1 / 27

THE AUSTRALIAN NATIONAL UNIVERSITY

THE AUSTRALIAN NATIONAL UNIVERSITY. Neural Regulation of Blood Pressure Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU Christian.Stricker@anu.edu.au http:/ /stricker.jcsmr.anu.edu.au/BPControl.pptx. Aims. The students should

caspar
Download Presentation

THE AUSTRALIAN NATIONAL UNIVERSITY

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. THE AUSTRALIAN NATIONAL UNIVERSITY Neural Regulation ofBlood PressureChristian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/BPControl.pptx

  2. Aims The students should • know the sensors, integrators and effectors of the regulating reflexes; • be cognisant of the anatomy of the reflex pathways; • understand which and how the three system parameters HR, SV and TPR are regulated; • be able to explain the nature and effect of the arterial baroreflex (AB) and the cardiopulmonary reflexes (CP); • realise that AB controls HR, SV and TPR, and CP mainly venous return; • know how AB establishes beat-to-beat control; and • appreciate how these reflexes can be modulated by respiration and asphyxia/shock.

  3. Contents • Overview of control of blood pressure • Principles of negative feedback • Regulated variables • Arterial baroreflex (AB) • Components: detectors (high pressure baroreceptors) and afferents, efferents (sympathetic and parasympathetic output to cardiovascular system), integrator. • Cardiopulmonary reflexes (CP) • Low pressure baroreceptors • Atrial stretch and release of ANP • Respiratory and chemoreflex modulation of baroreflex activity.

  4. Topic Coverage • In this session, we ONLY look at BP control on the short-term (seconds). • Long-term BP control via volume control will be done in relation to kidney (see later “Volume regulation”). • How vessels locally regulate resistance will be subject of the next lecture.

  5. Overview of BP Control • In Block 1, mean arterial pressure was defined as is mean arterial pressure (MAP), TPR total peripheral resistance, CO cardiac output, SV stroke volume and HR heart rate. • controlled by 3 variables: TPR, SV and HR. • Which system can control these parameters? • Autonomic nervous system: • HR: sympathetic (+), parasympathetic (-; vagal) at level of heart • SV: sympathetic (+), parasympathetic (-) at level of heart; sympathetic (+) at the level of TPR (afterload); and others. • TPR: sympathetic (+) ONLY at level of vessels (and other factors like volume, etc.).

  6. Principles of Feedback Control • Positive and negative feedback. • Feed-forward: central command before exercise starts. • Functional characteristic is feedback time: from few 100 ms to hours or even longer.

  7. Overview of Reflex Control Levick, 5th ed., 2010 • Two reflexes involved: • arterial baroreflex (AB) and • cardio-pulmonary reflexes (CP). • Modulated by respiration, chemoreceptors and muscle activity.

  8. Arterial Baroreceptor Reflex (AB) Mostly negative feedback to brainstem, sympathetic nervous system and heart: Depressor reflex. If BP↑ → HR↓, SV↓, TPR↓ and vice versa if BP↓. Most important contributor to short-term homeostatic control of BP (s - min).

  9. Locations of AB Sensors Levick, 5th ed., 2010 • Pressoreceptors in aortic arch and carotid sinus. • Afferent nerves: IX (carotid sinus) & X (aortic arch).

  10. AB Sensor Properties Modified from Levick, 5th ed., 2011 • True mechanoreceptors (wall tension in vessel). • Sensitive over large BP range • Low pressure range: A-fibres; high pressure range: C-fibres. • Linear response in “normal” range. • Respond better to pulsatile than steady pressure: adaptation under steady (static) conditions, much less under pulsatile (dynamic): poor at relaying absolute BP information.

  11. AB Set-Point and Sensitivity • Set-point = maximal slope in HR vs BP plot. • Maximal sensitivity 80 – 100 torr. • Exercise resets set-point: work in higher range without incurring depressor response; i.e. at same BP, a higher HR can be achieved. • Reset in proportion to work intensity. • AB active but HR is per-mitted to increase (muscle spindles). Levy, 5th ed., 2010

  12. AB Efferent Activity • SY output over whole sympathetic chain (T1 – L3). • Parasympathetic output via VA and lumbosacral cord (S2 – S4). • Both systems are activated wholly. • VA and SY outputs act together but in opposite directions: • VA↓ → SY↑ or • VA↑ → SY↓. • For > 180 torr, VA is maximised and SY minimised. Kollai & Koizumi, Pflügers Arch (1989), 413:365-371

  13. Central AB Pathways Levick, 5th ed., 2010 • Control area in brainstem (several nuclei) receives input from / sends outputs to many brain parts (hypothalamus). • VA relay simpler (NTS → NA) than that of SY pathway.

  14. Medullar Interactions Levick, 5th ed., 2010 • VA→ SY inhibition (dashed lines) via • caudal inhibiting rostral vasopressor area (via GABA) and • raphe nucleus to spinal cord (via serotonin).

