Neural and Chemical Control of Respiratory and Cardiovascular function PDF

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Neural and Chemical Control of Respiratory and Cardiovascular function...

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Neural and Chemical Control of Respiratory and Cardiovascular function Respiratory Control ◊ ◊ ◊ ◊ ◊

At rest, body cells use around 200mls of oxygen each minute Strenuous exercise - typically increases 15-20-fold in normal healthy adults Elite endurance trained athletes- typically increases 30-fold The challenge for the body? Match respiratory effort to metabolic demand

Rhythm of respiration 

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Controlled by inspiratory area – brain stem  Normal cycle 5-6 seconds (2/3 or 2/4)  Progressive increase in strength of excitatory signals to the inspiratory muscles Contraction of the inspiratory muscles Thorax increases in size, internal pressure drops and air enters passively Suddenly the respiratory signal ceases. Relaxation Expiration – a passive process (normally) Relaxation of Inspiratory muscles (switch-off) Thorax decreases in size, pressure rises and air leaves the lungs

Controlled by 2 separate interacting neural mechanisms 1. Voluntary system – cerebral cortex - Corticospinal tracts eg. Playing a wind instrument 2. Automatic System – medulla and pons - Matches respiration to metabolic needs Basic Rhythm of Respiration  Controlled by two specialised groups of neurons in the brain stem  Medulla Oblongata – controls the basic rhythm of respiration (Respiratory Rhythmicity Centre – RRC). Dorsal group – inspiration. Ventral group - expiration  Pons- adjusts the activity of the RRC in response to input from other areas of the brain – rate and depth of inspiration PONS 

Pneumotaxic area - Helps to turn off the inspiratory area “prematurely” - Shortens the duration of inhalation

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- Increases breathing rate Apneustic area - Sends excitatory impulses to inspiratory area to “keep going” - Facilitates respiration - Prolongs inhalation - Long deep inhalation Controlled mechanically to prevent over-inflation and Barotrauma (Hering-Breuer Reflex)

Basic Rhythm of respiration   

Basic rhythm set and co-ordinated by respiratory area in Medulla Oblongata However, we do not breathe at a set rate Rate is modified by: ◊ Chemical Stimuli ◊ Non-chemical stimuli ◊ Other factors (medication, limbic system)

Chemoreceptor Regulation    

Certain chemical stimuli determine how quickly and deeply we breath Chemicals regulating activity are:- O2, CO2, H+ ions Raised PCO2 /H+ ions or lowered O2 levels – increase respiratory centre activity Lowered PCO2/H+ions or raised O2 levels –decrease respiratory centre activity

Central Chemoreceptors o Found on the ventral surface of the medulla near, but not part of the respiratory centre o Superficial enough to be bathed in CSF, as well as being surrounded by the brain’s extra cellular fluid o Respond to a change in concentration of H+ ions and levels of CO2 in these fluids. o These are produced as a response to aerobic respiration in the cells (activity – muscle)

Location of the peripheral chemoreceptors  Two carotid bodies – near the bifurcation of the carotid artery at each side  Two or more aortic bodies- near the aortic arch  Mainly react to arterial hypoxemia , less so to H+ ions and CO2  Carotid bodies more sensitive than aortic bodies (consider brain) CO2 Hydration Blood brain barrier impermeable to H+ ions but CO2 readily diffuses across then undergoes hydration  CO2 + H2O H2CO3 H+ + HCO3 Changes in CO2 causes local changes in H+ ion concentration  Central chemoreceptors sensitive to this change  Effects of CO2 on respiration depend on the hydration and subsequent dissociation of CO2 into H+ ions which then stimulate the central chemoreceptors.  Exercise and activity will increase production of CO2 and therefore also increase in H+

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Increased CO2 o PCO2 = 5.3 KPa (40 mmHg) o Increase in PCO2 ( hypercapnia)- increases H+ ions- stimulates central and to a lesser extent peripheral receptors. o Highly activated inspiratory area – rate and depth of breathing increases – hyperventilation o Continues or increases as output of PCO2 and decreased pH is produced Low CO2 o Below 5.3 KPa (40mmHg) – hypocapnia o Central and peripheral receptors not stimulated – lack of stimulus to inspiratory area. o Inspiratory area sets a slow pace (rate/depth) until CO2 accumulates and rises to 5.3 KPa (40mmHg) Severe deficiency of O2      

