pressure increases above 160 do not increase discharge further. Figure 18-6 ∆ I = Change in carotid sinus nerve impulses per second ∆ P = change in arterial pressure per second Normal MAP Slide 91 Baroreflex Control Baroreceptor Signaling Baroreceptor Activation - ­ BP ¯ ­ receptor discharge ¯ ­ sympathetic inhibition ¯ ¯ sympathetic activity ­parasympathetic activity ¯ ¯Vasoconstriction ¯HR, ¯cardiac contractility (Leads to ¯ BP) “Negative feedback system” Slide 92 Figure 18-6 Baroreflex Control Baroreceptor Activation Resetting Baroreceptors “reset” when exposed to sustained changes increases or decreases in blood pressure. “Attenuate their potency” Figure 18-6 Slide 93 Baroreflex Control Baroreceptor Activation Resetting Baroreceptors “reset” when exposed to sustained changes increases or decreases in blood pressure. ­ in BP, rightward shift ¯ in BP, leftward shift Figure 18-6 Why is it important that baroreceptors can reset? Slide 94 Examples of baroreflex in BP control Carotid Occlusion: typical response Figure 18-7 Carotid occlusion leads to loss of baroreceptor mediated inhibitory actions on vasomotor center. Slide 95 Carotid occlusion ¯ ¯pressure in carotid sinus ¯ ¯ baroreceptor discharge ¯ ­ sympathetic nerve activity ¯parasympathetic nerve activity ¯ ­BP Examples of baroreflex in BP control Pressure Variability In normal dogs, MAP remains constant despite changes in feeding or posture, defecation and excitement. In baro-denervated dogs, MAP fluctuates widely during normal activity. Baroreceptors buffer “activity” related changes in pressure. Primary purpose of Baroreceptor System: To reduce the minute to minute variation in arterial pressure. Slide 96 Figure 18-8 Baroreflex Control Important Reminders…….. Arterial baroreceptors are activated with increases in pressure. • Baroreceptors are primarily important in the short term, or beat-to beat, control of blood pressure. • Baroreceptors are the principal means of short term BP control. • Importance of baroreceptors in the long term control of BP is questionable. Slide 97 Baroreceptors and orthostatic hypotension. From supine position to suddenly standing up Significant reduction in blood volume 300-800 mls. ¯ Venous return ¯ Cardiac Output Patients with baroreflex failure typically present with postural lightheadedness and orthostatic hypotension. Supine hypertension ¯ Blood Pressure or Hypotension Slide 98 Baroreflex Control Baroreceptor Signaling - ¯ BP ¯ ¯ receptor discharge ¯ ¯ sympathetic inhibition ¯ ­ sympathetic activity ¯ parasympathetic activity ¯ ­ Vasoconstriction ­ HR, ­ cardiac contractility (Leads to ­ BP) “Negative feedback system” Slide 99 Chemoreflex Control Chemoreceptor Anatomy Chemoreceptors initiate the response. Chemosenstive cells: ↓O2 ­CO2 ­H2 Chemoreceptor Signaling ¯O2, ­ C02, H+ ­ ChRc discharge ­ increase sympathetic activity ­ increase blood pressure ­perfusion, O2 delivery, CO2 and H+ removal Slide 100 Other Reflexes Low Pressure Receptors • atria and pulmonary arteries. Potentiate the BR response • activated by volume changes. • detect pressure changes in “low pressure” vessels. ↓BF and cerebral ischemia = “intense” response CNS Ischemic Response • activated when MAP falls below 60 mm Hg. • elicits massive sympathetic response. • extremely powerful, can occlude vessels. • LAST DITCH effort to control pressure and prevent loss of blood to brain. Slide 101 Other Reflexes Cushing Response • increased intracerebral pressure cuts off blood supply to activate CNS ischemic response. ↑BP to > than CSF to ↑BF in the brain Bainbridge Reflex (Atrial Reflex Control of the Heart) • increase in atrial pressure causes an increase in heart rate. • activated by stretch of the sinus node (15% increase). • afferent signals from sinus node can transmit via the vagus nerves to the brain, efferent signals increase heart rate and strength of heart contractility. Slide 102 Neural Reflexes •Arterial Baroreflex mechanism •Arterial Chemoreflex •Low pressure receptors (atria and pulmonary arteries)) •CNS Ischemic Response •Cushing Response •Bainbridge Reflex (Atrial Reflex Control of the Heart) Slide 103