  15. AB Function • AB helps to stabilise BP control beat by beat (fast response) within narrow range – if taken away, BP fluctuates widely. • Effect via VA↑ and SY↓ activity when BP↑: HR↓, SV↓ and TPR↓ and vice versa. • VA and SY work together in opposite direction. • During exercise, set-point shifted such that BP can rise without HRinhibition. • Can be modulated via inputs from respiratory and other centres. • Activated in orthostasis, dehydration, blood loss, shock, etc.

  16. Cardiopulmonary Reflexes (CP) Collection of various reflexes based on sensor types. Low-pressure receptor reflexes. Most afferent input is also inhibitory. Work predominantly on VR

  17. Cardiopulmonary Reflexes (CP) • Cardiac de-afferentiation (transplant) reveals tonic inhibition of HR and TPR only. • Intracoronary injection of solute causes bradycardia, vasodilation and hypotension: • Depressor reflex • Important to know some specific reflexes as some can cause only SY↑ activity: • Activation of veno-atrial stretch receptors causes tachycardia and diuresis.

  18. Locations & Types of CP Sensors Levick, 5th ed., 2010 • Mostly small, unmyelinated (80%) fibres from • cardiac mechano-receptors (wall tension in ventricle); • ventricular chemosensors (mediate pain via SY fibres): SY activity↑; • coronary artery baroreceptors (perfusion pressure); and • Some myelinated (20%): veno-atrial mechanoreceptors.

  19. 2. Veno-Atrial Stretch Receptors • Measure atrial blood volume, i.e. central venous pressure/volume in low pressure part of circulation (central veins, atria); control venous return (VR). • Upon activation (via infusion - ‘Bainbridge effect’) • tachycardia: SY activity↑ to SA node without change inVA activity; and • diuresis and natriuresis: control of blood volume via renal vasodilation (renal Na+ excretion↑), anti-diuretic hormone release↓ (hypothalamus) and release↑ of atrial natriuretic peptide (ANP) by atrial cells. • Part of long-term volume regulation response (see volume control in kidney section, later).

  20. Modulation by Respiration & Arterial Chemoreceptors

  21. Respiratory Modulation • During inspiration, HR↑: • Inspiratory centre inhibits vagal output (motoneurones shortly unresponsive to AB input) → disinhibition of SA node → HR↑. • Converse is true during expiration. • Emotional faint: VA output↑ → HR↓↓. Levick, 5th ed., 2010

  22. Arterial Chemoreceptors (+) • Carotid and aortic bodies (see respiration, later): sense and . • If < 80 torr (asphyxia, clinical shock, …): • TPR↑ as renal, splanchnic and muscle vascular beds are constricted; • Splanchnic veins constrict → pooling↓ → PMSF↑ → SV↑ → CO↑. • BP↑ as TPR↑ and CO↑. • Tachycardia due to resp. rate ↑ (lung stretch receptors inhibit VA output).

  23. Take-Home Messages • Under resting conditions, VA output on heart is more effective than that of SY. • Both, VA and SY nerves are tonically active and oppose each other; VA effect is fast, SY effect appreciably slower. • AB provides important short-term regulation of BP: • If BP↑, high pressure receptors in aortic arch and carotid sinus cause HR↓, SV↓ and TPR↓ and vice versa. • Cardiac de-afferentiation reveals tonic inhibitory effect on HR and TPR – part of CP reflexes. • Veno-atrial stretch receptors help control BP↑ mainly via circulating blood volume↓ / VR↓; i.e. renal vasodilation, hypothalamic release↓ of ADH and atrial release↑ of ANP. • When BP falls < 80 torr, peripheral chemoreceptors modulate BP via vasoconstriction plus tachycardia.

  24. MCQ Jack West, a 28 year-old bike rider was involved in a car accident on the road to the snowfields. He suffered a broken femur and was immediately transferred to Cooma Base Hospital. On admission, his HR was 112 bpm, and his blood pressure 65 / 45 torr. Which of the following statements best describes the state of the arterial baro- and cardiopulmonary reflex? • Arterial baroreceptor activity↓; sympathetic outflow↓; parasympathetic outflow↑; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↑; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↓; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↑; sympathetic outflow↓; parasympathetic outflow↑; low pressure mechanoreceptor activity↑. • Arterial baroreceptor activity↑; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↑.

  25. That’s it folks…

  26. MCQ Jack West, a 28 year-old bike rider was involved in a car accident on the road to the snowfields. He suffered a broken femur and was immediately transferred to Cooma Base Hospital. On admission, his HR was 112 bpm, and his blood pressure 65 / 45 torr. Which of the following statements best describes the state of the arterial baro- and cardiopulmonary reflex? • Arterial baroreceptor activity↓; sympathetic outflow↓; parasympathetic outflow↑; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↑; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↓; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↓. • Arterial baroreceptor activity↑; sympathetic outflow↓; parasympathetic outflow↑; low pressure mechanoreceptor activity↑. • Arterial baroreceptor activity↑; sympathetic outflow↑; parasympathetic outflow↓; low pressure mechanoreceptor activity↑.

More Related