Depresses the central chemoreceptor activity and inspiratory area Stops responding effectively to any inputs and sends fewer impulses to respiratory muscles Decreases breathing rate – worsening situation Hypoxaemia can kill in minutes If PaO2 drops to 8KPa (60mmHg) the peripheral chemoreceptors in carotid bodies will stimulate the inspiratory centre. Patients with chronic respiratory illness may rely on this “back-up” system

Other Factors Limbic system in brain – emotional anxiety /exercise anticipation/performance stress - Excitatory – increases rate and depth  Temperature – increases (fever and vigorous muscular exercise) – increases rate  Temperature – decrease (Hypothermia) – decreases rate  Proprioceptors – exercise, your rate and depth of breathing increases even before changes in chemical – proprioceptors that monitor joint and muscle movement send signals to medulla

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Terms     

Hypoventilation; normal respiration is between 10-16 breaths per minute Hyperventilation Tachypnea – fast breathing Dyspnoea – disordered breathing Apnea – patient not breathing for a period of time

Lungs & Heart o o o o

Work together One can help other (to a certain extent) Both fulfil same overall role in homeostasis However, heart has specific and specialist mechanisms for ensuring cardiovascular demand meets supply

Regulation of the heart o Adjustments to the heart rate are important in the short-term control of Cardiac Output and Blood Pressure. Cardiac output – the amount of blood that is being taken out of the heart every minute o CO = SV x HR o BP = CO x SVR o Adjustments to the heart rate are important in the short term control of Cardiac Output and Blood Pressure o CO = SV x HR o BP = CO x SVR Systems of regulation  Autonomic Nervous System  Hormones released by Adrenal Glands Autonomic Nervous System  Nervous system regulation of heart originates in CV centre of Medulla  Receives input from a variety of sensory receptors and higher centres in brain – cerebral cortex and Limbic system

 CV Centre – directs appropriate output by increasing and decreasing frequency of nerve impulses sent out to ANS.  Both Sympathetic and Parasympathetic branches

Sympathetic System    

CV centre to Sympathetic neurons to heart Via cardiac accelerator nerve Innervates conducting system - atria and ventricles of heart Cardiac accelerator nerve releases nor-epinephrine (nor-adrenaline) to increase heart rate

Parasympathetic Nervous System    

CV centre to Parasympathetic neurons to heart Via Vagus nerve – (cranial nerve X) Innervates conducting system – atria Neurotransmitter they release is – acetylcholine – slowing activity of SA node.

Normal resting heart rate o Balance between SNS and PNS o To increase HR -  SNS activity, -  PNS activity, or both o To slow HR – -  SNS activity, -  PNS activity, or both Stroke Volume Control ◊ ◊ ◊ ◊ ◊ ◊

 SNS activity enhances myocardial contractility Leads to better emptying of the ventricles  Venous return  greater stretch of ventricular walls prior to contraction Further increases SV Known as Frank-Starling Law of the Heart Ensures demand and supply meet

Sensory Receptors ◊ ◊ ◊ ◊ ◊ ◊ ◊ ◊

Baroreceptors (pressure sensors) – neurones sensitive to changes in BP Located in the Arch of the Aorta and Carotid Arteries Increased BP – baroreceptors send impulses along sensory neurons to CV centre in medulla CV centre responds- more impulses along parasympathetic (motor) nerve and decrease in accelerator output Heart rate, decreases CO, and thus decreases BP Baroreceptors do not stimulate CV centre Lack of stimulus Increase HR, Increases CO and this leads to increasing BP to normal levels

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Chemoreceptors

Sensitive to O2, CO2, H+ Located in Carotid Arteries and Arch of Aorta Hypoxia, Acidity (pH), Hypercapnia Stimulate the chemoreceptors to send information to CV centre in medulla Increased Sympathetic stimulation to peripheral arterioles and veins, producing increases vasoconstriction and increases BP  Also increased HR and force of contraction  Supply matched to demand

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Chemical Regulation 

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Hormones - Epinephine and norepinephine – Adrenal medulla - Enhance the hearts effectiveness as a pump - HR and force of contraction Excitement, Stress , Exercise - Adrenal Medulla – produce the hormone Longer-term changes (revising for exam!)...