Autonomic Control of the Heart PDF

Autonomic control of the heart Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Define Autonomic nervous system Identify which ANS component is dominant at rest and during exercise Explain the role of sympathetic and parasympathetic nervous system in t...

Autonomic control of the heart Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Define Autonomic nervous system Identify which ANS component is dominant at rest and during exercise Explain the role of sympathetic and parasympathetic nervous system in the control of heart rate. Describe the mechanisms of ANS on heart rate conductivity and contractility. Describe the effect oa ANS on blood vessels Autonomic nervous system Autonomic means involuntary or subconscious activity. Autonomic nervous system ANS= ANS is the neural network that controls these subconscious functions. Sympathetic Nervous system Controls many functions under times of acute stress ‘’ Fight and Flight” Parasympathetic Nervous system Control the body function during rest “rest-and-digest” response. (Cardiovascular centers) Vasomotor center VMC Cardiac accelerator center (CAC )or Vasoconstrictor center Vasodilator center Cardiac Inhibitory Center CIC (Cardiovascular centers) Definition: collection of neurons in the reticular formation of the medulla oblongata. Cardiac Acceleration Centre It contains VMC and CIC 1- Vasomotor center is divided into CAC and vasodilator Center A- Cardiac acceleration center (CAC),or vasoconstrictor center. Stimulation of CAC →increase sympathetic discharge to Adrenal medulla → Catecholamine release Blood vessel → Vasoconstriction (Cardiovascular centers) B- Vasodilator center (VDC). Stimulation of VDC→ Generalized vasodilation through inhibition of Vasoconstrictor center. 2- Cardiac Inhibitory Center (CIC) Contains the Dorsal motor nucleus of the vagus → Sends inhibitory signals to the heart via the vagus nerve Cardiac Inhibitory Centre  Sympathetic & parasympathetic supply atria (SAN & AVN) with predominance of the vagus nerve on S.A. Node during rest (vagal tone).  No vagal fibers supply the ventricles ( Ventricular vagal escape). So strong vagal stimulation →The ventricle continuous beating (25-40/min) after short time by idioventricular rhythm Sympathetic nervous system on the heart  It is derived from (T1-T5).  Sympathetic nervous system supply all part of the heart (atria, ventricles, conduction system and the coronary vessels). Effect of sympathetic stimulation Increase All Cardiac properties (Excitability, Rhythmicity , Conductivity and Contractility) 1-increase the heart rate in young adult humans from 70 beats/min up to 180 to 200 beats/min and, rarely, even 250 beats/min. 2- double the force of heart contraction, increasing the volume of blood pumped and the ejection pressure. 3- increase the cardiac output as much as twofold to threefold. 4- increase cardiac output, heart work and O2 consumption. 5- Vasodilation of the coronary blood vessels Parasympathetic nervous system on the heart  It is derived from the vagus nerve.  Parasympathetic nervous system supply part of the heart (S.A Node , A.V. Node, atria, conduction system and the coronary vessels).  Right vagus supply S. A Node, While left vagus supply A.V. Node. Effect of parasympathetic stimulation Depression of All Cardiac properties (Excitability, Rhythmicity , Conductivity and Contractility) 1- it cause –ve chronotropic effect, decrease atrial contraction (-ve inotropic), decrease atrial excitability, and decrease conductivity. 2- decrease cardiac output, heart work and O2 consumption. 3- Vasoconstriction of the coronary blood vessels Sympathetic vs Parasympathetic on Heart rate Sympathetic Post-ganglionic sympathetic fibres released norepinephrine (NE)→activate β1 adrenergic receptors located in the SA node → opens sodium (Na+) channels, “funny” current (If)→increase pacemaker potential Parasympathetic Post-ganglionic parasympathetic fibres released Acetylcholine (ACh)→activate Muscarinic receptors M3 located in the SA node → closes sodium (Na+) channels →decrease “funny” current (If)→decrease pacemaker potential Sympathetic vs Parasympathetic on Conductivity Sympathetic Parasympathetic Post-ganglionic sympathetic fibres released Post-ganglionic parasympathetic fibres norepinephrine (NE) → activate β1 released Acetylcholine (ACh)→activate adrenergic receptors located in the A.V node Muscarinic receptors M3 located in the A.V. → opens (Ca++) channels → faster Node → closes (Ca++) channels → Slow conduction through the AV node. conduction through the AV node. Sympathetic vs Parasympathetic on Contractility Sympathetic Post-ganglionic sympathetic fibres released norepinephrine (NE) → activate β1 adrenergic receptors located in the myocyte→ Open more Ca2+ channels , Increase the activity of the Ca2+ pump and Ca+2 storage in the sarcoplasmic reticulum → elevation of the plateau →increase Ventricular contraction. Parasympathetic PSNS does not innervate ventricular myocytes →NO direct effect on ventricular contractility. Autonomic Nervous System Regulation of Vascular Tone The main ANS factor altering vascular tone is the sympathetic nervous system. Post-ganglionic sympathetic fibres to the blood vessels releases norepinephrine (NE) → binds strongly to α1 and α2 receptors or weakly to β2 receptors. α1 and α2 receptors → vasoconstriction of the blood vessel → increase Arterial blood pressure. β2 adrenergic receptors → vasodilation of the blood vessel. Overall, the predominate effect of norepinephrine on the vasculature is vasoconstrictive → increasing vascular tone. Review Questions The sympathetic nervous system alters the electrical transmission of the sinoatrial node by influencing which of the following mechanisms? A. B. C. D. E. Decreasing Ca 2+conductance Decreasing Na + conductance Increasing Ca 2+ conductance Increasing Cl–conductance Increasing Na+ conductance Which of the following correctly lists the effects of the sympathetic nervous system (SNS) and parasympathetic nervous system (PSNS) on the heart? A. SNS: ↑ heart rate, ↑ contractility; PSNS: ↓ heart rate, ↓ contractility B. SNS: ↑ heart rate, ↑ contractility; PSNS: ↓ contractility C. SNS: ↑ heart rate, ↑ contractility; PSNS: ↓ heart rate D. SNS: ↑ heart rate; PSNS: ↓ heart rate ↓ contractility E. SNS: ↑ contractility; PSNS: ↓ heart rate, ↓ contractility Summary Define Autonomic nervous system Identify which ANS component is dominant at rest and during exercise Explain the role of sympathetic and parasympathetic nervous system in the control of heart rate. Describe the mechanisms of ANS on heart rate conductivity and contractility. Describe the effect of ANS on blood vessels Learning Resources  Hall JE, Hall ME. Guyton and Hall textbook of medical physiology e-Book. 14ed, Elsevier Health Sciences; 2021, Chapter 9, 113-126. https://www-clinicalkey- com.gmulibrary.com/#!/content/book/3-s2.0-B9780323597128000096  https://exchange.scholarrx.com/brick/autonomic-regulation-of-the-cardiovascular-system  Power-point presentation in the Moodle. www.gmu.ac.ae COLLEGE OF MEDICINE Thank you